9.23 Waters Divided: A History of Alluvial Fan Research and a View of Its Future

9.23 Waters Divided: A History of Alluvial Fan Research and a View of Its Future JD Stock, US Geological Survey, Menlo Park, CA, USA Published by Elsevier Inc.

9.23.1 Introduction 9.23.2 Formative Boundary Conditions for Alluvial Fan 9.23.3 Processes that Supply Sediment to Alluvial Fans 9.23.4 Processes Observed on Fans 9.23.5 Hypotheses Guiding Field and Experimental Work 9.23.6 Morphometry 9.23.7 Hydraulic Geometry 9.23.8 Sedimentology 9.23.8.1 Quantitative Sedimentology 9.23.8.2 Descriptive Stratigraphy 9.23.9 Geologic Record of Fans 9.23.9.1 Surficial Mapping and Dating 9.23.9.2 Deeper Stratigraphic Record 9.23.10 Experimental Approaches 9.23.11 Models of Fan Evolution 9.23.12 The Record of Hazards on Alluvial Fans 9.23.13 Discussion 9.23.13.1 What Generalizations Can We Make? 9.23.13.2 Needs for the Future Acknowledgments References

Glossary Alluvial fan Radial landform built of sediments deposited during the progressive downstream division of sediment, wholly or largely by the tractive forces of water. Commonly found where channels emerge from confined valleys onto plains or wider valleys, thus resulting in unconfined flows that spread laterally by avulsion. Armor Streambed surface layer of particles that is relatively coarse compared with the subsurface deposit. It is observed to form in flumes and during streamflow by the removal of fine particles until the surface becomes resistant to further entrainment or by the frictional aggregation of coarser particles. Avulsion Formation of a new channelized flow by breaking out of an existing pathway. Bajada Depositional plain adjacent to an eroding highland, typically occupied by multiple, coalesced alluvial fans. Debris flow fan Fan-shaped accumulation of deposits primarily from debris flows and associated inertial transport processes (e.g., avalanches). Distinguished often by the presence of debris flow deposits with boulder levees. There

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is no published guidance on how to define fans with deposits from both debris flows and traction transport, but one practice is to reserve the term debris flow fan for landforms above 10–15% where boulder levees of debris flow deposits characterize the surface. Fan head Uppermost point on the fan at which water and sediment begin to diverge. Precise usage varies because the modern channel network carrying sediment may be deeply inset within the older fan landform, so that modern sediment diverges downfan from an older fan head. Froude number A ratio of the influences of fluid inertial forces to gravitational forces, approximated by dividing flow velocity by the square root of the product of the gravitational acceleration (g) and mean flow depth. Local Froude numbers in excess of 1 represent supercritical flow conditions, commonly accompanied by antidunes, chutesand-pools, or upper planar beds in sand-bedded channels. Grainflow Lobes of sliding or rolling grains moving under largely dry conditions, often observed on steep, postwildfire hillslopes. Hyperconcentrated flow Flows in which the concentration of solids is sufficient to materially affect flow

Stock, J.D., 2013. Waters divided: a history of alluvial fan research and a view of its future. In: Shroder, J. (Editor in Chief), Wohl, E. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 9, Fluvial Geomorphology, pp. 413–458.

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

properties by damping turbulence, decreasing settling velocity, or creating finite yield strength. Costa (1984) proposed that flows with 40–70% solids by weight met some of these criteria. The upper boundary of 70% approximates flows sampled in the Rio Puerco River of New Mexico, USA. In the absence of further work this range has endured, with higher concentrations thought to be debris flows. Imbrication Stacking of elongate particles against each other by fluid stresses in a geometry analogous to books tilted together on a shelf. This geometry, with the large flat side-tilted downflow, maximizes resistance of the particle to entrainment. Imbrication is a characteristic of traction transport processes. Intersection point A term introduced by Hooke (1967) to describe the location at which incised fanhead channels rejoin the fan surface. Mudflow Debris flow composed predominately of sand or finer particles. The term has been applied indiscriminately in the media and the vernacular to describe a wide range of

storm-generated fine-grained debris flows. As a consequence, its meaning is often vague. Ravel Movement of individual particles down steep slopes by rolling, bouncing, or sliding. Commonly observed on post-wildfire hillslopes steeper than common friction angles. Sieve deposit Lobes of clast-supported, matrix-free gravels and cobbles inferred to represent the deposition of coarse debris as the water moving it infiltrated rapidly into a permeable subsurface. Observed in flume studies (Hooke, 1967) and interpreted in the field. Traction transport Grain by grain transport of sediment as surface drag forces roll, slide, and bounce particles. Resulting deposits show imbrication of particles from pea gravel to some boulders, and deposits may show bedding. Unconfined flow General term for flow lacking welldefined banks or walls, characteristically observed on hillslopes as Horton overland flow or saturation overland flow, or in distributary environments as threads of fast moving flow within slower moving flow.

Abstract Flows exiting confined valleys tend to deposit sediment in fan-shaped landforms. Where deposition is wholly or largely by the tractive forces of flowing water, these landforms are called alluvial fans. They are the product of the progressive division of water and sediment downfan, from slopes that may exceed 0.10 to distal slopes that may be below 0.01. Channel depths also tend to decline, from values that approach one to several meters at steep fanheads, to a few decimeters at distal fan margins. The result is a radiating, depositional ramp where confined or unconfined flows transport sediment from source basins to bounding streams, subsiding basins, or stable platforms. Where streams or subsiding basins consume the sediment supply from the source basin, the fan may approach a steady form whose extent and distal slope are set by stream location or subsidence rate. Where boundary conditions do not remove sediment, the fan may prograde out to long distances and low slopes (o0.01). Theoretical and experimental work over the past several decades support the notion that alluvial fan long-profiles become steeper as sediment supply increases or transport capacity decreases, and increasingly concave upward as the rate of bed material deposition decreases downfan. Grainsize distributions of alluvial fans seem to span the range observed in alluvial rivers, with no processes that uniquely identify them, apart from the distributary pattern of deposition. Bed sand cover tends to increase downfan in arid-region fans, with an absence of systematic downfan fining of coarser grain sizes. Surficial mapping and geochronology have demonstrated that fan deposition varies greatly through time, arguably from climate variations that alter hillslope sediment supply. The combination of surficial mapping and hydraulic modeling with high-resolution topography can now produce detailed flood susceptibility maps. The effective use of these maps to protect lives and property, however, depends on answering many of the enduring questions about the mechanics of how water and sediment divide down alluvial fans.

9.23.1

Introduction

The man who introduced alluvial fans to the geologic literature spent a decade in the steeplands of Kashmir in the service of its Maharajah. During his investigations of mineral and forest resources for the Maharajah, and eventually as Governor of the area of Ladakh, Drew (1875) observed the geography and culture of Kashmir. In his travels along recently deglaciated valleys, Drew (1873) observed that radial cones of sediment accumulated where steep tributary valleys joined lower gradient mainstem valleys (Figure 1(a)). Drew called these features alluvial fans. At the time he wrote, alluvium was a general term for sediment deposited by flowing water (derived from Latin roots meaning ‘to wash against’). He used the term fan to convey the essence of the shape of these deposits. Although

other workers had recognized and described similar features they called alluvial cones in the European Alps (e.g., Blanqui, 1843; Surell, 1844; Marsh, 1874), New Zealand (Haast, 1864), and the western United States (Figure 1(b); Gilbert, 1882), Drew was the first to focus on their mechanics, and to propose a coherent hypothesis for their form. In nearly 140 years since the publication of his paper (Drew, 1873) describing alluvial fans, fan-shaped depositional features have been described from every continent on the Earth, and on Mars. Starting in the 1930s and accelerating to the present, features called alluvial fans have been studied to understand their hazards, their sedimentology as it informs the search for ancient reservoirs of oil or water, as a geologic record of tectonic and climatic change, and more abstractly, for their role in shaping landscapes. They have been studied by

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Figure 1 Early illustrations of alluvial fans. (a) Illustration from Drew (1873), who coined the term alluvial fan. (b) Illustration from Gilbert (1882) of an alluvial cone along the Wasatch Mountains, UT, USA. The term fan won out in the literature. Note that neither of these early works specifies the processes that deposit material on fans.

sedimentologists, physical geographers, geomorphologists, hydrologists, geophysicists, petroleum engineers, and many other disciplines in an effort to understand their genesis, architecture, and physical properties. As a consequence, there are many views of what constitutes an alluvial fan. Drew’s original definition is arguably vague enough to encompass a number of fundamentally different processes that deposit sediment in fan-shaped geometries. A modest summary of these processes would at least include rockfall, ravel, grainflow, landslides, rock avalanches, debris flows, hyperconcentrated flows, and conventional fluvial transport by traction (see Figures 2–5). The features he described in the Kashmir could have been formed in part by nearly all of these processes, although he describes talus fans separately. His use of ‘alluvial’ would be inconsistent with the strictly gravitational processes of rockfall, ravel, and grainflow particular to talus fans, although one could debate whether water’s role in debris flows would exclude them from the definition. As a matter of practice, alluvial fans are widely regarded as those fans deposited wholly, or in part, by the tractive forces of flowing water. There is an additional debate of whether fans that evolve along or at the terrestrial boundary of fluvial networks are properly called alluvial fans. Although these fans

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share geometric properties of their steepland cousins, their sediment is commonly better sorted by fluvial processes. This debate is further complicated by the reality that some fans have had significant changes in process or boundary condition over their depositional lifetime; many fans in the Great Basin were at one time either subaqueous or had lakes as their boundary condition (e.g., Reheis, 1999). Changing source and depositional conditions imply that some fans may be a palimpsest of different processes, leading to the difficulty of using a single process to characterize them. This chapter will be limited to the history of work on fan forms made primarily by traction transport (i.e., particle-byparticle transport by flowing water). In doing so it omits a large number of important studies on fans wholly created by inertial transport processes, including debris flows, rockfalls, ravel, grainflow, and other processes characteristic of the steepest landscapes (e.g., Figures 2 and 3). These processes encompass fundamentally different physics from those of traction transport of hyperconcentrated and fluvial processes. In particular, we have yet to understand the physics of inertial processes well enough to be able to characterize them with transport equations, as we commonly do (however crudely) with excess shear stress formulae for traction transport. For the sake of brevity the chapter also omits discussing fans deposited at the margins of the ocean (fan-deltas) or in deep subaqueous environments (lacustrine or marine) where a different set of processes or boundary conditions must also be considered (e.g., Nemec and Steel, 1988; Mohrig et al., 1998; Sun et al., 2002). Readers will also notice the omission of a substantial Japanese-language literature on alluvial fans (e.g., Yazawa et al., 1971; Saito, 1988), in large part because this important work is inaccessible to me. This chapter attempts to update the many fine reviews and special volumes on alluvial fans (e.g., Bull, 1977; McGowen, 1979; Nilsen, 1982; Ethridge, 1985; Rachocki and Church, 1990; Blair and McPherson, 1994a, b, 2009; Harvey et al., 2005) by including newer experimental and surficial mapping work. Many of the hypotheses from newer experimental work have yet to be tested in the field, and recent mapping and geochronology will likely form a basis for future flood hazard estimates. The reader will recognize that there is an inherent bias toward fans of the American southwest, from where much of the literature and the author’s own experiences originate. They may note an encouraging trend in alluvial fan investigations outside this region, a trend that will hopefully lead to new insights from fans with very different climatic and tectonic boundary conditions. In the following review I describe the boundary conditions necessary for fan deposition, which lead to a crude geography of alluvial fan deposits. I use these to focus a description of source and depositional processes on fans as we understand them from observation and field work. I then describe the existing set of hypotheses that have guided studies on alluvial fans since Drew’s work. I describe the models of morphometry and sedimentology that have evolved from these hypotheses, and how they have been used to construct a geologic view of alluvial fans as it informs the search for water and oil resources in ancient fan deposits, and the record of environmental change stored in some fans. A number of elemental problems that are difficult to solve from geologic evidence

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Figure 2 Images of fans dominated by inertial depositional processes, at successively lower average slopes. (a) Fan constructed by postfire rockfall, grainflow, and debris flows, 2009, Mill Creek below Morton Peak, San Bernardino Mts, CA, USA. (b) Detail of small (B1 dm wide) debris flow on (a). (c) Larger example of a fan constructed in the Sierra, NV, USA, by similar processes. (d) Debris flow fan sourced in limestones of the Bay of Kotor, Montenegro. Note the surface morphology of nested lobes. Generation of debris flows does not require fine sediment. (e) Postfire debris flow deposit on a steep fan in the Big Tujunga Canyon, San Gabriel Mts, CA, USA. (f) Image of peak flow coming from postfire hyperconcentrated flow in the Harvard Hills, southern California, courtesy of staff at Stough Canyon Nature Center, City of Burbank, CA, USA. This and other images show peak flows that have characteristics of fluids rather than debris flows. (g) Fan deposited by hyperconcentrated or debris flows, margin of Mill Creek, San Bernardino Mts, CA, USA. Deposit from (f) and (g) are shown in Figures 3(e) and 3(f).

alone have led to an important body of new experimental work, the goal of which is to understand as precisely as possible how boundary conditions affect fan deposition and architecture. Much of this work has yet to find its way into published field applications. From well-constrained experimental models, I turn to the record of recent hazardous events on alluvial fans, which argue for a deeper marriage between field and experimental work. In the Discussion section I attempt to generalize from the existing body of work and point towards the use of new technologies to solve outstanding problems.

9.23.2

Formative Boundary Conditions for Alluvial Fan

Drew (1873) proposed that the slope of streams was proportional to the transport rate of sediment (implied to be bedload). Figure 6(a), modified from Drew (1873), shows a concave-up longitudinal profile whose slope declines as the transport rate declines downstream due to bedload deposition. Ikeda and Iseya (1988) demonstrated Drew’s hypothesis in a flume, where they showed that bed slope increases as sediment supply (and hence steady-state transport

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Figure 3 Inertial and hyperconcentrated deposits of steepland fans. (a) Sequence of 2009–10 deposits on a steep (351) postfire fan in the San Gabriel Mts, analogous to deposits in Figures 2(a) and 2(b). Grainflow and ravel deposits of fired-material (dark bottom layers), overlain by a thin grainflow/ravel deposit of subfire grus, capped by a nonsorted, nonimbricated debris flow deposit. (b) Boulders of a debris flow terminal deposit, from the August 1999 storm in Valley of the Falls, San Bernardino Mts, CA, USA. (c) Deposit from a 2004 debris flow redundant, on a fan in southern Nevada, USA. The deposit is clast-supported with a sand matrix, but pebble and coarser particles lack persistent imbrication. (d) Postfire debris flow deposited by the 2009 storm in the San Gabriel Mts on a fan like that of Figure 2(e). A small hollow B100 m upvalley failed, and this coarse, clast-supported, unimbricated material was deposited at a tributary junction. (e) Clast and matrix-supported, unimbricated material deposited by the hyperconcentrated flows of Figure 2(f). These deposits indicate that some hyperconcentrated flows may leave deposits that are difficult to separate from debris flows. (f) Debris flow or hyperconcentrated deposits from the fan in Figure 2(g).

rate) increases. In confined valleys, an increase in bedload without a counterbalancing change in hydrology leads to aggradation, and fill terraces. One view of alluvial fans is that they represent processes building a slope of transport by deposition across a surface whose slope is insufficient to transport the imposed sediment load. This slope of transport is reflected in the continuity of slope values (Figure 7)

from headwaters, down to the terminus of the fan at slopes that vary depending on the boundary conditions, but which can be well below 0.01. The process of avulsion (or the formation of a new channel by breaking out of an existing one) leads to a number of these channels, which form a fan when coalesced. In short, alluvial fans form in landscapes where a stream flows from a confined valley out into

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Figure 4 Images of fans whose active transport processes are largely by fluid traction. (a) Hanaupah fan, west side of Death Valley, USA (e.g., Denny, 1965; Stock et al., 2007). Active channels are high albedo, whereas older surfaces are varying degrees of brown from desert varnish. (b) Fan, east side of Death Valley, USA. Note the lack of confined channels in lower fan, a characteristic of fans on the rapidly subsiding east side of the valley. (c) Axi-symmetric fan in Death Valley, USA, prograding into a former lake basin. Note the lack of confined channels and the abandoned, higher varnished surfaces. (d) Fan near Baker, CA, USA, with a steep axial wash as its boundary. (e) Globe fan, Providence Mts, CA, USA. This channelized fan feeds into a steep axial wash (Stock et al., 2007). (f) Flow moving from source area across fan to Mojave River, 2006. Photo by Marith Reheis, USGS.

an unconfined area. A reduction in slope is a common, but not required condition. This geometry occurs in a number of geologic settings. Boundary conditions also matter. For instance, in many humid settings, rivers and streams act as a boundary condition that removes all of the sediment transported to the fan margin. Once the channel has built a slope of transport to these boundaries, most of the source catchment’s sediment probably bypasses the channelized fan, at least so long as sediment supply rates do not exceed channel transport capacity. Arid region fans bounded by ephemeral streams are often similarly channelized, and locally incised. These boundary conditions can be contrasted with fans prograding into subsiding basins (e.g., Figures 4(a)–4(c)), which seem to have a tendency for reduced channelization. Experimental work by Nicholas et al. (2009) described in Section 9.23.10 explores this issue. Where fans are bounded by subsiding basins, fan extent is likely limited by the pattern and magnitude of subsidence for a given supply rate (e.g., Whipple and Trayler, 1996; Parker et al., 1998a).

‘Tectonic’ boundary conditions dominate fan formation in areas of active extension, compression, and transtension, where normal or high-angle reverse faults juxtapose confined, steepland valleys with unconfined depositional surfaces (i.e., piedmonts). Areas of widespread extension (e.g., Basin and Range, Hellenic back arc, Afar rift valley, Tibetan Plateau, and Andean back-arc) create a series of mountains separated by subsiding basins, ideal boundary conditions for alluvial fans. These areas seem to be disproportionately arid because of rain-shadow effects from bounding mountain ranges (e.g., Sierra Nevada, Himalaya, and Andes) and current global circulation patterns (Afar, Hellenic back arc). Along with the absence of vegetation cover, this condition arguably leads to the predominance of arid-region fan studies. Some of the largest fans are found in convergent tectonic settings such as the depositional fringe from the Himalayan thrust front (e.g., Kosi fan), the west of Taiwan bordering the thrust front, or the Zagros Mountains and Iranian plateau. The geometry of some of these latter systems favors large fans because the outboard depositional area is not constricted by the periodic mountain ranges of extensional

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

environments. Transtensional basins form along releasing bends in strike-slip faults, like the San Andreas or Anatolian faults. These basins frequently subside rapidly although they are surrounded by steeplands, boundary conditions that favor small but thick alluvial fans (e.g., Ridge Basin group along the southern San Andreas, Crowell, 2003). In contrast, Quaternary strike-slip faults in the Betic Cordillera of Spain have produced shallower basin fills and alluvial fan sequences. The ‘legacy of tectonism,’ or ancient base-level changes, creates landscapes that favor alluvial fan formation. Retreating escarpments such as the Drakensberg of South Africa, the Australian escarpment, and other retreating escarpments juxtapose steeplands with relatively flat piedmonts, leading to formation of local alluvial fans. The western edge of the Sierra Nevada range is a curious case of juxtaposition of confined valleys with a subsiding basin that leads to large fans on the borders of the Great Valley. The legacy steeplands of ancient mountain ranges such as the Appalachians or Rocky Mountains are bordered by piedmont environments that accommodate alluvial fans, particularly where abrupt changes in rock resistance to erosion juxtapose steep valleys with piedmonts (e.g., Blue Ridge of Virginia). ‘Glacial’ erosion in the Northern and Southern Hemisphere carved wide, low-gradient trunk valleys. Where steep tributary valleys join, the abrupt change in gradient and loss of confinement often favors formation of alluvial fans. In these environments, fan formation can be especially rapid following deglaciation because of the large amounts of relict glacial sediments available for entrainment. Topologic conditions favor fan formation. Where tributary valleys with episodic high supplies of bed material (e.g., valleys with large landslide sources) join mainstems, alluvial fans often form marginal to mainstem floodplains. These fans are common in dissected steeplands in many active and inactive tectonic environments.

9.23.3

Processes that Supply Sediment to Alluvial Fans

The texture and stratigraphic architecture of alluvial fans are strongly influenced by the processes that supply sediment. Steepland catchments (e.g., Figures 2–5) have a variety of hillslope processes that produce a wide range of sediment sizes, from clay to boulders. In temperate soil-mantled steeplands, soil creep moves mud, silt, sand, and gravel from hillslopes into valley networks at rates that depend nonlinearly on slope and a coefficient that arguably describes the frequency of perturbation (e.g., Roering et al., 1999). Over geologic timescales, these rates appear as slow, low-magnitude inputs of primarily soil (i.e., mud, silt, and sand) to the valley network. Primarily (though not exclusively) in arid regions, overland flow can transport large volumes of sediment to the valley network during intense storms, usually as sand and finer particles. In the steepest landscapes, rockfall, ravel (the motion of individual grains downslope), and grainflow can rapidly deliver immense amounts of sediment to valleys, particularly after fires (Figures 2(a) and 2(b)). In some fireprone steeplands (e.g., San Gabriel Mts, USA), postfire ravel

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from hillslopes accumulates in valley bottoms and is entrained by hyperconcentrated flow and debris flows in large postfire rainfalls. The resulting deposits have a wide range of grain sizes, although the deposits are typified by sand (e.g., Chawner, 1935). The foregoing hillslope processes act to fill in valley networks with a wide mixture of sediment sizes. Deepseated landslides that commonly involve bedrock as well as soil can deliver huge volumes of sediment to valley networks, overwhelming the stream’s transport capacity. These events can result in large increases in sediment supply downvalley that lead to aggradation within the valley, and further down on the alluvial fan. Episodic, intense rainfalls (commonly above 10 mm h1) generate shallow landslides that may mobilize as debris flows, fast-flowing mixtures of soil rock, and water. In regions where soil cover is thin, sparse, or absent, debris flows can be triggered where jets of water from intense rainfalls run over rocky ground and impact colluvium as they cascade down steep rocky valleys (so-called firehose initiation). Neither landslides nor debris flows require fine sediment to initiate or mobilize (e.g., limestone-sourced debris flows in Figure 2(d)). Debris flows carve many steepland valleys above 3–5% gradient (e.g., Stock and Dietrich, 2003, 2006), and can deliver thousands of cubic meters of unsorted sediment (mud to boulders) to alluvial fans in a single event. As Iverson and colleagues have shown (e.g., Iverson, 1997; Iverson and Denlinger, 2001; Iverson and Vallance, 2001), the physics of such processes involve substantial particle inertia, rather than tractive forces applied to particle surfaces by a fluid. For example, the leading edge of many experimental debris flows has been shown to be a granular front of negligible pore pressure (e.g., Iverson, 1997). Debris flows tend to gain most of their material by entrainment along their runout path (e.g., Stock and Dietrich, 2003). As a consequence, they transfer hillslope sediment from the catchment, out onto fans or the fluvial system. Debris flows that are composed of coarse, granular fronts that lack substantial quantities of clay or silt do not appear to be mobile much below slopes of 0.03–0.05 (e.g., Stock and Dietrich, 2003; Figure 8). Where debris flows have more substantial amounts of silt or clay, however, they may be mobile to very low slopes (e.g., o0.01). For instance, one can find muddy debris flow deposits along the Amargosa wash (Death Valley area, CA, USA) at a slope of 0.005. Iverson (1997) proposed that some very fine-grained debris flows like these are mobile at low slopes because of long timescales for pore water pressure diffusion. In addition to debris flows, hyperconcentrated flows (20–40% sediment by weight), and the transport of individual particles by the traction of flowing water transfer sediment from source catchments onto fans. As a result, steep alluvial fans tend to have mixtures of these processes, whose sedimentology is not always readily interpreted. It is not clear that an unambiguous criterion exists to distinguish all hyperconcentrated flow deposits from debris flow deposits. For instance, Figure 2(f) shows peak flows inundating the Stough Creek Nature Center, City of Burbank, CA, USA. Images of these flows show fluid-like characteristics, including waves and particles rolled by the fluid. Yet the deposits from this event lack any kind of imbrication or sorting

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Transport hypothesis: slope declines with lower fluxes

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Figure 6 Two contrasting hypotheses for alluvial fan long-profiles. (a) Illustration of Drew’s (1873) transport hypothesis showing decline in slope as the unit bedload flux declines downfan due to deposition. Inset shows data from Ikeda and Iseya (1988) illustrating this effect in a laboratory flume. (b) Threshold hypothesis illustrating the decline in fan slope as particle size decreases due to declining flow depth (transport capacity).

(Figure 3(e)) that would distinguish them from debris flow deposits. To distinguish inertially dominated deposits (e.g., debris flows) from traction-dominated deposits, Costa (1984) and

Stock and Dietrich (2003) made observations on a number of recent debris flow deposits. They argued that criteria for unambiguous field determination of debris flows include: no sorting of source-area materials, matrix support of coarse

Figure 5 Deposits of traction transport fans. (a) Example of imbricated pebbles, deposited by supercritical flow in a 5–7% sloped flume. (b) Imbricated pebbles from an outcrop at Hanaupah fan, Death Valley, CA, USA. Note that both large and smaller particles are imbricated. (c) Imbricated boulders forming a bank on a Hanaupah channel. Their position downstream from creosote bushes suggests they were deposited in the wake of the bushes. (d) Section adjacent to Figure 4(c), showing imbricated cobbles in the lower third, a layer of imbricated boulders above (analogous to Figure 4(c)), and a top layer of matrix supported, unimbricated gravel that represents pedogenic alteration of traction or debris flow deposits. (e) Cross section through a modern bar on Lucy Gray fan, Mojave, USA, showing a coarse armor layer on surface, underlain by finer imbricated sandy gravel. (f) Cut-bank exposure on Globe fan, Mojave, USA, with alternating fine/coarse layers, arguably a record of armor formation seen in (e). (g) Cut-bank exposure at Globe fan (Stock et al., 2007) with similar alternation. (h) Cross section through modern Globe wash, showing lobate bar with gravel clasts (top center), and sandy fill on either side of bar, with floating coarse clasts.

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Figure 7 Graph of valley slope from mainstem headwater to distal reaches of five alluvial fans in the American southwest. Slope is measured between each contour interval from USSG 7.50 maps along the main channel of the source catchment, and the alluvial fan (see Stock et al., 2007 for details). Note (1) continuity of slope from catchment to fan; (2) low terminal slopes for Hanaupah and Lucy Gray, which are bounded by depositional basins and (3) higher terminal slopes for the other three fans which are bounded by axial washes. The concave-up long-profiles of these systems reflect Drew’s original insight into fans as engines of transport and deposition.

Legend Debris flow levee Slope over 5-m 0–5 5.1–6 6.1–8 8.1–10 10.1–20 20.1–30 30.1–40 40.1–50 50.1–60 60.1–80 80.1–100 100.1–120 Figure 8 Shaded relief of 1-m LiDAR from an alluviaI fan on Molokai, HI, USA, with slope value superimposed (slopes below 0.05 are in gray). Bouldery, debris flow levees (red lines) mapped in the field do not extend below slopes of B0.05, suggesting that on many fans, granular debris flows with minor amounts of silt and clay are probably not mobile far below the 3–5% slope limit proposed by Stock and Dietrich (2003) for channelized, granular debris flows.

particles, and a lack of pervasive imbrication (i.e., imbrication of inequant particles at all grain scales). By contrast, deposits from traction transport commonly have imbrication at fine scale (e.g., fine pebble) up to cobble (hence, pervasive), although not every particle need be imbricated. Some coarse debris flow levees also have particles that are stacked against each other (Figure 3(b)), although their matrix will lack imbrication. Some debris-flow deposits lack fine particles, and

appear as clast-supported deposits with no imbrication. As many have argued, presence of bedforms (e.g., slip faces and laminae) is also not consistent with debris flow deposition. Grain size alone is probably not a sufficient condition for discriminating between inertial and traction transport. Lone cobbles or boulders on fans need not indicate inertial transport, since such particles can be exceptionally mobile because of their low effective friction angle (e.g., Solari and Parker, 2000).

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

9.23.4

Processes Observed on Fans

There are a limited number of direct observations of transport and depositional processes on alluvial fans. These events are so unpredictable that there are almost no monitoring data beyond those taken during the brief observations outlined in the following paragraphs. In the first widely reported observations on alluvial fan deposition, Eckis (1928) described a flood on a southern California alluvial fan on 16 February 1927. He noted that where streams lost confinement, deposition occurred as their waters spread, and that this reduction in local gradient from accumulation forced continued deposition and the formation of a low ‘debris dam.’ Flow subsequently bifurcated around the dam, creating two channels that forced the same process of deposition further down. Eckis hypothesized that this mechanism was responsible for both the occurrence of local boulder bars and the distribution of sediment across the fan. His rare firsthand observations make his work relevant today, particularly because the bifurcation process he describes is one of the most widely reported events in experimental work. Bull (1964a) reports observations of flow of B15 cm maximum depth, as surges of water that spread over fans on the western margin of the San Joaquin Valley, CA, USA. The author is aware of only two direct measurements for flow on alluvial fans (see Figure 9). Both indicate flow at supercritical conditions, with Froude numbers Fr greater than 1.

The Froude number is defined as a ratio of the influences of fluid inertial forces to gravitational forces, approximated by dividing average flow velocity by the square root of the product of the gravitational acceleration g and mean flow depth. Rahn (1967) measured stream flow from summer thunderstorms in two fan channels in southern Arizona. His photographs show standing waves on the San Tan channel, where he measured an average velocity of 1.9 m s1 and a flow depth of 30 cm (Fr ¼ 1.09). On a smaller, sand-bedded channel at Joshua Tree State Monument, he recorded an average velocity of 1.8 m s1 at a flow depth of 8 cm (Fr ¼ 2.11). Beaumont and Oberlander (1971) witnessed flow and sediment transport at the fanhead of Mosaic Canyon, Death Valley. They measured the velocities (B1.1–2.0 m s1) and depths (B6–10 cm) in a rapidly changing braided gravel channel with a slope of 0.079. For the deeper, faster thread, these values indicated supercritical flow (1.3oFro2.0). Downstream from this reach, they observed cobbles being rolled in standing waves in thin flow (5.1–12.7 cm). They extracted these particles and found that their intermediate axes ranged from 8.4 to 14.0 cm, equivalent or larger than flow depths. These rare measurements indicate that flow in steep alluvial fan channels can (1) plausibly be supercritical and (2) transport particles whose axes approach or exceed flow depth. Such flow likely involves standing waves, antidunes and chutes-and-pools, at Fr greater than 0.8 (e.g., Karim, 1995). Flume experiments on sand transport under supercritical flow represent some of the only data on likely flow conditions in some steep, sandy alluvial fan channels. Experiments reported by Kennedy (1963) illustrated that antidune minimum wavelength l in supercritical flow was a function of mean flow velocity U as l ¼ 2pU 2 =g:

(a)

(b)

Figure 9 Images showing (a) standing waves on a channel flowing across a fan in southern Arizona (Rahn, 1967; see text for hydraulic details). (b) Supercritical flow moving gravel across a fan head channel in Death Valley (Beaumont and Oberlander, 1971). These two images represent some of the only hydraulic measurements taken during sediment transport on alluvial fans.

423

½1

Alexander and others (2001) used an experimental flume to detail sand bedforms preserved by supercritical water flow, including antidunes and chutes-and-pools. They reproduced Kennedy’s relation [1], and made peels of the sand deposits in an effort to find what preserved bedforms were unique to supercritical flow. They found that many of preserved bedforms, including lenticular laminsets with troughs, and rare convex-upward laminsets that recorded the shape of the antidune, could be confused with hummocky and swaley cross-strata from subcritical flow. They highlighted, however, the presence of sets of downstream dipping laminae produced by bedwaves following wave breaking under supercritical flow. Although these features are somewhat subtle, they argued that their presence is the only preserved structure distinctive of supercritical flow in sand-bedded systems. Stock and others (2007) used flume data from Ikeda and Iseya (1988) to test typical excess shear stress gravel transport equations (e.g., Parker, 1990; Wilcock and Kenworthy, 2002) under supercritical flow conditions. They found that the close fit of supercritical transport rates to conventional bedload equation predictions was consistent with their use in steep fan channels where gravel transport is likely under supercritical flow (e.g., Beaumont and Oberlander, 1971).

424

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Wasson (1974) describes direct observations of water and sediment transport from a flood on an alluvial fan in New South Wales, Australia. Clearwater flow commenced in the wake of an intense convective storm (132 mm per 2 h), with subsequent onset of more turbid flow, which Wasson interpreted as evidence for active landslide sediment sources in the catchment. Both water level and sediment concentration oscillated during the event, which was followed by reappearance of relatively clearwater flow. Wasson described the aggregation of the coarsest material into lobes or bars early in the event, with subsequent flow bifurcated on either side. The observations are significant because they indicate fluctuations in water and sediment supply during one event, formation

of coarse bars by gradual particle accretion, and subsequent diversion of active sediment around these coarse barforms. The author observed flow and sediment transport over a small alluvial fan near Pescadero, CA, USA, in 1998 (Figure 10). Shallow (o3 cm), rapid flow rolled waves of gravel down a flow thread, with particle tops often exposed above water as they rolled along a sandy bed. In this case the combination of steep slope (B0.05), fast flow, and smooth channel bed surface resulted in gravel moving at speeds that approached that of the fluid velocity. Where flow was obstructed by debris, a thread would avulse and particles would begin to roll into that thread, accumulating behind the largest or most inequant particle, and eventually forming a coarse

(a)

(b)

Figure 10 Flow across an alluvial fan during a 1998 storm near Pescadero area, CA, USA. Flow is from subsurface stormflow, exfiltrating at a seepage head. (a) Flow rolling emergent pebbles (lower center) over a bed that is comparatively smooth. (b) Lobate deposits of well-sorted pebbles just upflow from a narrow flow constriction. These depositional tongues of gravel represent failed avulsions, but leave behind lobes that appear similar to those described as sieve deposits. The author watched them deposit grain-by-grain, not en masse.

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

tongue of well-sorted particles that would back up until it resealed the channel wall. In 2009, staff at the Stough Canyon Nature Center, City of Burbank, California, photographed hyperconcentrated flows depositing sediment at a fanhead downstream from an area that had burned 2 years ago (Figure 2(f)). Images show large particles being rolled along by a fluid mass, although the resulting deposits lack sorting or imbrication (Figure 3(e)).

9.23.5

Hypotheses Guiding Field and Experimental Work

Observations and measurements on alluvial fans have aimed at explaining how they fit in with general geomorphic theory, why they occur where they do, their shape, why particle sizes vary downfan, how particle size controls fan shape, and why the depositional areas of so many fans seem to have decreased with geologically recent incision. The brief discussion that follows introduces some of these hypotheses to frame the following sections on morphology, sedimentology, and experimental work. At least some early workers thought of alluvial fans as part of Davis’ geographic cycle, with a build up followed by long-term dissection (e.g., Eckis, 1928). By contrast, Denny (1965) hypothesized that the alluvial fans of the Death Valley region had approached a dynamic equilibrium where over geologic time, addition of material from catchments was balanced by erosion of material from dissected fan areas. Bull proposed that alluvial fans reflect a tendency toward equilibrium among a complex set of controlling factors (Bull, 1964b) including area, lithology, mean slope, vegetation, stream channel slope, climate tectonism, and geometry. He later wrote that it was unlikely that all of these variables would remain steady, so that alluvial fans would probably always be in a transient state (Bull, 1975). Much of the quantitative sedimentology work has been guided by the expectation that the researcher could find a relation between downfan decrease in sediment size and downfan reduction of transport capacity, and that this relation could be tested by plotting slope against the relevant grainsize metric (a- or b-axis of maximum clast size, b-axes of mean or median grain size). Table 1 records this effort. Figure 6(b) illustrates one formalization of this hypothesis using Shields’ critical stress for entrainment. Shields’ criterion predicts that at the threshold of motion for a uniform grain size D, tc ¼ tc ðrs  rw ÞgD

½2

where rs and rw are sediment particle and water density, tc is the critical fluid shear stress for initial motion, and tc is a dimensionless number characterizing resistance to motion. Approximations of shear stress using the product of hydraulic radius R and slope S (rwgRS) are known to be locally inaccurate, but may capture reach-scale variations in shear stress. Using this approximation, [2] can be recast in terms of a threshold slope at bed-material entrainment:   r  rw D S ¼ tc s R rw

½3

425

so long as form drag is insubstantial. If fan slope is set by threshold conditions, and D is represented by the median grain size d50, a plot of d50/R against S should be nonrandom for a constant tc . Alternately, Drew’s (1873) paper on alluvial fans presents the hypothesis that the slope of streams is a statement about their transport capacity, and that as deposition removes progressively more sediment and bedload sediment flux decreases, slope declines (Figure 6(a)). Much of the morphologic work in Table 2 has been directed at some elemental relation between fan area and source basin area that would give insight into the competing roles of geology, climate, and erosional processes in controlling fan size. A number of workers have argued that the crude power law relation between drainage area and fan area represents an approach toward fan steady state. Much of this work has evolved with the increasing availability of topographic maps from around the world. Substantial work has focused on how and why many alluvial fans are now entrenched. Fanhead trenching has been widely observed in the western United States (e.g., Eckis, 1928; Bull, 1964a, b; Lustig, 1965) and Spain (e.g., Harvey, 1990), and there are a wide range of hypotheses (see Table 9.1 in Schumm and others, 1987). For instance, Eckis (1928) argued that entrenchment is part of a geomorphic cycle as a basin erodes downward and sediment load decreases. Bull (1964b) found evidence for both tectonic and climatic forcing in fans of the western San Joaquin Valley, USA. Bluck (1964) argued that entrenchment followed a change from mudflows to streamflows. Lustig (1965) proposed that past climates had higher rainfalls, leading to greater discharges, higher water–sediment ratios, and frequent floods that spilled sediment across fan surfaces. Reductions in rainfall led to reductions in water-tosediment ratio, and the onset of mudflows that incised fanhead trenches. For example, Harvey (1990) has argued that late Pleistocene and Holocene environments in Spain have decreased sediment supply, leading to fan incision.

9.23.6

Morphometry

The US Geological Survey produced some of the first widely available maps of alluvial fans in the American west, leading to a surge in the mid-twentieth-century literature on alluvial fans (see Table 2). For instance, during his studies of alluvial fans on California’s Great Valley margin, Bull (1964b) used topographic maps to divide fan surfaces into linear segments, which he interpreted as relict signatures of rock uplift. In the same area, he found that fans with mudstone and shale sources had approximately twice the planform area as those derived from sandstone basins (Bull, 1962a, 1964b). He proposed that the ratio of source to depositional area of alluvial fans was a statement about both the relative erodibility of the source catchment (e.g., Bull, 1964b; Hooke and Rohrer, 1977) and the efficiency of sediment transport. He reasoned that fans whose catchments had high erosion rates and high transport rates would have proportionally large fan planform areas as a consequence. He proposed the expression Af ¼ c1 Adn1

½4

426

Observation

Variable/equation

Interpretation/ generalization

Size range (mm)

Example quantification

Location

Reference(s)

Grain size downfan

Median of 10 maximum sizes (?-axis)

Maximum Grain size controls slope ? Maximum Grain size controls slope Pattern of debris flow deposition Selective sorting, physical weathering Weathering? Spatially varying hydraulics Debris flows?

203–3988

None

San Gabriels, USA

Eckis, 1928

Not given B20–400

None None

San Gabriels, USA West. USA

Chawner, 1935 Blissenbach, 1951, 1952

B50–1600

None

Beatty, 1963

B400–900

dmax ¼25 885 þ 2506f

White Mountains, CA, USA Las Vegas, NV, USA

1.35–0.18 0.07–0.53

d50 ¼2.24  1.24 log x No downfan pattern

New Mexico, USA Fresno, CA, USA

Ruhe, 1964 Bull, 1964a

30–1128

No downfan pattern

Lustig, 1965

Varying source area

0.1–0.6

No downfan pattern

Influence of hydrology? None Maximum Grain size controls slope ?

2–35 B150–560 2–450

Variable No downfan pattern Decreases downstream

mud to B4000

Decreases downstream

White Mountains, CA, USA White Mountains, CA, USA Death Valley, CA Elburz Mts, Iran Alaskan & Icelandic outwash Dam break flood, Colorado, USA

Greatest boulder est. weight Maximum grain size in area (?-axis) Mean of 10 maximum sizes (?-axis) Maximum grain size in area (b-axis) Median (wt. %) Median (?-axis) Maximum grain size in 500 radius (baxis) Median of granule-to-clay (wt. %) Geometric mean along transect (b-axis) Maximum grain size in area (a-axis) Mean of 10 maximum sizes (a-axis) Characteristic size

Bluck, 1964

Lustig, 1965 Denny, 1965 Beaumont, 1972 Boothroyd & Nummedal, 1978 Tunbridge, 1983

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Table 1 Summary of sedimentologic observations of alluvial fans

Mean of 10 maximum sizes (b-axis) Mean along transect sample Maximum grain size (b-axis) Mean of 10 maximum clasts (a-axis) Mean (of 48 mm fraction b-axis) Maximum grain size in cut bank (b-axis) Maximum grain size in cut bank (b-axis) Maximum grain size in cut bank (b-axis) Mean of 5 maximum sizes (?-axis) Maximum grain size (a-axis) Maximum grain size in cut bank (b-axis) Median (wt. %) surface Median (wt. %) subsurface

Sand % downfan

Transition from debris flow to fluvial Process transition/short distance Lack of supply?

B500–3000

Decreases downstream

73–505

Costa Rica SW AZ, USA

Kesel, 1985; Kesel & Lowe, 1987 Mayer et al., 1984

B40–400 540–330 42–65 380–3560 230–2240 80–860 B10–500 600–11 500 100–3650 0.8–1.8 B0.002–7

No downfan pattern dmax ¼50.4  2.11 log x dmean ¼5.9  0.22 log x No downfan pattern No downfan pattern No downfan pattern No downfan pattern No downfan pattern No downfan pattern No downfan pattern No downfan pattern

NW England Montana, USA Montana, USA Death Valley, CA, USA Death Valley, CA, USA Death Valley, CA, USA Appalachia, USA Venezuela Eureka Valley, CA, USA AZ, USA Coast Range, CA, USA

Wells & Harvey, 1987 Ritter et al., 1993 Ritter et al., 1993 Blair, 1999a Blair, 1999b Blair, 2000 Mills, 2000 Wieczorek et al., 2002 Blair, 2003 Vincent et al., 2004 Florsheim, 2004

B60–250

Variable

De Scally & Owens, 2005

No No No No No

Southern Alps, New Zealand Chile SE CA, USA Chile SE CA, USA Death Valley, CA

Mather & Hartley, 2005 Stock et al., 2007 Haug et al., 2010 This publication Blair, 2000

Increase down fan Increase down fan

Eureka Valley, CA, USA SE CA, USA

Blair, 2003 Stock et al., 2007

Maximum grain size (a-axis) Median (42 mm, b-axis) Maximum grain size (?-axis) Median (wt. %) Weight %, granule and finer, cut bank

? Sand

B5–450 1–61 B60–800 0.8–1.44 14.3–44.1%

Weight %, granule and finer, cut bank Bed percent area via pebble count

m-vc Sand o2 mm

B39 to 69% 14–100%

downfan pattern downfan pattern downfan pattern downfan pattern generalization

Notes: Bindicates value estimated from graph in publication; dmax is the maximum grain size; dmean is the mean grain size; x is the distance downfan, see source reference for precise definition; m-vc is medium to very coarse sand. a4 b 4 c axis.

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Mean of 50 samples

Maximum Grain size controls slope Selective sorting, physical weathering ? None None Facies distribution Debris flows? Lack of supply? Influence of debris flows Debris flows ?

427

428

Observation

Equation

Fan depositional area is power function of source area

Afan ¼c1An1 drainage

Fan volume function of fan area Fan slope S decreases with drainage area A

Savg ¼ c2An2

Example quantification 0.88

Afan ¼(1.3–2.4)Adrainage Afan ¼(0.1–0.5)Adrainage 0.80–1.0 Afan ¼(0.15–1.05)Adrainage 0.24–1.13 Afan ¼(0.16–0.95)Adrainage 0.89–1.07 Afan ¼0.48Adrainage 0.32 Afan ¼0.17Adrainage 0.48 Afan ¼1.181Adrainage 0.946 None given Afan ¼0.115Adrainage 0.488 Afan ¼0.229Adrainage 1.08 Afan ¼0.807Adrainage 0.675 Afan ¼0.84Adrainage 0.75 Afan ¼(0.40–0.41)Adrainage 0.68–0.94 Afan ¼0.376Adrainage 0.861 Afan ¼0.18Adrainage 0.85 Afan ¼(0.15–0.18)Adrainage 0.68–0.77 Vfan ¼(7.91–10.2)Adrainage 0.95–1.04 No generalization given Savg ¼(0.025–0.034)Ad  (0.28–0.34) Savg ¼4.112Ad 0.391 Savg ¼42.27Ad 0.128 Savg ¼0.079Ad 0.208 Savg ¼0.066Ad 0.197 Sch ¼0.039Ad 0.334

Location

Reference(s)

Fresno, CA, USA Death Valley area, CA, USA Eastern CA, USA Death Valley area, CA, USA Rocky Mts, Canada Rocky Mts, Canada Iran Appalachia, USA Argentina Appalachia, USA SE Spain SE Spain NV, USA NV, USA UAE and Oman Canada and USA Canada and USA Himalaya/west US Fresno, CA, USA AZ, USA Iran SE Spain SE Spain SE Spain

Bull, 1962a, 1964b Denny, 1965 Hooke, 1968 Hooke & Rohrer, 1977 Kostaschuk et al., 1986 Kostaschuk et al., 1986 Beaumont, 1972 Kochel, 1990 Milana & Ruzycki, 1999 Mills, 2000 Harvey, 2002a Harvey, 2002b Harvey, 2002b Harvey, 2005 Al-Farraj & Harvey, 2005 Giles, 2010 Giles, 2010 Drew, 1873; Eckis, 1928; Melton, 1965 Bull, 1962a, 1964b Melton, 1965 Beaumont, 1972 Harvey, 1984a Harvey, 1987 Harvey, 1987

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Table 2 Summary of morphometric observations of alluvial fans

Savg ¼ a(DH/A0.5)n

Fan depositional process covaries with basin relief (Melton’s number M)

M ¼(DH/A0.5)

DH 4 300 Ad 0.69 Fan width increases with length Fan channel width w increases with drainage area

SE Spain SE Spain Compilation Argentina Argentina SE Spain SE Spain NV, USA UAE and Oman NV, USA Philippines, Taiwan, Japan AZ, CA, USA Rocky Mts, Canada Rocky Mts, Canada Canada and USA Eastern Alps, Italy Eastern Alps, Italy Eastern Alps, Italy Calabria, Italy Calabria, Italy SE Spain Global compilation Death Valley area, CA, USA SE Spain SE Spain SE Spain

Note: bold indicates English units. Savg is slope averaged over the fan surface; Sch is slope averaged along a channel on the fan; Wfanchannel is the width of a channel on the fan.

Harvey, 1990 Harvey, 1990 Harvey, 1992 Milana & Ruzycki, 1999 Milana & Ruzycki, 1999 Harvey, 2002a Harvey, 2002b Harvey, 2002b Al-Farraj & Harvey, 2005 Harvey, 2005 Saito and Oguchi, 2005 Melton, 1965 Kostaschuk et al., 1986 Kostaschuk et al., 1986 Giles, 2010 Marchi et al., 1993 Marchi et al., 1993 Marchi et al., 1993 Sorriso-Valvo et al., 1998 Sorriso-Valvo et al., 1998 Harvey 1984a Heward, 1978 Denny, 1965 Harvey, 1987 Harvey, 1990 Harvey, 2002a

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Fan slope S increases with source basin relief DH

Sch ¼0.042Ad 0.22 Savg ¼0.066Ad 0.20 Savg ¼c2An2 Savg ¼0.161Ad 0.218 Sch ¼0.201Ad 0.215 Savg ¼0.068Ad 0.249 Savg ¼0.058Ad 0.37 Savg ¼(0.093–0.097)Ad (0.114–0.158) Savg ¼0.092Ad 0.23 Savg ¼0.095Ad 0.135 Savg ¼0.039Ad 0.29 6.0oao9.0; 0.62ono0.88 Savg ¼0.71(DH/A0.5)1.76 Savg ¼0.19(DH/A0.5)2.33 Savg ¼0.22–0.23(DH/A0.5)(0.99–1.30) 0.10oMo0.28 fluvial 0.36oMo0.52 mixed 0.49oMo1.74 debris flow 0.25oMo0.79 fluvial 0.25oMo1.18 debris flow Threshold for debris flows No generalization given Wfanchannel ¼ 122A0.5 Wfanchannel ¼7.12A0.251 Wfanchannel ¼7.69A0.26 Wfanchannel ¼6.68A0.319

429

430

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

where Af is the fan area, Ad is the drainage area of the catchment feeding sediment to the fan, and c1 and n1 are the parameters that describe the intercept and slope of a power law relation. The power law slope n is commonly (not always) less than 1, indicating a tendency for increasingly large basins to produce proportionally smaller fan areas. Hooke (1968) argued that this relation was also a consequence of a tendency toward steady-state deposition rates among adjoining fans. For any adjoining fans with unequal deposition rates, the fan with the lower rate would tend to lose depositional area to its advancing neighbor. The fact that he found geographic clusters of parameter values suggested to him that his examples had approached a steady state. He also reasoned that geographies with small mountain ranges and large piedmont areas would tend toward higher c-values, purely as a consequence of accommodation area. This equation is arguably the most widely studied aspect of alluvial fans, in part because of the wide availability of topographic maps (see Table 2). Despite its wide use, there are no guidelines about how to define the limits of fan depositional area, or the precise location of the fanhead, sometimes nontrivial questions. It is also clear that boundary conditions matter greatly. For instance, Whipple and Trayler (1996) formulated a conservation of mass framework to predict the ratio of fan to catchment area as a function of the geometry and subsidence rate of depositional basins. They contrasted this ratio between fans feeding a low subsidence rate basin on the western bajada of Owens Valley, with fans feeding the high-subsidence rate basin of Owens Lake. They found that fans building out into the high subsidence rate basin were B1/10th the size of those prograding onto the more slowly subsiding bajada. Dade and Verdeyen (2007) proposed that some of the variance in the ratios of fan area to source area could be explained using a ratio of basin subsidence rate (equivalent to rock uplift rate they assumed) to the product of precipitation and basin relief. They compiled data from terrestrial and Martian fans to argue that the ratio of fan area to source catchment declined exponentially as subsidence rate (normalized by precipitation and basin relief) increased. Giles (2010) questioned the implicit assumption that fan area is a consistent proxy for fan volume, and hence a measure of the time-integrated erosional flux from the source catchment. He estimated fan volumes from postglacial valleys to avoid the issue of the fan volume buried in subsiding basins. He found that a plot of fan area against fan volume had an exponent not significantly different from 1, validating the use of fan area as a proxy for volume, at least in nonsubsiding basins. Another widely reported metric is the power law relation between source area and some measure of slope averaged over the fan: Sf ¼ c2 Adn2

½5

where Sf is the fan slope, Ad is the drainage area of the catchment feeding sediment to the fan, and c2 and n2 are the parameters that describe the intercept and slope of a power law relation (see Table 2). We might expect this relation to follow from the fact that basin channels that are concave up have slopes that decrease with increasing drainage area

so that there is a tendency for average fan slope to vary inversely with source basin drainage area. The variation in intercept c2 is arguably analogous to the channel steepness index (e.g., Whipple and Tucker, 1999) in that it is a statement about the fanhead slope at a reference drainage area. The power law slope n2 seems to vary between  0.1 and  0.4 (Table 1), which is arguably a statement about the distributions of channel concavities in the sample sets. Harvey (1992) found that alluvial fans in southwest United States, Spain, Greece, and England tended to have higher mean fan slopes when they were dominated by debris flow deposits (0.09oSmeano0.37) rather than fluvial deposits (0.04oSmeano0.14). This tendency manifested itself as higher intercepts (c2) for fans with debris flow deposits as opposed to fluvial deposits. Power law slopes of debris-flowdominated fans also tended to have less negative exponents than those of fluvial fans, a finding that echoes the observations that channel concavities tend to appear lower in debris-flow-dominated networks (e.g., Lague and Davy, 2003). In an earlier work, Harvey (1984a) proposed that debris-flow-dominated fans (420% debris flow deposit outcrops) had source basins with systematically higher relief. He used the expression DH ¼ 300Ad0.69 to separate debrisflow-dominated fans from fluvial or mixed-process fans in southeast Spain. Saito and Oguchi (2005) explored the morphometry of 690 alluvial fans in the humid regions of Japan, Taiwan, and the Philippines. They used 1:25 000 and 1:50 000 maps to delineate fan forms, and calculated mean fan slope as the elevation difference between fanhead and fan boundary, divided by fan length. They found that mean fan slopes had a lognormal distribution of values that ranged from 0.002 to 0.123, with no significant gaps. Hashimoto et al. (2008) continued this analysis with DEM-derived slopes of 430 alluvial fans in Japan and the American southwest. They used a 50-m DEM for the Japanese fans, and a 30-m discretization of the SRTM (Shuttle Radar Topographic Mission) to characterize the American fans. They used a variety of methods (interpretation of topographic maps and satellite imagery, stream-nets derived from the DEM, and shaded relief maps) to delineate the margin of the fan. They created two 1-km-wide radial sectors, 500 m above and below the fan margin, and calculated the mean slope of both polygons. They computed the overall fan slope by dividing the elevation difference between fan apex and toe by fan length. They found that the American fans had a much wider distribution of upper, lower, and total fan slopes, including many more steep fans than the Japanese sample. For instance, distal slopes on the Japanese fans had a mean of 0.581, compared with 2.831 for the American fans. These studies illustrate that fan-shaped features have a wide, continuous range of slopes. Melton (1965) proposed that fans with greater relief in their source catchment would have steeper slopes. He normalized relief by the square root of drainage area, in order to collapse variability in relief due to basin size. In alluvial fans of southeast Arizona, and in Death Valley, Melton found that fan slope increased as a power law function of this ratio. Melton also evaluated fan slope over ‘the upper fourth or so of each fan.’ Like expression [3], we might expect this relation to follow from the fact that basin channels that are concave up have

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

slopes that decrease with increasing drainage area, and increasing basin relief leads to steeper channels. From this geometry alone, there is a tendency for average fan slope to vary inversely with basin drainage area, and directly with basin relief, as pointed out by Hooke (1968). Kostaschuk and others (1986), Jackson et al. (1987) and Marchi (1993) used the ratio of basin relief to square root of basin area, which they called the Melton number, to separate fluvial from debrisflow-dominated fans. Kostaschuk and others (1986) calculated average fan slope by dividing fan relief by fan length. Marchi and others (1993) calculated fan slope as the average slope between fan apex and toe along a bisector. Sorriso-Valvo and others (1998) used discriminant analysis of the morphometry of a number of Italian fan-basin systems (e.g., fan area and slope, and basin relief, length, and Melton number) to separate debris flow from fluvial fans. They found that these metrics could classify approximately half of the debris flow and fluvial-dominated fans correctly. Doehring (1970) proposed using contour crenulations that increased in frequency upslope to distinguish pediments from alluvial fans. Hooke and Rohrer (1979) plotted a measure of fan slope versus degrees from the axis to illustrate the tendency for some fans to have increasingly steep slopes approaching their radial margins. They found variations up to 0.06 between the axial slope and the radial margin slope, with a tendency for steeper fans to have larger deviations. They interpreted this tendency as a reflection of the tendency for steeper fans to have flows with higher inertia, and a reduced tendency to divert from the direct axial path. Another approach is to estimate fan volume versus source catchment characteristics. This has been done in a very precise way for some small fans (e.g., Granger et al., 1996) to verify cosmogenic radionuclide estimates of long-term erosion rate. It has also been done at larger scales to look for variations in erosion rate, and faulting along active escarpments. Hooke (1972) mapped six units of progressively older age in alluvial fans of Death Valley. He correlated some of these units with dated Lake Manly deposits. By assuming that the active surfaces of alluvial fans should have constant slopes, he projected five of these units below the modern topography to estimate the volume of material deposited through time. He proposed that these units recorded an exponential increase in tilt rate through time. Hooke and Dorn (1992) subsequently revised the ages for tilting based on rock-varnish dating techniques. Jayko (2005) proposed that lake beds provided a bounding surface under fans along the east side of Death Valley. She approximated their geometry as cones, and estimated fan volumes in an effort to understand variations in erosion rate (and hence fault offset rate) along the eastern margin of Death Valley.

9.23.7

Hydraulic Geometry

Gohain and Parkash (1990) report bank depths and width/ depth ratios for B160 km of the main channel of the Kosi alluvial fan of India. This channel drains the high Himalayas, has a bimodal discharge regime with very large discharges and suspended sediment concentrations (up to 10 000 mg l1) during monsoons, and has substantially smaller discharges

431

during the dry season. Starting near the fanhead at slopes of B0.0005, bank depths decline from just under 10 m at fanhead, to just under 2 m at B34 km, in a reach that is largely a braided, gravel channel. Width-to-depth ratios climb over this reach, from just above 40 to values approaching 1000. Downstream, gravel beds transition to sand, silt, and mud, and bank depths rise to values of B4–6 m, and width/depth ratios decline to values of 100–200. Vincent and others (2004) reconstructed peak flow hydraulics of a 1988 flood down 3.5 km of the Wild Burro fan, near Tucson, AZ, USA (Figure 11(a)). This work probably represents the most detailed reconstruction of an alluvial fan flood to date (compare with Figure 11(b)). They used highwater indicators to map flow depth estimates down nearly 5 km of the sand-bedded, B2–3% sloped fan. As the fan width expanded from B200 to B1900 m, they found that total flow path width increased from 144 to 716 m, although most of this increase occurred in the last 3.5 km. The majority of this expansion occurred in shallow flow threads of less than 30 cm depth. Total inundated deep channel width increased, and then decreased downfan. They found that the peak flood discharges (estimated at 200–300 m3 s1) inundated half of the fan area, with 35% channelized flow, and the remainder unchannelized flow. Peak unchannelized flows were commonly less than 30 cm deep, with channelized flow in excess of 200 cm locally. Widths and flow depths tended to decrease down fan, although deep channels could be found all along the floodway. Similar to many flume studies (see Section 9.23.10), they found expansion reaches with wider, shallower channels that alternated with narrow reaches with deeper, narrower channels and well-defined banks. They found that flood flow branched preferentially at the expansion reaches where banks were low. They compared their inundation map to a surficial geologic map and found that 98% of the flood inundation occurred within middle and late Holocene mapped units. They used a step-backwater hydraulic model with an assumed Manning’s n of 0.035 to estimate discharges down 20 of the narrow reaches. The resultant Fr numbers were all above 0.9, indicating the likelihood of upper flow regimes given the assumed roughness value. They graphed discharge estimates against measured flow width w and mean depth h, and estimated flow mean velocity v, to yield the hydraulic geometry of both expansion (exp) and narrow (nar) reaches: wexp ¼ 15:83Q0:45

½6

wnar ¼ 4:05Q0:40 ; hnar ¼ 0:17Q0:37 ; vnar ¼ 1:45Q0:23 ½7a; b; c These expressions represent the only known estimates of hydraulic geometry for alluvial fan channels. Vincent and others (2004) argued that these values were similar to those observed in other ephemeral streams. They pointed out that width/depth ratios that ranged from 6 to 36 in the narrow channels were far lower than the value of 200 assumed by FEMA (2002), and that the confinement of flood waters to existing channels was also inconsistent with FEMA’s guidelines for flood hazards on fans. By contrast, Field (2001) and Pearthree et al. (2004) found that a flood on the Tiger Wash fan in Arizona avulsed out of preexisting channels. They used imagery and high-water

432

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Maximum flow depths > 100 cm 50−100 cm 30−50 cm 10−30 cm < 10 cm No inundation

0

0.5

1

km

Z

(a)

(b)

Figure 11 (a) Image showing maximum flow depths during flow on an alluvial fan in southern Arizona, 1988. Data showing this level of detail for a fan flood are exceedingly rare, and this represents one of the best documented examples in the natural world (Eckis, 1928, Vincent et al., 2004). (b) Image showing inundation of fan during the 1969 floods, Los Angeles, CA, USA (Singer and McGlone, 1971). Note the contrast between the portrayal of unconfined flow in (b) and channelized flow in Figure 9(a).

indicators to map flow into four categories: (1) channel flow from 20 to 200 cm depth; (2) unconfined flow of more than 20 cm; (3) unconfined flow less than 20 cm depth; and (4) undifferentiated unconfined flow. Over the limits of their mapping, they found that B50% of the area was inundated, with channel and deep unconfined flow restricted to 8% of their detailed map area. They too found that inundation was concentrated in the youngest of their surficial units, although they reported a few areas of Pleistocene deposits that were inundated. Field (2001) used repeat aerial photography to study the locations of channel avulsions on five fluvially dominated alluvial fans in southern Arizona. Using photography that started in 1936, he found that avulsions occurred where bank heights were comparatively low (0–32 cm) in aggrading reaches or along the outside perimeter of channel bends. Avulsed flow reoccupied narrower, often lower elevation channels, whose width subsequently increased by threefold or more. Figure 12 reproduces his conceptual model

for avulsions, illustrating reaches of net transport or erosion alternating with less confined reaches of aggradation where avulsion occurs preferentially. Arzani (2005) describes channel dimensions from a large, low-slope fan in central Iran. He measured bankfull widths from 3 to 45 m, and bankfull depths from 0.4 to 2.0 m, although he does not report any downfan patterns. Stock et al. (2007) measured bankfull width and depth along four alluvial fans in the Mojave and Death Valley region. They used the presence of recent deposits to define bank tops, finding that width/depth ratios for fan channels ranged from 10 to 100, far smaller than those of axial washes (41000). They reported that the hydraulic radius decreased down all four fans, from 0.5 to 0.9 m at fanheads, to 0.1–0.2 m at distal margins. They found no systematic pattern in relative roughness (hydraulic radius divided by median gravel diameter), whereas Shield stresses t declined on all four fans (1.8–0.1) because of the reduction in hydraulic radius.

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

433

Discontinuity Erosional channel

Sheetflood zone

Depositional channel

Erosional channels

Sheetflood zone

Depositional channel

Plan view

Longitudinal profile

Gravel bar Depositional channel seds. Sheetflood zone sediments Overland flow zone Headcut Top of channel bank Older fan sediments Figure 12 Distributary channel geometries observed by Field (2001). This alteration of expansion and contraction of flow geometry has been noted by other field workers (e.g., Vincent et al., 2004), and in numerous laboratory analogs of alluvial fans (e.g., Schumm et al., 1987; Whipple et al., 1998; Sheets et al., 2002; Reitz et al., 2010). It represents one the most robust patterns found in both kinds of studies.

9.23.8 9.23.8.1

Sedimentology Quantitative Sedimentology

Drew (1873) observed that the longitudinal profiles of streams were concave up (Figure 6(a)). He proposed that the decrease in slope downfan was a statement about the decrease in sediment transport rate with progressive deposition. Blissenbach (1951, 1952) revisited this observation in the American southwest, after a long hiatus. He proposed that concave-up longitudinal profiles resulted from the progressive decline in particle-size downfan. In the wake of Blissenbach’s proposal, a number of field efforts have focused on the relation between grainsize metrics (usually maximum or mean grain size) and fan slope (Table 3). This outstanding question of whether fan long-profiles are shaped by a gradient in transport rate, or a gradient in the threshold of motion continues to be debated, but it has generated a number of attempts to quantify transport capacity on the basis of grain size, reviewed in the following paragraphs. For instance, Eckis (1928) mapped recent fan alluvial deposits along the margin of the San Gabriel Mountains at Cucomonga. On active channels he noted that the median of the 10 largest fragments decreased with slope downfan, perhaps the first quantitative observation of downfan fining. From the context, his choice of maximum particle size may reflect

a concern with impact forces on structures. In the wake of the destructive alluvial fan floods of 1933/1934 in the La Crescenta/Montrose area, Chawner (1935) presented the first grain size distributions for alluvial fan deposits. He argued that sorting distinguished mudflow deposits from ‘turbulent water,’ and noted that estimated boulder weights decreased downfan. He used a crude postflood survey of deposits to estimate that the source catchment had lowered by an average of 2.5 in., a strikingly large number. Damage from this storm and from the subsequent 1938 storm led to continued development of checkbasins to catch water and sediment. Blissenbach (1951, 1952, 1954) returned to the problem of fan long profiles in papers exploring the relationship between maximum particle size and surface slope on fans in the Aubrey Cliffs and Santa Catalina and Tucson Mountains of Arizona. In his Master’s thesis, he mapped fan deposits of the Santa Catalina Mountains, distinguishing recent wash fill from three older stages of alluvial fan deposits. He drew a straight transect down fan, and estimated the largest particle at selected exposures. His data are notable for the smooth decrease in maximum grainsize downfan. He is also possibly the first to articulate criteria for distinguishing stream-deposits (imbricated and bedded) from mudflow deposits (no imbrication or internal structure). He argued that mudflow deposits were characteristic of arid climates. His work remains significant today because it influenced much of the work that followed,

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

Table 3 Some studied events on alluvial fans Date

Location

Type of damage where known

Process

Example reference(s)

1847–1938;1939–69

Utah, USA

2/16/1927

Deer Canyon, San Bernardino’s, USA northern Utah

Structure, transportation, lives Unknown

Mixture of traction and debris flows Traction

Woolley, 1946; Patton & Baker, 1976 Eckis, 1928

Residential structures

Pack, 1923

Structure, transportation, lives Structure, transportation, lives ?

Mixture of traction and debris flows Primarily traction/ hyperconcentrated Primarily traction/ hyperconcentrated Traction

?

Traction

Miller, 1977

Sructure, transportation

Traction

Singer& McGlone, 1971

Structure, transportation

Traction

None

Traction

Waanamen cited in NRC, 1996 Hooke cited in NRC, 1996

?

Traction

Structure, transportation ?

Traction Traction

None Residential structures Levee damage

Traction Traction Mixture of traction and debris flows Traction Traction

8/13/1923 12/31/1933–1/1/1934 March, 1938 7/25/1950

San Gabriels, Montrose, CA, USA Southern California, USA

June,1982

Furnace Creek, Death Valley, CA, USA Furnace Creek, Death Valley, CA, USA Deer Canyon, San Bernardinos, USA Deer Canyon area, San Bernardinos, USA Hanaupah, Death Valley, USA Santa Monica Cr., Carpinteria, CA, USA Tujunga Wash, CA, USA Wadi Mikeimin, Sinai, Israel NSW, Australia Palm Desert, CA, USA Mt Cook area, New Zealand Tabernas, Spain Horseshoe/Estes Park, CO, USA Howgill Fells, England

May–June, 1983

Utah, USA

Structure, transportation

9/2/1984

? ?

Traction

? ?

Traction Traction

1992 onward

Saddle Mtn. Arlington, AZ, USA Tortolita Mts, Tucson, AZ, USA Sydney, Australia Carefree fan, Arizona, USA Mt Pintubo

Mixture of traction and debris flows Mixture of traction and debris flows Traction

12/15/1999–12/16/1999

Venezuela

August, 2008

Kosi River, India

Structure, transportation, lives Structure, transportation, lives Structure, transportation, lives

Lahars/hyperconcentrated flows Mixture of traction and debris flows Traction

July, 1968 1/25/1969 1/25/1969 January, 1969 January, 1969 Jan-, Feb-1969 January, 1971 January, 1972 9/9–9/10, 1976 December, 1979 9-28 to 9-29, 1980 7/25/1982

7/27/1988 2/2/1990–2/4/1990 10/6/1993

Structure ? None

which continued to argue the problem of climatic controls on alluvial fans, used his metric of maximum grain size and was an attempt to verify his observation that fan slope is a function of maximum grain size. For instance, Beatty (1963) premised his study of alluvial fans in the White Mountains, California, on the hypothesis that arid region fans were primarily composed of debris flow deposits. Following Blissenbach, he plotted the decrease in the

Chawner, 1935; Taylor, 1934 Troxell, 1942 Anstey, 1965

Fenzel & Price cited in NRC, 1996 Scott, 1973 Schick & Lekach, 1987 Wasson, 1974 Magura & Wood, 1980 Whitehouse and McSaveney, 1990 Harvey, 1984c Tunbridge, 1983; Blair, 1987 Wells & Harvey, 1987 Wieczorek et al., 1989 Hjalmarson cited in NRC, 1996 Pearthree et al., 2004 Vincent et al., 2004 Scott & Erskine, 1994 Hjalmarson cited in NRC, 1996 Newhall & Punongbayan, 1996 Larsen & Wieczorek, 2006 Sinha, 2009

average sizes of the 10 largest particles in a 5-ft square against distance from fan apex. He argued that the irregular decrease was a result of debris flow processes. He described two firsthand accounts of debris flows triggered by the 1952 thunderstorms. These accounts and subsequent mapping by Beatty (1963) illustrate that the 1952 debris flows deposited Btwothirds of the way from the fanhead to the axial stream, and that subsequent flood flow dissected debris flow deposits,

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

leaving tongues of silt and sand on the remaining one-third of the fan. He emphasized that local deposits that dammed the valley could trigger avulsion. His work is notable for its clear description of historic debris flow deposits and their role in shaping some fans, and for his argument that isolated boulders on fans are plausibly from channelized water flow or debris flows. He is the first to propose that lobes of boulders on fan surfaces represent debris flow deposits whose fine matrices have been removed by subsequent water flow. Bluck (1964) explored the maximum particle size in streamflow and mudflow deposits on a fan north of Las Vegas, Arizona. He showed that particle sizes in both deposits decreased exponentially downfan. He attributed the fining of the maximum particle size to selective sorting and abrasion and to physical weathering. In southern New Mexico, Ruhe (1964) characterized the downfan decrease in median diameter of weathered alluvial fan deposits of the Jornada fan in an effort to understand the distribution of piedmont soils. In the course of dissertation work in the White Mountains of eastern California, Lustig (1965) sampled 202 sites to map the granulometry of a 3–8% slope alluvial fan. At each station, he measured the b-axis of the largest particle within a 50-ft radius and collected grab samples of granule-to-clay sizes. He found no systematic variations in these parameters downfan, and no systematic relation between maximum particle size and local slope. Denny (1965) mapped and measured alluvial fans of the Death Valley region. He mapped fan surfaces as modern washes, abandoned washes, desert pavements, or pediments. Within the first three units, he sampled at 25 equally spaced points along a tape and calculated the geometric mean of the surface clast population. Of the 13 active channels in which he collected data, only two show a persistent monotonic decrease in mean grain size with distance from drainage divide. Mean clast size in most of the samples does not change systematically until the last sections of the fan where it declines to values of B2–10 mm. He noted that d90 (90th percentile) values were three to five times larger than mean diameters, and that no systematic relation existed between channel slope and bed material size. By comparison to humid region streams, Denny observed that arid fan channels were steeper and finer grained. He wrote that the lack of a clear pattern in grain size downfan reflected the vagaries of hydraulic and sediment supply patterns. His work is notable for its use of geomorphic mapping to stratify sampling, and for its careful application of fluvial techniques to sediment sampling. Mayer et al. (1984) measured the mean particle size in Pleistocene, Holocene, and modern channel deposits in SW Arizona, USA. They hypothesized that hillslopes under different climates would produce different grain size distributions, a tendency that would be reflected in clast sizes and downfan fining rates depending on abrasion and sorting. On two fan channels they found that modern channel fining rates were the highest, followed by Holocene and Late Pleistocene rates. Late Pleistocene deposits had the finest particle size distributions, an observation they attributed primarily to hillslope supply. Working in glacial outwash fans in Alaska and Iceland, Boothroyd and Nummedal (1978) reported consistent reductions in maximum clast size on bars with distance from the source. Wells and Harvey (1987) found no systematic downstream changes in maximum clast size on

435

fluvial fans deposited during a 1982 thunderstorm in northeast England. Kesel (1985) and Kesel and Lowe (1987) were among the first to measure grain sizes on alluvial fans in the tropics (Costa Rica). At one active alluvial fan draining a volcanic edifice and three inactive fans draining steeplands, they measured the 10 largest clasts at sites of opportunity downfan. They reported systematic decreases in the average of this population downfan, and a power law relation such that channel slope increased with maximum clast size. They argued that these relations were the signature of fluvial transport processes, and illustrated that maximum clast size on these tropical fans was systematically larger than most previously studied arid-region fans. They found that the rate of decline, however, was not separable from that observed in arid-region fans. There appears to be a discrepancy between Toro Amarillo grain sizes reported in the 1985 paper versus the 1987 paper. Ritter and others (1993) characterized the gravel (48 mm b-axis) of a glacio-fluvial alluvial fan in Montana. They found no systematic changes in either maximum or mean gravel diameter downfan in cutbank exposures. Along bankcuts in two traction-transport fans (Hell’s Gate and Anvil Springs Canyon) in Death Valley, Blair (1999a, 2000) measured the intermediate diameter of the largest exposed clast. In the B9–11 km he measured down from the fan apex, he found no systematic changes in maximum clast size. He also measured the grain size distributions of granule and finer particles in deposits exposed in cutbanks. There is a slight tendency for a downfan increase in weight percent sand (Table 2) in the cutbank deposits of Hell’s Gate (Blair, 2000) and Cucomungo fans (Blair, 2003). Wieczorek and others (2002) measured the dimensions of the maximum clast sizes deposited in floods and debris flows that inundated 10 fans of coastal Venezuela. They found no strong downfan patterns, although a contour plot of maximum clast size for the Caraballeda fan shows that the very largest boulders (up to B5 m) are concentrated near the fan apex. Florsheim (2004) explored sidevalley tributary alluvial fans in the Navarro River of northern California, USA. She sampled subsurface sediment using 30 auger and core samples. She found that median grain size of bulk samples decreased with radial distance downfan in three of the four fans, coinciding with a decrease in gravel fraction. She attributed this decline to a transition from debris flow to stream flow deposition, and to selective sorting. Her median values are significantly finer than most of the other sources in Table 2, in part because she sampled the bulk distribution of sediment rather than point counting. De Scally and Owens (2005) examined particle size and shape trends down four fans in the Southern Alps of New Zealand. They hypothesized that different depositional processes on each fan would produce different trends in these metrics. They found no patterns in mean grain size or shape along the fan dominated by fluvial processes and downfan coarsening on fans dominated by snow avalanches and other inertial processes. Mather and Hartley (2005) measured maximum clast sizes along a Chilean fan and found no systematic changes. Stock and others (2007) used pebble counts to characterize the grain size distributions of active channel deposits at four fans in the Mojave and Death Valley regions of California, USA. At 68 sites, they reported bed percent sand cover, d16, d25,

436

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

d50, d75, d84, mode, interquartile range (IQR), and standard deviation of grain size distributions 42 mm. They found that nonparametric values stabilized after 75 counts, rather than the conventional 50 counts in better-sorted perennial channels. They reported that median gravel diameter and IQR did not vary systematically down the upper 60–80% of the fan channel length, whereas bed percent sand cover increased down fan from fractions less than 20% to distal fan values in excess of 70%. In an unpublished work on an additional sandbedded fan (Table 1), the author found no detectable change in grain size down the New York Mountains fan, Mojave area, California, with median grain size varying from 0.8 to 1.44 mm as the slope varied from 3 to 6%.

9.23.8.2

Descriptive Stratigraphy

Bull (1964a) sampled and described ‘mudflow’ and traction deposits emplaced over the course of several years on low sloped (0.003–0.029) fans in the western margin of the San Joaquin Valley, CA, USA. He described the traction deposits as sheets of well-sorted sand and silt deposited by shallow braided streams. Maps of these deposits show no systematic change in median grain size (0.07–0.53 mm) or clay content (5–11%) over B1.5 miles of deposition. Bull (1964a) found that ‘mudflow’ deposits had higher clay contents (average of 31%) and he proposed (Bull, 1962b, 1964a, b, 1972) separating these deposits from traction deposits using a plot of median grain diameter (‘M’) versus coarsest 1% of the distribution (‘C’), as developed by Passega (1957). Kochel and Johnson (1984) used this plot to separate out mudflow from streamflow deposits in alluvial fan deposits of the Appalachian Mountains, USA. By contrast, Wasson (1977) found that CM diagrams did not separate debris flow from traction deposits, a failure he attributed to subsequent deposition in interstitial fines into the traction deposits. In steep fans of New South Wales, Australia, he described a downfan decrease in debris flow deposit abundance. Downfan, he found poorly sorted, sub-horizontal sheets of imbricated gravel (20–30 cm thick) alternating with moderately well-sorted, well-bedded lenticular gravels and sands with tabular cross-bedding. Gole and Chitale (1966) described the Kosi fan of India, whose headwaters include rapidly uplifting peaks of the Himalayas. They emphasized the enormous annual quantities of sand and finer grain sizes deposited on the fan (B96 000 acre-feet per year) during monsoons, and the damages from flood-driven deposition of this material. They reported a decline in grain sizes from boulders at the fanhead to clay at the distal margins, 198 miles away. They noted the westward migration of active fan channels from the easternmost A.D. 1731 channels, to the westernmost channels of A.D. 1963, with a 101 cone of deposition coincident with each active phase. They argued that as deposition within each cone advanced over one to many decades, lowered areas to the west attracted the next phase of activity. Wells and Dorr (1987) elaborated on this earlier work. They used Landsat images to map fan channels and described the stratigraphy from a reconnaissance trip. They distinguished between a modern upper fan with braided channels and a maximum slope of B0.007, which extended B147 km from the fanhead, and a lower fan of meandering

streams with oxbow lakes and wide floodplains at slopes as low as 6 105. Gohain and Parkash (1990) further elaborated on this division, mapping changes in planform geometry down fan from gravelly sandy braidplain channels to sandy braided channels, to straight channels, and finally meandering channels in the distal margin. They describe a wide range of bar and bedforms along this transition, encompassing a gradual change from gravel- and sand-dominated barforms. Using sedimentology described from a number of braided stream and alluvial fan studies, Miall (1977, 1978) developed lithofacies codes for braided river deposits that he extended to alluvial fan deposits. Miall (1978) initially proposed three type-localities that could encompass alluvial fan depositional environments: (1) alluvial fans subject to debris flows (Trollheim type), (2) shallow, proximal rivers or alluvial fans whose deposits were exclusively traction (Scott type), and (3) deeper, distal gravel-bedded rivers or alluvial fans (Donjek type). The Trollheim type was initially based on alluvial fans with a predominance of debris flow deposits, as described by Hooke (1967) and by Wasson (1977). Miall’s Table 4.1 (1996) divides the sedimentology of debris flow deposits (facies code SG, or sediment gravity flows) into lobes or sheets of clast or matrix-supported gravels that can be massive (pseudoplastic debris flow), weakly graded (plastic debris flow), inversely graded (pseudoplastic or clast-rich debris flow), or normally graded (pseudoplastic debris flow). In addition to debris flow deposits, Trollheim-type deposits may contain lesser amounts of gravel bars and bedforms (facies code GB) as massive or crudely bedded imbricated gravels, or stratified gravels with trough or planar cross-beds. Finally, a variety of sandy bedforms (facies code SB) may be present as planar or troughcrossbeds, ripples, or laminae deposited in lenses, sheets, wedges, and other small pocket geometries. The Scott type was based on work on glacial outwash fans in Alaska and Iceland by Boothroyd and Ashley (1975) and Boothroyd and Nummedal (1978). It lacks the debris flow deposits of the Trollheim type and is composed primarily of gravel bars and bedforms (GB) as described in the previous sentences, with minor amounts of sandy bedforms (SB) deposited in small pocket geometries. Finally, the Donjek type is based largely on work by Williams and Rust (1969), and Rust (1972, 1978) on glacial outwash streams of Alaska. This type contains gravel bars and bedforms (GB) and sandy bedforms (SB) of the facies above, with the addition of downstream accretion macroforms (facies code DA), sandy bedforms that rest on a flat or channeled base rather than in small pocket geometries. Work on alluvial fans deposited in glacial and periglacial environments includes steeper fan system as well. Ryder (1971) studied mixed debris flow/fluvial fans deposited at steep glacial valley tributaries in south-central BC, Canada. He described well-bedded and sorted gravels (fluvial) interbedded with nonsorted, unstratified angular gravels with a silt and clay matrix (debris flow). On 2–101 fans with both materials, he found 40–60% of each deposit. He interpreted fanhead trenching as a result of reduced sediment supply, or base-level lowering. Kostaschuk and others (1986) reported the sedimentology and morphology of alluvial fans in recently glaciated valleys of Alberta, Canada. Unlike Ryder, they found that fan deposits were either dominantly debris flow or fluvial

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

materials. On fluvial fans, they described longitudinal bars of imbricated gravel that graded into distal lobes of well-sorted, sheetlike, imbricated gravels. They also describe distal laminated sands that they interpreted as overbank deposits. They proposed that debris flow fans originated in basins that were smaller and steeper than fluvial fan source basins. In the Blue Ridge Mountains of VA, USA, Kochel and Johnson (1984) noted that debris flow deposits were restricted to uppermost alluvial fan areas, with imbricated cobbles and sand composing 80% of the fan deposits. In the same fluvial fans, Kochel (1990) reported the downfan transition from proximal poorly sorted angular boulders, to medial interbedded sands and gravels, and finally to distal gravel and sand sheets. In the mountains of southeast Spain, Harvey (1984a) examined incised alluvial fans and classified their sediments into debris flow deposits, sheet and channel gravels, and silts. He showed that the proportion of channel gravel deposits grew downfan at the expense of debris flow deposits, consistent with other work showing the transition from debris flow deposition to fluvial processes down some fans (e.g., Beatty, 1963). McCarthy and others (1988, 1991, 1992) and Stanistreet and McCarthy (1993) described the low-gradient (B3 104 slope) distributary fan system of the Okavango River of Botswana, Africa. This large (B18 000 km2) alluvial fan is deposited in a graben from an arm of the East African Rift. Its sediments are almost entirely aeolian sands from the Kalahari, in association with biologically produced fines from peatlands. The fan itself is a mixture of seasonal and permanent swamplands, with meandering and low sinuosity channels transporting sand bedload, confined by peat levees. Both water discharge and channel width decrease progressively downfan as evaporation consumes 95% of the annual inflow. The Okavango fan arguably represents amongst the lowestgradient fan depositional environments described to date, and is notable for the large impact that vegetation plays on its channel pattern. Blair and McPherson (1992) revisited the stratigraphy of the Trollheim fan in Deep Springs Valley, CA, USA. On the surface, they described the prevalence of varnished, angular, matrix-free gravel, deposited in lobes or levees. In channel cuts, they observed similar angular gravel, lacking bedforms, supported by a matrix of finer silt and clay. They interpreted the subsurface units as debris flow deposits, and argued that the surface lobes interpreted by Hooke (1967) as sieve deposits were actually old debris flow deposits whose matrix fines had been removed by winnowing. As a consequence, they argued that sedimentological models based on Hooke (1967) and other’s interpretations would need revision. Tunbridge (1983) and Blair (1987) described fan deposits from a dam-break flood in Horseshoe Park Valley, CO, USA, on 15 July 1982. These deposits form the basis of much of their later interpretation, so it is worth describing two views of them in detail. Tunbridge (1983) separated the deposits into five facies: boulder beds, gravel braid bars, braided stream sands, sandy flood sheets, and distal fine sands and muds. He described the boulder beds as massive 6- to 10-m-thick deposits of boulders with a gravel and coarse-sand matrix. Downfan he described pebbly, gravelly sands forming

437

longitudinal bars (his Figure 5) up to 20 m long and 8 m wide. Bar sediments fine upward from a pebbly, cross-stratified base to a planar-bedded sandy top with low angle downstream-dipping cross laminations. Where the fan deposits impinge on Fall River channel, this facies grades into pebbly sands of B2-m thickness, deposited in a braided channel. Tunbridge described these deposits as gravel foresets on the downstream slip-face bars, capped by parallel-laminated sands and ripple cross-laminated fine sands. At the distal extent of deposits, he described planar-laminated sand sheets, and finally laminated and rippled fine sands and muds. Blair (1987) used air photos taken late in the Horseshoe Park dam-break flood to help divide deposits of the flood onto three lobes and characterized the sedimentology of 50 depositional sites. In his resulting map, he divided deposits into 13 facies with progressively declining grain sizes downfan. In the initial lobe, Blair interpreted the levees in the coarse boulder deposits as ‘noncohesive sediment-gravity flow’ because the tops of the boulder levees were 2-m above adjacent highwater marks and were poorly sorted. He characterized the remaining boulder deposits as the products of sheetflood deposition, and illustrated the down-lobe fining from sandy boulder gravel to distal muds. He characterized the second lobe as sandy boulder gravels that fined down-lobe to granular coarse to medium sand, and interpreted their deposition by sheetfloods. In the third lobe, deposited late in the flood, he found planar-bedded ‘interstratified cobble or pebble gravel and granular sand’ couplets, dipping parallel to the fan surface. He subdivided these deposits into six units of decreasing maximum grainsize down-lobe. Where flow ‘rechannelized’ at the distal end of lobe three, he described lenticular pebble–gravel beds interstratified with inclined beds of granular sand, which dipped both up- and downstream. He interpreted these units as deposits formed by antidunes. In the distal-most units of lobe three, he described ripple-, trough-, and planar-crossbedded sands, and planar-crossbedded sandy pebble gravels units dipping upstream. He interpreted the latter as antidune deposits. He mapped waveforms present on a low-altitude air photograph taken late in the flood. He assumed these were migrating antidunes rather than standing waves, and used Kennedy’s flume relation between antidune wavelength and average flow velocity to back-calculate velocity. He estimated flow depth using Costa’s (1983) empirical relation between maximum clast dimension and flow depth. He combined these results to argue for Froude numbers of 2.0. He asserted that the gravel and sand couplets of the third lobe were deposited under these flow conditions. This assertion forms the basis for much of his later models for upper regime flow conditions accompanying alluvial fan deposition, where he assumes a one-to-one correspondence between alternating gravel sand couplets and upper flow regime sheetfloods. His interpretations of upstream-dipping planar crossbeds at the distal ends of lobe three as antidune deposits are another leg of this hypothesis. Blair (1999a, b, c, 2000) detailed the sedimentology of alluvial fans (2–51 slopes) in the Death Valley area, contrasting deposits from waterflow and debris flow processes. On the Anvil Spring Canyon (38 sample sites) and Hell’s Gate (45 sample sites) fans, he divided waterlaid deposits exposed in cut-banks into three main, and several subsidiary facies.

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

The most common facies (A) he described as a clast-supported, imbricated, sandy, granular, fine-to-medium pebble gravel alternating with cobbly, coarse to very coarse pebble gravel in planar–stratified couplets dipping at the fan surface slope. In both fans, this facies comprised the majority of cutbank deposits, from 60 to 95% abundance. Less commonly (0–25%), he found deposits of ‘wedge-planar beds of sandy pebble gravel in backsets dipping 5–241’ (facies B). He also described a facies of pebbly, cobbly, boulder gravel deposited in irregular and lenticular beds (facies C in Blair, 1999a, and D in Blair, 2000). On Anvil Spring Canyon fan, he described massively bedded cobble–boulder gravel deposited in radially aligned ribbons, and on Hells’ Gate fan, he described distal deposits of planar laminated sand. He plotted the abundances of all of these facies down each fan, and found no strong gradients in any of them. He interpreted facies A and B as products of deposition from unconfined (i.e., sheetflood) upper flow regimes with antidune bedforms. He argued (e.g., Blair, 2000) that the lack of deep scours, lenticular beds, rapid lateral changes in texture, or muddy overbank deposits indicated that these two facies were deposited in sheetfloods, rather than in channelized flow. In his interpretation (e.g., Blair, 1999a), the upslope dipping beds of facies B represented stoss-side deposition of upslope migrating antidunes, and the planar couplets of facies A represented plane-beds deposited during the washout of antidunes. He based these inferences on comparisons with deposits from historic events on the Roaring River and Little River fans (Blair, 1987; Blair and McPherson, 1994a, b), and on experimental results from flumes (e.g., Gilbert, 1914; Fahnestock and Hauschild, 1962; Simons and Richardson, 1966; Shaw and Kellerhals, 1977; Langford and Bracken, 1987; Alexander and Fielding, 1997). For instance, he argued that the Roaring River deposits of alternating fine and coarse sandy cobble–pebble gravel couplets with occasional backset wedges were deposited by antidunes and breaking antidunes, on the basis of the occurrence of waves in an air photograph. He argued that flume results cited just above indicated that only antidunes could produce the observed backset beds. He did not articulate why the flume or Roaring River results rule out other origins for these deposits. He generalized the interpretations above to argue for a waterlaid alluvial fan depositional model where channel deposits were infrequent, and deposition is dominated by unconfined ‘sheetflow’’’ rather than channelized, braided stream flow. Blair (1999b) characterized the majority (490%) of sediments of the Warm Spring Canyon alluvial fan in California as debris flow deposits. He divided these into two facies, a matrix-supported unit of primary debris flow deposits and clastsupported, imbricated deposits that he interpreted as the eroded remnants of debris flow deposits, winnowed of fines by subsequent water flow. Within the active channel of the fan, he found clast-supported, imbricated gravels, cobble and boulders deposited by modern channel flows. He observed that these deposits were restricted to this incised active channel and argued that they represented reworking of older debris flows by streams. He also described planar beds of gravelly, sandy mud downfan from the incised channel, deposits he interpreted as the final surges of debris flows. In the Cucomungo fan of Eureka Valley, California, Blair (2003) described

a similar set of facies, dominated by debris flow deposits. Here too he interpreted the imbricated, bedded deposits as winnowed lags from water flows across debris flow deposits. He argued that debris-flow-dominated fans predominate where source catchments have significant silt and clay in the soils of their hillslopes, leading to the generation of debris flows (Blair, 1999c). Mather and Hartley (2005) described the sedimentology of an arid region alluvial fan in Chile. They divided deposits into (1) imbricated, bedded sheetform gravels with alternations of coarse and fine beds, (2) matrix- to clast-supported sheetform gravels, and (3) clast-supported channelized gravels. On the basis of Blair (1999a, 2000), they interpreted their facies one as the washout deposits of antidunes, their facies two as debris flow deposits, and their facies three as low-flow reworking on the previous two deposits. Arzani (2005) described a large (45-km radius), low-sloped (B0.008) alluvial fan from central Iran. He divided deposits into (1) imbricated, clast-supported gravels with alternations of coarse and fine beds, (2) lenticular, imbricated gravels and sandy gravels deposited as sinuous lobes or sheets, (3) calcareous, silty marls, and (4) calcareous marls with aeolian sand. He found facies one present in the active channel as armored bars with finer subsurface deposits, facies two present as meandering gravel bars and sheets downstream from facies one, and facies three and four as overbank deposits of silt and sand, and interfingered fluvial and playa deposits, respectively. His work demonstrates that where the boundary conditions allow, alluvial fans may propagate out to large diameters at comparatively low slopes, and that the alternating coarse/fine couplets can be found as an armor over a finer-grained subsurface.

9.23.9 9.23.9.1

Geologic Record of Fans Surficial Mapping and Dating

Following World War II, the availability of maps and air photographs for the American southwest triggered a surge of surficial mapping of alluvial fans. Albedo and surface texture contrasts observed on air photographs could be correlated with ground observations of desert soils developed on alluvial fans to create maps of relative surface deposit age. These were significant advances over previous generations of maps (e.g., Trowbridge, 1911). When combined with absolute and relative age dating, surficial maps can be used to understand the role of climate and tectonism in shaping alluvial fans; to understand how pedogenic processes influence the distribution of hydrology and ecology, including endangered species like desert tortoises; to estimate fault offset rates; and to map relative flood hazards of alluvial fans. In his maps of alluvial fans in the Death Valley region, Denny (1965) distinguished between modern washes (no varnish on clasts, little vegetation), abandoned washes (varnished clast and vegetation), desert pavements (armored surfaces of rock fragments lacking depositional topography like bars and swales), and pediments (dissected fan deposits). He used this mapping to argue that older fan pavements and dissected surfaces were a substantial source of runoff and

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

sediment to the active washes. Hooke (1967) used clast weathering and varnish, presence of desert pavement and topographic position to map six units (recent channel, overflow channel, oldest channel, youngest fan surface, older fan surface, oldest fan surface) in fans of the White Mountains, California. His maps also show debris flow levees, and units he termed sieve deposits: clast-supported, matrix-free gravels and cobbles deposited in lobes. By analogy to his experimental work (see Section 9.23.10) Hooke inferred that these deposits represented the deposition of coarse debris as the water moving it infiltrated rapidly into a permeable subsurface. This initial barrier causes subsequent deposition, creating a matrix-poor, lobate deposit. He argued that debris flows required fine sediment in the source area, and that sieve deposits would not form if such fine material was present. In addition, he argued that sieve deposits would lack the matrix of fine sediment, and the coarse boulders that characterize debris flow deposits. However, he acknowledged that distinguishing sieve deposits from debris flow deposits whose fines had been removed by erosion might not be possible. Quantitative and qualitative work on weathering and soil pedogenic processes in arid lands has evolved to the point that it plays a fundamental role in surficial mapping. Following early work by Gile (1966), subsequent workers began to use relative dating techniques to test hypotheses about climatic controls on fan deposition. Harden (1982), Harden and Taylor (1983) and Harden and others (1991) developed and used soil profile indices as a tool to systematically explore the effects of pedogenesis on arid-region soils. McFadden and Tinsely (1985) and Machete (1985) explored pedogenic carbonate accumulation in desert soils. McFadden and others (1986, 1992), McFadden and Weldon (1987), McFadden (1988), and Reheis and others (1989) characterized development rates of Quaternary soils in the Mojave area. McFadden and others (1989) provide a review of the origin and history of soil development and other relative dating techniques as they have been applied to alluvial fan deposits. A number of studies (e.g., Wells et al., 1987, 1990; Reheis et al., 1989; Harvey and Wells, 1994; Ritter et al., 2000; McDonald et al., 2003) have demonstrated that Pleistocene and Holocene deposits can be distinguished from each other using soil and surface characteristics. In the subsurface these include degree and geometry of secondary carbonate and iron minerals, clay films, and thickness of aeolian deposits beneath desert pavements. On the surface these include presence and texture of pavements (e.g., Wood et al., 2002), degree of weathering of large surface clasts (e.g., relative abundance of rapidly weathered lithologies, varnish development, pit depth, rind thickness, angularity, grain relief, etc.), and relief of relict depositional features (e.g., fluvial bars and swales). For instance, Wells et al. (1987) showed that relict Pleistocene fluvial barforms had systematically lower relief than Holocene equivalents. Wells and Dohrenwend (1985) also mapped and characterized bedforms on Late Pleistocene and early Holocene alluvial-fan surfaces of southeastern California. They interpreted sub-centimeter amplitude bedforms as evidence for unconfined flow over desert pavements. There is a debate about how to use the intensity and chemistry of varnish deposits to interpret exposure age (e.g., Dorn et al., 1987; Wells and McFadden, 1987). Development of a suite of relative age dating techniques, largely

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in the American southwest, provided a basis for regional mapping to distinguish Pleistocene and Holocene deposits. Harvey hypothesized that climatic conditions lead to fan dissection or aggradation in alluvial fans of Spain, the American southwest, and elsewhere. In southeast Spain, he documented aggradation on alluvial fans culminating in development of pedogenic carbonate horizons. He documented the degree of surface calcrete on these fans and used a threestage calcrete development index (e.g., Dumas, 1969) locally correlated with absolute age dating to assign relative ages. He found that calcrete development (and hence aggradation) had ceased by the beginning of the Wurm period (B110 ka). During the last glacial and continuing in Holocene time, he documented widespread trenching of alluvial fans (Harvey, 1978, 1984a, b, 1987, 1990), coinciding with a progressive decrease in sand- and finer grain sizes, and an increase in gravel deposition (Harvey, 1990). He proposed that fans with significant calcrete crusts tended to have more confined channels, which led to continued distal channel lowering (Harvey, 1987). Harvey (1984b) also reported a progressive decline in slope from fan surface, to terrace, to modern channel, also consistent with a decline in sediment supply. He argued that the last interglacial climate in Spain led to high hillslope sediment production rates that resulted in fan aggradation (e.g., Harvey, 1990). By contrast, late Pleistocene and Holocene fan incision was a result of a reduction in this sediment supply or an increase in transport capacity (e.g., Harvey, 2002a). He extended this work to the southwest United States where Harvey and Wells (1994) and Harvey and others (1999) used relative dating and absolute ages of lake shorelines to map alluvial fan deposits. They argued that an early Holocene reduction in hillslope sediment supply and an increase in storm runoff led to dissection of alluvial fan surfaces in the Mojave. The application of absolute dating tools (e.g., radiocarbon, Optically Stimulated Luminesence, or OSL, and InfraRed Stimulated Luminescence, or IRSL) to units mapped using relative age dating has improved our ability to test hypotheses for climatic control of fan deposition. Wells et al. (1987) used radiocarbon to date Lake Mojave shorelines, and tied alluvial fan depositional units to these shorelines. They reported that most of the alluvial fan deposition occurred following the beginning of the Holocene at c. 10 ka. They proposed that climate changes leading to reduced vegetation density increased sediment yield and runoff. Reheis and others (1996) studied alluvial fans in western-most Nevada to test this hypothesis. They reported depositional ages that spanned the last c. 12 ka. Importantly, they reported an absence of depositional ages from c. 12 to c. 50 ka, an observation they attributed to the presence of stable, vegetated slopes prior to Holocene climate change. Ritter and others (2000) used radiocarbon to date shells in beach ridges and bedforms proximal to alluvial fans in central Nevada, USA. They found depositional ages of c. 11–15 ka, and proposed a regional correlation with fan deposits triggered by climate change in other areas of the western United States (their Figure 11). Miller and others (2001) characterized and dated the stratigraphy of side-valley fans in steeplands of central Nevada. They found that fan depositional activity was largely late-Holocene (c. 1930–2550 years before present),

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

and attributed this to reduction in vegetation cover with the onset of late-Holocene droughts. McDonald and others (2003) report regional alluvial fan deposition in the eastern Mojave c. 9.4–14 ka. Working in the northeastern Mojave, Mahan and others (2007) used OSL techniques to date Quaternary units, finding concentrations of alluvial fan depositional activity in the mid-Holocene (c. 3–5 ka), and of groundwater deposits in the Pleistocene from 20 to 52 ka, and from 140 to 185 ka. Working on alluvial fans in Queensland, Australia, Nott and others (2001) used OSL to date depositional events between 14 and 27 ka. They interpreted that these deposits represented a time of increased sediment supply when reduced plant cover and intense rainfall events triggered slope instability. The technique has also been applied to fans in Argentina (Robinson et al., 2005) to show mid-Pleistocene depositional ages. Frankel and others (2007) used 10Be clast exposure ages and 36Cl depth profiles to date alluvial deposits offset along the Death Valley fault zone. They bracketed deposition between maximum exposure ages of c. 70 ka, and minimum burial ages of c. 63 ka, and used LiDAR topography to estimate channel offset. A number of recent USGS workers have used pedogenic and geochronologic techniques to create detailed maps of the surficial geology of alluvial fans in the Mojave (Bedford, 2003; Amoroso and Miller, 2006; Dudash, 2006; Schmidt and McMackin, 2006; Miller et al., 2009; Bedford et al., 2010). They divided alluvial fan deposits into young, intermediate, and oldest. They subdivided young deposits into four categories: active washes and three higher terraces whose deposits had weak to absent desert pavement, and increasing degrees of surface varnish and pedogenic horizons of calcite, aeolian silt (Av), and cambic material (Bw). Luminescence, radiocarbon, and U/Th dates (e.g., Clarke, 1994; Wang et al., 1994; Miller et al., 2007; Sohn et al., 2007) indicate latest Pleistocene to Holocene deposition times for similar deposits. They subdivided intermediate deposits into three categories, with increasing thicknesses of aeolian silt, and development of varnish, pavement, Bt layers, and calcic cementation. Most deposits with desert pavements have been dated as late Pleistocene, with some falling in the range of late-middle Pleistocene (Bull, 1991; Wang et al., 1994; Sowers et al., 1988; Miller et al., 2007; Maher et al., 2007; Sohn et al., 2007). The oldest deposits appeared infrequently as dissected whaleback ridges. For each geologic unit, these USGS workers measured characteristic hydrologic parameters (e.g., Ksat, depth and duration of moisture storage) and ecology. They used their surficial maps to project characteristic hydrologic values across the region to model the distribution of ecosystems as a consequence of geologically inherited hydrology (e.g., Bedford et al., 2009). Miller and others (2010) found that a number of Mojave alluvial fan deposits clustered during 3–6 ka and 9–14 ka, a clustering they attributed to the influence of monsoonal thunderstorms. Bacon and others (2010) mapped alluvial fan deposits in the Yuma area, and dated inset terraces at c. 2.9–3.2 calendar years before present. They summarized mid-Holocene alluvial fan depositional ages within the American southwest from a variety of sources and argued that widespread deposition occurred during more frequent, intense rainfalls associated with an intensified ENSO (El Nino

Southern Oscillation) cycle. Walker and Fattahi (2011) reviewed the geochronology (OSL, IRSL, 10Be, 36Cl, U-series) of alluvial deposits in Iran, finding that most of the deposition occurred prior to c. 9 ka, with sustained channel incision thereafter. They interpreted this sequence as a result of decreased sediment supply coinciding with increased flow, resulting in channel incision.

9.23.9.2

Deeper Stratigraphic Record

A number of researchers have used fan deposits to explore the geologic record of environmental and other climate change. At Holocene timescales, Meyer and others, (1995), Meyer and Wells (1997), Pierce and others (2004), and Pierce and Meyer (2008) have dated charcoal in alluvial fan deposits in tributary fans in Yellowstone National Park and central Idaho, USA. They used these dates and the associated deposits to reconstruct a history of fire frequency, finding that the bulk of fire-related debris-flow deposition occurred during warmer periods that likely included droughts and large forest fires. Postfire deposition during these times accounted for 24% (Pierce et al., 2004) to 30% (Meyer et al., 1995) of the stratigraphy. Frechette and Meyer (2009) extended the technique to document Holocene fire frequency of a steepland in New Mexico, and found a peak in fire-related deposition c. 650 calendar years BP, corresponding to a severe drought. Scharer and others (2007) used radiocarbon dating techniques to reconstruct the depositional history of an alluvial fan just off of the San Andreas Fault. They reconstructed the recurrence interval of large slip events and found a relatively constant recurrence rate of B110 years. Cherven (1984) and Weissman and others (2002, 2005) explained the stratigraphy of large alluvial fans of the San Joaquin Valley, California, as a result of fluctuations in accommodation space driven by Sierran glaciation. Cherven (1984) found three facies (silt- and fine sandstone, mediumgrained sand, and gravelly sand, and gravel and sand), which coarsened upward, indicating progradation of the fans out into the San Joaquin Valley. Weissman and others (2002, 2005) argued that high sediment supply during glaciations led to valley aggradation and widespread deposition across high fan surfaces. Conversely, reduced sediment supply during the onset of interglacial times led to incision and movement of sediment deposition to distal fan regions. They found that areas of higher subsidence had more vertically stacked fan deposits than those of lower subsidence where fans tended to prograde laterally. At longer timescales, dissection or burial obliterates fan forms. Then the most reliable evidence for ancient alluvial fans is the presence of radiating paleocurrents (e.g., Williams, 1969; Howard, 1966). Nilsen (1982) writes that the bestdescribed ancient fan deposits come from sandstones associated with the Caledonian, Appalachian, and Hercynian orogenic belts. Sedimentologists have explored the architecture of such deposits to understand how they evolve with tectonism, sea-level change, and longer term climate variations. For instance, Heward (1978) described work on fining and coarsening upward alluvial fan sequences preserved in the

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

geologic record (see his Table 1). He reported examples of alluvial fan deposits with sustained, progressive changes in grain size, bed thickness, and depositional process over hundreds of meters. He contrasted this apparent continuity with the existing short-term record of depositional events on alluvial fans, which seemed stochastic by contrast. He proposed that over longer timescales, depositional events would tend toward a central tendency that reflected climatic and tectonic controls on deposition. To illustrate, he compared the vertical stacking accompanying different types of faults. In the Ridge Basin of California (e.g., Crowell, 2003) he described a 412-km-thick section of alluvial fan deposits from the Violin Breccia (Figure 13), which record a long-lived steepland basin margin due to offset on the San Gabriel fault. He contrasted this relatively vertical stacking pattern with the

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laterally extensive alluvial fan deposits that accompany Basin and range-style normal faulting (e.g., Steel and Wilson, 1975). McGowen (1979) summarizes reports of ancient alluvial fan deposits from South Africa (Pretorius, 1976), Norway (Nilsen, 1969), and Europe (Glennie, 1972). In deepest time, alluvial fan deposits have been inferred from well into the Proterozoic (see Long, 1978, for a summary of examples). Uranium placer deposits in Proterozoic alluvial deposits of South Africa (e.g., Pretorius, 1976; Minter, 1978) and Canada (e.g., Robertson, 1976) have spurred exploration of ancient alluvial fan deposits bordering Precambrian shield areas. Ethridge and others (1984) investigated the Precambrian Vallecito conglomerate of Colorado, USA, finding three types of braided river deposits (cobble–boulder, pebble–conglomerates, and pebbly–quartzites), which they interpreted as proximal, medial,

(a)

(b)

Figure 13 Illustration showing (a) alluvial fan deposits (Violin Breccia) of the Ridge Basin, southern California. Imbrication and sandy channel fills are widespread within this unit, suggesting its deposition by traction transport. (b) Stratigraphy of the Ridge Basin Group (Crowell, 2003). Note the persistence of alluvial fan deposition over almost 12 km of deposits. Sections like this illustrate that fans may persist over long geologic periods, so long as subsidence and rock uplift permit.

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

and distal facies of an alluvial fan. Exploring for natural gas reservoirs in the Canadian Rockies, Varley (1984) found a similar set of lithofacies in a wedge-shaped Cretaceous deposit, which he interpreted as an alluvial fan deposit. He described downfan decreases in maximum clast size and increases in matrix sand (5–60%) that influenced reservoir characteristics. Burgisser (1981, 1984) describes Miocene alluvial fan deposits of the Hornli fan, part of a series of molasse deposits on the north side of the European Alps. He reported a gradual change from conglomerate beds at the paleofan head, through conglomerate–siltstones, to sandstones and siltstones at the paleo-fan terminus. Middleton and Trujillo (1984) used maps of maximum clast size to reconstruct the planform geometry of a Proterozoic alluvial fan in Arizona. Nemec and Steel (1984) presented graphs of conglomerate bed thickness versus maximum particle size in Ordovician to Paleogene conglomerates. Following Bluck (1967), they hypothesized that the transport capacity of a Bingham fluid (i.e., a fluid with a yield shear strength) would covary with its thickness. Therefore, one would expect that debris flows that exhibit Bingham behaviors would generate deposits whose maximum grain size increased with bed thickness. They argued that such a relation should not occur in traction transport. With some caution, they proposed this linear correlation as one technique to identify ancient mass-flow deposits in alluvial fan deposits. Wells (1984) described Cretaceous alluvial fan deposits from Antarctica, composed of both ‘sheetflow’ and debris flow deposits, neither of which had significant correlations between bed thickness and maximum clast size. Nichols (2005) summarized work on Tertiary alluvial fan deposits of the Ebro Basin, Spain. He described growth strata, and a lack of deeply incised valleys in the depositional sequence, as well as a progressive increase in sandstone bodies down depositional slope. He used the chronostratigraphy of these alluvial fan deposits to constrain the emergence of highrelief source basins in the late Oligocene, and the absence of incised valleys in the depositional sequence as evidence for a rising base level.

9.23.10

Experimental Approaches

Many of the variables and boundary conditions that early workers identified as controlling factors on fan evolution are challenging to measure in the field (e.g., water and sediment supply and discharge rates, grainsize distribution, subsidence patterns and rates). As a consequence, flume studies starting in the 1960s have attempted to explore the problems of fan longprofile evolution, fanhead trenching, and the effect of varying processes on fan morphology. Hooke (1967) used a flume in the Keck Laboratory at the California Institute of Technology to create alluvial fan deposits, which he used to help interpret his field observations. To create traction deposits, he used a constant head tank to release water into a 0.16-sloped inlet box, filled with coarsesand to granules. To create debris-flow deposits, he mixed water and sediment in a can, and poured the mixture into the inlet box. The resultant flows deposited in an area with a low slope (0.0076). He introduced the term intersection point to

describe the location at which his incised fanhead channels rejoined the fan surface. He found that intersection points tended to migrate further downfan as debris flows predominated in upper fan portions, in part because his debris flows did not produce an incised channel. In his traction runs with coarse sand and granules, he found that where water infiltrated rapidly, granule lobes formed a barrier against which subsequent material deposited. He termed these features sieve deposits, calling on the analogy of a sieve of coarse material through which the water passes. He believed that lobes of matrix-free gravel, cobbles and boulders in the fans of eastern California represented the field equivalent of his laboratory features. Later in 1968 he expanded the depositional area and conducted a series of experiments to explore the effects of varying discharge on fan slope. He found that the mean slope along the upper fan axis decreased monotonically by a factor of approximately two as discharge increased. Hooke and Rohrer (1979) continued these experiments by exploring how water and sand size influenced the slope of a small fan in a 1.5  2.7 m box. They found that coarser sand (geometric mean diameter of 1.3 mm) generated a steeper fan than finer sands (0.54 or 0.17 mm), and that increasing water discharge for the same sand size resulted in lower slopes. They also explored the role of dominant discharge, the effects of fan azimuth on slope, and compared the latter to measurements of fan slope with azimuth in the arid lands of California Schumm and others (1987) reported on unpublished experiments by Weaver (1984) using a rainfall–erosion facility to simulate basin erosion and fan deposition. These experiments rained on a 56.1-m2 source area, creating fans composed of sand and silt over a 69.7-m2 depositional area. The experiments were intended to investigate rates and patterns of deposition, fanhead trenching, the effects of reducing sediment supply, and the effects of delivering both fluvial sediment and mudflows on fan morphometry. After an initial period of lateral expansion, they imposed a lateral boundary that trapped sediment coarser than silt size. They imposed a constant rainfall rate on the source catchment, and lowered the basin base level three times. Local peaks of sediment yield are superimposed on an overall declining trend. The locus of deposition was largely controlled by episodic fanhead trenching, which increased in frequency as sediment supply declined. In the absence of a trench, numerous channels spread over the apex and midfan, depositing sediment. This deposition increased slopes downfan from deposition areas, which resulted in trenching, and subsequent migration of depositional zones to the mid- and distal fan. Trenched channels then backfilled as the aggrading lower boundary condition reduced slopes, and the cycle began anew. This alternation changed the amount of the fan surface occupied by water, from peaks approaching 80% to lows of B15%. They also recorded the movement of sediment downfan as flow converged into steeper erosional channels, and diverged again into local distributary lobes. In map view, this redistribution process is identical to that described in natural alluvial fans by Field (2001). Using episodic rainfall, they found that small events deposited at the fan apex, whereas larger events deposited in mid- to distal fan regions. Using a smaller, steeper catchment, they introduced varying numbers of mudflows to the fan and found that overall fan slope increased with the

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

abundance of mudflow deposition. Schumm and others (1987) argued that these experiments showed that episodic fanhead trenching could occur in the absence of external changes (e.g., boundary level changes or changes in water supply), as an internal dynamic of sedimentation interacting with water. Bryant and others (1995) created laboratory-scale fluvial fans to explore the effect of deposition rate on avulsion frequency. For each experiment, they fed a constant supply of quartz sand and water into a stream box and recorded the number of avulsions on the developing fan. They found that avulsion frequency increased with increasing sedimentation rate, and then stabilized as inertial flows began to redistribute material at the steepest slopes. Whipple and others (1998) created sand-dominated fans in two experimental basins (2.1 and 5.2 m dimensions) to test a theory developed by Parker and others (1998a). During each of their 51 experiments, they fed in a constant grain size distribution at a constant supply rate and water discharge. They used ponded water to provide a constant base level until the fans had prograded to the experimental margins, and then raised this base level to create a steadily aggrading experimental fan. In their sandy experimental fans they observed the transient formation of mid-fan and distal-fan depositional lobes at sites of flow expansion down fan from concentrated channel flow. This sequence of distal channel aggradation was followed by channel backfilling to the fanhead. In agreement with Parker and others (1998a), they found that fan slope increased with the ratio of sediment to water supply. They found that the degree of channelization in their experiments tended to increase with water discharge and increasingly fine grain sizes below 0.16 mm, but was independent of sediment supply rate, effects that could not be explained by channel geometry rules used in Parker and others (1998a). Paola and others (2001) developed an experimental facility (‘eXperimental EarthScape basin’) where they could vary subsidence rate and pattern using 432 subsidence cells over a 13  6.5 m depositional area. As a consequence, researchers can explore the quantitative effects of subsidence and water and sediment supply on the resulting deposits. For instance, Sheets and others (2002) conducted four successive experiments with quartz sand and crushed coal to examine which flow events were preserved in the deposits, and the characteristic timescale for individual events to integrate to basin-scale stratal patterns. Their experimental fans had some of the same processes observed by earlier workers, including the alternation of confined, scouring channels with lobate depositional areas downfan. They found an inverse relation between channel flow occupation and deposition rate, indicating that most of the deposition in their experiments occurred during short-lived, unchannelized flow events. On the basis of their experiments, they proposed that the time for individual depositional events to create basinscale stratal patterns could be approximated by a multiple of the time to form a deposit of one channel depth. Cazanacli and others (2002) used the same experimental facility to explore the rate at which flow occupies the experimental fan surface. They found that the fraction of the experimental fans surface that remained untouched by flow (‘dry’ fraction) declined harmonically with time. They characterized this rate

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with a decay time, which represented the time over which the original ‘dry’ fraction of the fan surface was reduced by one half. They reasoned that this time was proportional to the ratio of the difference between channel width and fan width along contour, and the lateral flow migration rate. They proposed that the lateral migration rate could be approximated by dividing the volumetric sediment flux with the cross-sectional area of high-flow occupation. Jerolmack and Mohrig (2007) used the EarthScape basin to explore the problem of channel migration. They defined a timescale as the ratio of average channel depth to in-channel aggradation rate, and proposed that it could be used to help characterize the likelihood of avulsion processes. They defined a lateral migration timescale as the ratio of channel width to bank erosion rate, so that continuously migrating channels would be characterized by a short migration timescale. They hypothesized that channels whose avulsion timescale was much shorter than their lateral migration timescale would have a tendency to bifurcate, leading to branched or distributary channel networks. Using a compilation of laboratory and field studies, they found that a ratio of just below 1 tended to separate these systems. Davies and Korup (2007) explored the results of episodic, large inputs of sediment on the long profile of a small, sand (d50 ¼ 0.19 mm) laboratory fan. They fed an avalanching sandpile into an intermittent water discharge to form an experimental fan, bounded by a free-fall edge to maintain the fan boundary at a fixed elevation and position. They found that large inputs of sediment aggraded the fanhead and did not seem to influence distal fan elevations. The fan channel subsequently entrenched these deposits. They used these experiments as analogs for field data from New Zealand alluvial fans and argued that fanhead deposits in some areas reflect infrequent, catastrophic inputs of sediment from large events in the source basin (e.g., landslides). Nicholas and others (2009) and Clarke and others (2010) used a 3  3 m basin to create experimental alluvial fans using controlled inputs of sediment (unimodal sand) and water. They used an arcuate drainage channel that removed sediment as a boundary condition, and explored the effect of aggradation rate on flow width and autogenic behavior. They found that as aggradation decreased, with increasing amounts of sediment bypassing the fan to reach the boundary, flow tended to channelize at decreasing widths. They proposed that flow width is a function of aggradation rate, in agreement with their theoretical model (Nicholas and Quine, 2007), and consistent with results from Jerolmack and Mohrig (2007). Reitz and others (2010) used laboratory experiments to explore avulsion patterns in sand fans fed by a flume with constant water and sediment supply. Their experiments indicated that fanhead entrenchment is the normal state, and the only time the head is not entrenched is when a channel backfilled all the way to the apex. They found that once a network of four to five channels developed on their experimental fans, channelized flow oscillated between these threads indefinitely. Similar to Schumm and others (1987) and Whipple and others (1998), they observed that as aggradation at the distal fan channel decreased slope, within-channel deposition backfilled upslope, driving flooding and avulsion at the fan head. On this basis, they proposed that the time

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Waters Divided: A History of Alluvial Fan Research and a View of Its Future

required to backfill such a channel to the fan apex approximates a timescale for avulsion: hwr=Qs

½8

where h and w are representative channel depths and widths, r is the radial length (which may vary with time), and Qs is the volumetric sediment supply to the fan. The implication is that tectonically active source basins whose fans have narrow, shallow channels should be more prone to relatively frequent avulsions than tectonically inactive basins whose fans have wide, deep channels. The widespread presence of entrenched fans complicates this generalization.

9.23.11

Models of Fan Evolution

Price (1974) developed perhaps the first simulation model for fan evolution, with the goal of developing a statistical understanding of subsurface permeability variations in alluvial fan aquifers. His model is remarkable for its comprehensive attempt to link the mechanics of earthquake generation and surface uplift with erosion and depositional processes. He generates sediment using a time-dependent weathering function whose products are eroded either by water where they are below a threshold thickness, or by debris flows when they are above this value. Deposition occurs across a surface using a random walk method, where the probability that a flow will move into a given cell and the grain size of the resultant deposit are functions of local slope. The model illustrated that numerical techniques could be used to generate and explore realistic-looking alluvial fan deposits. Koltermann and Gorelick (1992) simulated alluvial fan deposition over geologic time using a numerical model that incorporated more evolved fluid and sediment transport equations. They parameterized sediment transport capacity as the product of the depth-averaged flow velocity and the boundary shear stress, where boundary shear stress was calculated as the square of the depth averaged horizontal velocity, fluid density, and a friction coefficient. In effect, sediment transport capacity was a cubic function of horizontal flow velocity. They tuned this sediment transport capacity to the dO18 curve to simulate effects of climate change on sediment supply, and deposited one of four grain size classes depending on a critical shear stress threshold. Their paper does not clarify hydraulic boundary conditions down fan, or how they generated deposition across the fan surface over time. Their model does reproduce observed alternations of coarse (gravel and sand) and fine (silt and clay) sediments in an alluvial fan in the San Francisco Bay Area, USA. Parker and others (1998a, b) and Whipple and others (1998) used sediment transport models with thresholds to investigate long-profiles of alluvial fans, the first time Drew’s (1873) hypothesis had been explored with a modern understanding of sediment transport. Parker and others (1998a) assumed constant subsidence of a fan of length L whose active sediment deposition area they idealized as a pie-shaped sector, with increasing width downfan. Using conservation of mass and assuming a constant sediment supply Qs0, and a uniform

grain size, they solved for the case where fan elevation reached steady state and found that   r 2  ½9 Qs ¼ Qs0 1  L where Qs was the sediment flux down the fan radius r. With zero-flux boundary conditions, they observed that the steadystate fan length L corresponding to a given subsidence rate ns was L¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Qs0 I 2 ns y ð1  lp Þ

½10

where I represents the fraction of time during which flow occurs (intermittency), y is the planform angle traversed by the fan, and lp is the sediment porosity. This result suggests that fans grow to a length that is dependent on the ratio of their sediment supply to subsidence rate, a result that might have implications for morphometric equations relating fan area to source basin area. By assuming relations for flow resistance and an excess shear stress sediment transport function, Parker and others (1998a) solved for equilibrium fan long profiles for unchannelized flow and examined the limiting case where the Shields’ stress was far in excess of the critical Shields’ stress for initiation of sediment motion. Assuming a Chezy resistance relation and a Meyer–Peter Mueller sediment transport relation, they found that fan slope S was S¼R

ar Qs as Qw

½11

where R is the submerged specific gravity of sediment, the a’s are coefficients in the relations between resistance and sediment transport functions, and Qw is the volumetric water flux. Under these conditions, slope is proportional to the ratio of sediment to water discharge, a finding consistent with earlier experimental work by Hooke and Rohrer (1979). The same relation held for channelized flow with high excess shear stresses. Equilibrium fan slopes declined in the cases where either Shields’ stresses were dropped to values closer to thresholds for sediment motion, or the exponent on Meyer–Peter Muller sediment transport equation was increased above 3/2. In field examples where these conditions are obtained, Parker and others (1998a) predict that channelized flow has the effect of reducing alluvial fan slopes. Oscillation between these conditions could lead to channel entrenchment. Recently, Stock and others (2007) attempted to test between two end-member theories for alluvial fan slopes, threshold versus supply (Figures 6(a) and 6(b)). They used down-fan measurements of grain size and hydraulic geometry data to estimate downfan thresholds for motion, and changes in bedload transport capacity. They found that median gravel diameters did not change along the upper 60–80% of fan channels and the threshold theory for grain size control of fan slope was not consistent with plots of eqn [3]. They tested Drew’s (1873) theory by estimating downfan changes in bedload transport capacity using a gravel transport equation

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

that Wilcock and Kenworthy (2002) modified after a surfacebased transport model proposed by Parker (1990). The expression was calibrated using measured bedload flux under a variety of flow conditions:   " #4:5 rs  1 gqbg r x w  Wg ¼ ½12  1:5 ¼ A 1   0:25 tb =trg Fsand tb rw

where Wg is a nondimensional unit gravel flux qbg, Fsand is the surface bed sand fraction, and the parameters A ( ¼ 115) and x ( ¼ 0.923) have values estimated in Wilcock and Kenworthy (2002). The quotient tb/trg is the nondimensional critical . shear stress tc for gravel entrainment, or trg  A further adjustment reduces trg as surface bed sand fraction increases using an expression proposed by Wilcock and Kenworthy (2002):    ¼ ðt Þ þ ðtrg Þ0  ðtrg Þ1 trg ½13 rg 1 exp½14Fsand   is the reference nondimensional stress for gravel at where trg  ) is this stress for a bed of 100% the threshold of motion, (trg 1 gravel and (trg)0 is the equivalent stress for a bed with negligible gravel. These transport models will match the observed fan long profiles, so long as the transport rate declines

exponentially downfan so that half of the bedload is deposited every 0.2–1.4 km. These calculations are consistent with Drew’s (1873) hypothesis that some fan long profiles are statements about the rate of deposition downfan. Although these calculations appear unwieldy, they represent the most evolved view of bedload transport, and their input parameters (grain size distributions, sand fraction of the bed, hydraulic geometry) represent the minimum amount of field data needed to estimate bedload transport with current understanding.

9.23.12

The Record of Hazards on Alluvial Fans

Alluvial (and some debris flow) fans offer low-relief development sites within steepland valleys (Figures 14(a) and 14(b)), and at the margin of mountain ranges (Figures 14(c) and 14(d)). Where flooding is sufficiently episodic, hazards may not be apparent to local government or to developers (Table 3). Landscape changes in the source catchments (e.g., deforestation, overgrazing, and fire) may increase the frequency and magnitude of destructive events. In the European Alps, a series of destructive floods on fans and elsewhere followed deforestation (e.g., Blanqui, 1843 and Surell, 1844, as summarized in Marsh, 1874). Damage and lost lives from these events drove the nineteenth-century programs to reforest

(a)

(c)

445

(b)

(d)

Figure 14 Humans in the alluvial fan environment. (a) Infrastructure damage from a high-intensity rainstorm led to flooding that damaged state and NPS infrastructure in Death Valley, CA, USA, 2004 (NPS photo credit). (b) Butter Creek at Cowlitz River, WA, USA, flood of February, 1996 that led to damaged homes. (c) Dunsmuir debris basin, San Gabriel Mountains, CA, USA. (d) Unprotected residential development expanding onto alluvial fans, Las Vegas area, NV, USA, 2009.

446

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

alluvial fan source catchments and to create engineered structures to retain floodwaters (e.g., Brown, 1880; Mougin, 1931). Early quantitative work by Surell (1844) on steep rivers and alluvial fans was perhaps a consequence of this hazard. In India, monsoonal flooding on the Kosi fan shifted channels westward over the period A.D. 1736–1964 (Gole and Chitale, 1966). Approximately 9700 km2 of sand deposition from these floods forced abandonment of many towns, and resulted in losses of property, livestock, and human life. In response, the government has leveed the river for over 150 miles, and built a gated structure to impound water and redirect it for irrigation. On 18 August 2008, floods breached one of the levees. The resulting inundation of the central regions of the fan led to thousands of deaths, and affected several million people (e.g., Sinha, 2009). In the new world, farms and settlements on fans along the Wasatch front in Utah experienced a series of destructive debris flows and floods during the nineteenth and twentieth centuries (e.g., Pack, 1923; Woolley, 1946). Residents of these fans argued that overgrazing in the source catchments had increased flooding. In response, these communities began to engineer a ‘barrier system of flood control’ in the 1920s (Winsor, 1938). Alluvial fan flooding damaged foothill communities of the Los Angeles basin, and began to be recognized as a threat in the wake of the 1916 floods (McGlashan and Ebert, 1918). In response, the Los Angeles Department of Public Works developed perhaps the most publicized system of catchbasins on alluvial fans (e.g., McPhee, 1989; Figure 14(c)). This system evolved as a response to episodic floods in storms of 1934, 1938 (Troxell, 1942), 1969 (Waanamen cited in NRC, 1996), and 1980 (Chin et al., 1991; NRC, 1996). As wildfires continue to burn source catchments for these fans, maintenance of these catch-basins remains the best protection for residents of steepland margins. Even with engineering resources, some events defy mitigation. The 1991 eruption of Mt Pinatubo in the Philippines triggered catastrophic sedimentation on the alluvial fans surrounding the volcano (see papers in Newhall and Punongbayan, 1996). Pyroclastic flows deposited on the flanks of Pinatubo eroded rapidly during heavy, monsoonal rainfalls following the eruption. Hyperconcentrated flows and debris flows composed of volcanic material (lahars) increased sediment supply to the fans surrounding the mountain, resulting in aggradation and flooding. Initial attempts to surround fan margins with levees did not account for continued high supply of sediment. As the leveed area infilled, it became a high point from which subsequent events inundated surrounding communities. Consequent floods and deposits forced the evacuation of tens of thousands of residents, and the abandonment of many communities. In some of the preceding examples, economic resources have been sufficient to build and maintain a level of engineering defense against flooding. Where communities develop on alluvial fans without these resources, episodic floods can cause natural disasters on par with earthquakes. Wieczorek and others (2002) and Larsen and Wieczorek (2006) describe alluvial fan flooding and debris flows in coastal Venezuela. Intense rainfalls of 15–16 December 1999 triggered flooding and debris flows that swept down onto the residences and commercial buildings of coastal towns, causing damages in

excess of US $2 billion and killing B15 000 people, B5% of the area’s population. Wieczorek and others (2002) mapped deposit depths on the Carballeda fan, one of the most severely impacted areas. They calculated B2 million m3 of sediment deposition by water and debris flows, one of the largest and most damaging recorded alluvial fan floods. Larsen and Wieczorek (2006) discuss structural and nonstructural measures to mitigate hazards from these events, including zoning and set-backs, street orientation, and the use of robust multistory buildings as shelters. As population pressure increases the value of alluvial fan lands, engineering solutions to fan hazards will be a primary recourse. Alluvial fan floods of 1976 overwhelmed flood control dikes and inundated the city of Palm Desert, CA, USA. Alluvial fan damage from these and other events in the western United States triggered a National Research Council review of the problem of estimating flood hazards on alluvial fans (NRC, 1996). Existing techniques of calculating the regulatory 100-year flood extent on alluvial fans are premised in part on work by Dawdy (1979). On the basis of intuition, he asserted that flood flow on alluvial fan channels approaches critical, and that dD/dW¼  0.005 governs the relation between critical flow depth D and width W. He reasoned that since fan channels must migrate across the fan over geologic time, each point on an alluvial fan contour has an equal probability of flooding. As the length of the contour increases with downfan distance, the resultant probability of flooding decreases. French (1987, 1992, 2001) reviews the resulting engineering methodologies to design for flooding hazards on fans and proposes some modifications. He observed a preferred direction of flow on many alluvial fans, down the medial axis. Given this, he pointed out that the probability of flooding along a contour was more likely a normal distribution about the medial axis, which results in a more rapid decline in flooding probability downfan. In the absence of detailed records of flooding, several workers have developed numerical models to predict flash flood hazards on fans. Mukhopadhyay and others (2003) used finite-difference approximations to the kinematic wave equations to simulate floods using rainfall and topographic inputs. To model alluvial fan floods, Pelletier and others (2005) coded a 2D raster model that solved the St Venant shallow water equations with a Manning’s equation friction law. They implemented the code on 3- to 5-m topography of two fans in Arizona, Tiger Wash (Pearthree et al., 2004) and Wild Burro Wash (Vincent et al., 2004), using boundary discharges corresponding to recurrence intervals of 10 and 100 years, and a maximum probable flood. They compared these model results to inundation and surficial geology maps developed by Arizona Geological Survey and US Geological Survey personnel (Pearthree et al., 2004; Vincent et al., 2004). They concluded that their flood simulations were a significant improvement over current FEMA maps because the model results agreed with both mapped floods and surficial geology. The California Geological Survey also has an ongoing effort to map the active portions of alluvial fans on southern California as a guide to the most likely areas of future flood hazards (e.g., Bedrossian et al., 2010). Flooding damage continues, both in arid regions and elsewhere (Figure 14). To provide protection, engineering

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

solutions will require answering a number of elemental research questions about alluvial fans and their catchments. To answer these questions, studies are needed to measure peak flows and subsequent fluxes of water and sediment for catchbasin capacity, to map and model floodways using surficial geology, and to quantify the frequency and magnitude of events recorded in historic accounts and the geologic record.

9.23.13 9.23.13.1

Discussion What Generalizations Can We Make?

The slopes, sediment sizes, and bedforms of traction-dominated alluvial fans span nearly the full range observed in fluvial systems, leading to the great variability of field results described above. This is no accident. Alluvial fans are the product of the progressive division of water and sediment downstream, primarily by traction transport. These processes of division remain obscure and have largely been studied only in laboratories to date. The result is a radiating, depositional ramp whose confined and unconfined flows transport material from catchments to bounding streams or subsiding basins. The sedimentology of the ramps depends strongly on source basin processes, which allow a wide range of grain sizes (Table 1) and bedforms. This wide range is reflected in the variability of grain sizes and bedforms that can be found on the sections on sedimentology. The long profile and degree of channelization appear to depend on the rate of bedload supply, the rate at which this supply is deposited downfan, the hydraulic geometry of transport system, and the downfan boundary conditions of subsidence rate and bounding streams and rivers. There is, consequently, great variability in the values found for morphometric relations between fan source area and fan area, and fan average slope versus source area (Table 2). These provide a useful scaling from which to put bounds on ancient source catchment properties, but it remains unclear how to connect them to a more mechanistic view of alluvial fans. Recent theoretical and experimental work (e.g., Whipple and Trayler, 1996; Parker et al., 1998a; Dade and Verdeyen, 2007) provide some guidance on the effect of basin boundary conditions on fan depositional area, and if these boundary conditions can be used to explain some of the variance in eqn [4], its utility would be enhanced. It is clear that traction-transport alluvial fan slopes vary across a wide range of values, from above 0.1 to below 0.01 (e.g., Figure 7; Saito and Oguchi, 2005). Slopes of traction-dominated fans tend to decrease from fanhead values that can be as steep as 0.15 or more (e.g., Harvey, 2005) to distal values less than 0.01, depending on whether the boundary condition is a stream or lake. The upper boundary of traction transport remains in doubt. The occurrence of lower slopes likely depends greatly on the boundary condition. Where high subsidence rates occur, such as in Death Valley, low slope sections of fans are short or absent (e.g., Saito and others, 2003; Stock and others, 2007) because these sections are consumed by the subsiding basin. The very few first-hand observations of transport indicate that on steeper alluvial fans, confined flows are supercritical

447

and can transport particles that exceed their depth (e.g., Rahn, 1967; Beaumont and Oberlander, 1971). With the limited number of studies on channel dimensions downfan (e.g., Vincent et al., 2004; Stock et al., 2007), the most that can be said now is that steep, channelized fans have hydraulic radii that tend to decrease from several meters at fanheads to several decimeters at distal margins (e.g., Vincent et al., 2004; Arzani, 2005; Stock et al., 2007). The deeper active channel widths also tend to decrease downfan to a point on channelized fans. A wide range of processes deposit sediment on steep fans. Granular debris flows appear to be mobile down to slopes of B0.05 (e.g., Figure 8). Their deposits occur as unsorted, unimbricated sheets of material which may be clast-supported. Where catchments produce debris flows that lack these frictional, granular fronts and possess substantial amounts of clay or silt, the resulting debris flows appear to be mobile down to slopes well below 0.01. These deposits appear to be sheets of un-imbricated materials supported by a matrix of silt, clay, or fine sand. Table 1 illustrates that grain sizes on alluvial fans range from boulders to sand. On steep alluvial fans above B0.05 slope, bed materials are largely cobbles, gravels, pebbles, and granules, with increasing amounts of sand downfan. Maximum clast sizes range from house-sized boulders to a few meters at fanheads, declining downfan to decimeters. Clast size is perhaps largely a statement about source catchment supply. Observations of particles rolling in shallow flows on fans (Beaumont and Oberlander, 1971; author at Pescadero) form part of a larger set of observations (e.g., Leopold and Miller, 1956; Fahnestock and Hauschild, 1962; Solari and Parker, 2000) that indicate that isolated large particles in steep channels are exceptionally mobile. Where decreases in maximum particle size prove enduring, they may be statements about the downfan decline in flow depth. It remains to be seen whether the grain size variation downfan reflects depositional processes or varying hydraulic geometry. Table 1 indicates that most recent attempts to find systematic downfan decreases in maximum clast size fail to do so. Although the fans that have been studied to date seem to lack strong downstream fining of bed material along much of their length, downstream fining is well documented in some perennial rivers (e.g., Ferguson et al., 1996; Rice, 1999). Perhaps the absence of sorting down some fans is a statement about the increased mobility of gravel in sand-rich environments. If so, fan channels with low sand supply or perennial flow that removes sand cover might exhibit stronger downfan fining because of size-selective transport. The occurrence of sand as matrix material, as channelized bedforms, and as bed cover seems to increase downfan on those fans (Stock et al., 2007) and fan deposits (e.g., Blair, 2000, 2003; Nichols, 2005) where it has been quantified. Bed percent cover seems to vary from values below 20% at steep fan heads to values in excess of 70% at distal margins. This decline is arguably a statement about the overbank deposition of coarser bed material. Where sand cover increases above values of B40–50%, open patches of sand occur, which likely increase the mobility of gravel substantially (the uncongested bed state of Ikeda and Iseya, 1988). Most fans studied to date do not appear to have systematic decreases in the size of fractions coarser than sand downfan, although there are a

448

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

minority that do (e.g., Blissenbach, 1952; Denny, 1965; Kesel, 1985), for reasons that remain obscure. Fans with perennial or intermittent channels (e.g., Kesel, 1985) likely have a tendency for coarser particle size distributions, if for no other reason than the absence of widespread sand on the bed. Where material is moved by traction transport, it tends to be deposited in lenticular sheets, often alternating between coarse and fine bed material (e.g., Figures 5(e)–5(h)). This sequence may be the most widely observed deposit on traction-dominated alluvial fans (e.g., Blair, 1999a, 2000; Arzani, 2005; Hartley and Mather, 2005). Recent studies (e.g., Blair, 1999a, 2000; Hartley and Mather, 2005) have interpreted these alternations as evidence for antidune washouts, with the coarser layers representing the washout phase. This is consistent with flume results, but there are alternate explanations that are also consistent with field evidence. Bars in the active washes of many alluvial fans tend to be armored (e.g., coarse bar tops of Figures 2(e) and 2(h), Arzani, 2005), with finer grained subsurface deposits. Preferential armoring of bar tops is not consistent with antidune-washout deposits, which would tend to destroy barforms. This alternation is consistent with the development of a surface armor from hydraulic sorting and decreases in sediment supply rate over the hydrograph. This process has been widely observed in streams (e.g., Parker and Klingeman, 1982; Parker and Sutherland, 1990; Lisle, 1995; Powell et al., 2001); the author watched such an armor layer develop on a bar during a 1998 flood on a small fan in the Pt Reyes area, CA, USA. The sheet form of these alternating coarse/fine layers described by other studies is consistent with thin, elongate bars that dip at the angle of the channel. Although transport of material into these bars may well have been under supercritical flow (e.g., Beaumont and Oberlander, 1971), their sedimentology does not require this condition. The coevolution of relative and absolute age dating techniques in the American southwest and elsewhere has led to detailed sedimentation histories paired with mapping. Relative age dating tools allow regional mapping efforts using pedogenesis, and the geochronology ties these local or regional units to an absolute timescale. A regional model has evolved that ties late Pleistocene and Holocene depositional episodes to episodic, high-intensity storms that erode hillslopes with reduced vegetation cover (e.g., Wells et al., 1987; Miller et al., 2001, 2010). This model has been proposed for a number of other areas, including Spain (e.g., Harvey, 1990) and Australia (e.g., Nott et al., 2001). The same mapping and dating effort has led to the understanding that active flow along many alluvial fans is channelized, with the implication that flood hazard models should start from surficial geologic maps (e.g., Pelletier et al., 2005). With the advent of LiDAR topography as a basis for mapping and flow modeling, the combination of surficial mapping with hydraulic modeling promises a new era of flood hazard models. It is not evident from studies of either modern or ancient deposits that alluvial fans have any depositional processes that uniquely identify them in the geologic record. Hydraulic geometry values (eqns [6] and [7]) reported by Vincent and others (2004), and bedforms observed in many alluvial fan channels (e.g., Figure 5) lead one to the conclusion that transport processes in traction-dominated alluvial fan

channels are much the same as other fluvial systems. In the absence of radiating paleocurrent or paleochannel data, the deposits that characterize modern alluvial fans can be found in other depositional environments as well, including confined valleys. We are not yet at the level of modern understanding that we can uniquely interpret the ancient record in terms of sediment supply, tectonism, or climate. As a consequence, interpreting the ancient record of fans remains an exciting subject for debate and exploration. Much of the experimental work has been directed at three problems: (1) controls on fan long profiles, (2) conditions for incised or channelized fans, and (3) role of channelization on fan dynamics. A number of experiments have now found that increasing sediment supply rates with a constant water discharge tends to steepen fan slopes (e.g., Hooke, 1968; Hooke and Rohrer, 1979; Whipple et al., 1998). This result is consistent with the excess shear stress theories of bedload sediment transport, which predict aggradation to a steeper slope to transport an increased sediment supply (e.g., eqns [11] or [12]). In unimodal fine sediments, coarser grain sizes at the same discharge, or debris-flow deposition, also tend to lead to steeper fan slopes (e.g., Hooke and Rohrer, 1979; Schumm and others, 1987). Experimental work has demonstrated that both on aggrading, poorly channelized fans, and on channelized fans, the alternation of flow expansion with flow concentration leads to a predictable sequence of deposition and channelization (e.g., Figure 12). This robust result has been observed in nearly every fan experiment and has also been observed in the field (Field, 2001). It seems to represent a characteristic of sediment transport in these poorly confined systems. Experimental work has also repeatedly shown that boundary conditions matter greatly to fan dimensions and to fan aggradation rates. For instance, episodic trenching at fanheads has been shown to occur with fluctuations in sediment supply (e.g., Schumm and others, 1987; Davies and Korup, 2007). Trenching, however, tends to occur in all traction–transport experiments, even in the absence of fluctuations of sediment supply. Results from compilations of experimental and field work by Jerolmack and Mohrig (2007) and Nicholas and others (2009) suggest that avulsion and multithread channels tend to occur at relatively higher aggradation rates, scaled by channel depth. However, once the experimental fan slope is sufficient to transport the supplied sediment to the lateral boundary of the experiment, channelization predominates as on-fan deposition decreases (e.g., Nicholas et al., 2009). This behavior follows from theory developed by Sheets and others (2002) and Jerolmack and Mohrig (2007) showing that lateral migration rate scales with the ratio of average channel depth to in-channel aggradation rate. As aggradation rate in the channel declines, lateral migration rates slow and the channel system locks in to its inherited position. Hence, on experimental fans with fixed boundary conditions (i.e., no subsidence), flow evolves to a channelized pattern. By analogy, where alluvial fans prograde out to rivers, they might tend toward incised, single-thread channels. The presence of perennial rivers in temperate climates might suggest that singlethread, incised alluvial fans would tend to predominate in temperate climates over arid climates with less frequent main channel transport.

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

At this time, small-scale experiments cannot practically be scaled in a rigorous hydrodynamic sense; translating these results to the field requires caution. However, a growing body of work shows that experiments can be very effective as scale models for natural systems when there is a strong separation of scales between sediment transport and the pattern of interest (e.g., Paola et al., 2009). For instance, it appears that the time and space scales of autogenic channel switching and lobe building are dictated principally by conservation of mass (e.g., ratio of in-channel aggradation rate to average channel depth) and the presence of a transport threshold (e.g., Reitz et al., 2010).

9.23.13.2

Needs for the Future

There is an enduring need to understand the deposits that result from varying sediment concentrations and flow states. Alluvial fan flows are sufficiently unpredictable that observers are rarely in-place to make detailed observations on flow properties and resulting deposits. This lack of event data remains an important barrier to further progress on the hydraulics and sediment transport formula for alluvial fans. As a consequence, we have few type-sections where a one-to-one correspondence exists between an observed flow property (e.g., hyperconcentration, or Fr) and the resulting deposit. In the absence of such type-sections, a wide number of interpretations exist in the literature, often presented as facts. For instance, our ability to discriminate between inertial and traction deposits and between hyperconcentrated and debris flow deposits is likely overstated at present (e.g., Figures 2(f) and 3(e)). An experimental facility with the capability of simulating many of these flow and sediment transport conditions and allowing subsequent deposition could help solve this issue. Research on debris flow dynamics has made substantial breakthroughs through the use of this kind of facility (see Iverson, 1997; Iverson and Denlinger, 2001; Iverson and Vallance, 2001). A related issue is that despite over 130 years of recognition and study, bedload transport equations have not been verified for use on steep alluvial fans. Although Stock and others (2007) used flume data to validate one possible transport equation, it is not clear how generalizable these results are. This is an important gap because of the role that inertia may play on particle motion in steep, supercritical flow conditions that likely occur on many arid region fans with substantial sand fractions on their bed. This problem could be solved in existing flume facilities, with the recognition that bed texture should approximate those of field studies. Revisiting some of the type fan localities with a modern understanding of sediment transport and processes might clarify what some of the older studies mean. Surprisingly little description of the sedimentology of active bedforms in alluvial fan channels exists, including armoring, bar dimensions, and internal architecture. These kind of data would be immensely useful in the debate about how to interpret the widely observed alternations of coarse and fine beds and bedforms observed in older fan deposits. For example, there is no fan equivalent to the bedform classifications of Montgomery and Buffington (1997) for fluvial systems. These classifications would need to be amended, at the very least for steep arid fans

449

where alternate bars tend to give way to plane bed at much steeper slopes (e.g., above 0.05 slope). No theory for the effect of exceptional grain sizes (e.g., maximum clast size) on channel reach slope exists. Equations [2] and [3] illustrate that in order to assess the role of grain size fining on fan slope in the context of the current understanding of sediment transport, grain size data must be collected as distributions (d50) at the same spot that some metric of reach average flow depth and slope are collected. With the exception of Stock and others (2007), none of the nearly 75 years of data collection represented in Table 1 meet these criteria, limiting our ability to test models for fan evolution. The evolution of newer geochronologic techniques like OSL and cosmogenic radionuclides has allowed us begin to assign absolute ages to many of the deposits that have been mapped using relative chronologies. The use of cosmogenic radionuclides to estimate catchment averaged erosion rates could be used to estimate the long-term supply rates of sediment to fans (e.g., Cyr et al., 2010). In combination with fan dimensions, these data could be used to test theories for fan extent (eqn [11]) proposed by Parker and others (1998a). Cosmogenic radionuclides can also be used to estimate paleoerosion rates where deposits can be independently dated. By choosing fan source areas that are tectonically inactive, variations in these paleoerosion rates through time may provide a damped view of the effects of climate change on hillslope sediment supply. The assumption that the fluctuation of hillslope erosion rates with climate drives fan deposition remains largely untested. Work on this problem is ongoing at the Globe fan in the Providence Mountains, California (e.g., Cyr et al., 2010). Newer experimental and theoretical work presents hypotheses that field studies could begin to test. This work emphasizes the importance of boundary conditions like aggradation rate, basin subsidence rate, and presence or absence of distal streams on channelization and long-profile development of alluvial fans. For instance, Parker and others (1998a) proposed that fan length is a function of the ratio of volumetric sediment supply to subsidence rate (eqn [10]). We now have sufficient tools (e.g., cosmogenic radionuclides) that we could begin to test this hypothesis in field sites like Death Valley, where the variability in high subsidence rates probably affects both long profile and length substantially. Experiments on the conditions for lateral migration and avulsion (Sheets et al., 2002; Jerolmack and Mohrig, 2007; Reitz and others, 2010) provide field workers with a theory for these rates that depends on local sediment supply rates and cross-sectional area. This theory could be tested by estimating long-term (cosmogenic radionuclides) or short-term (bedload traps) sediment supply rates and lateral migration rates (data from active alluvial fan channels or repeat imagery). Experimental results from Nicholas and others (2009) suggest that fans that are connected to streams that can transport their supplied load should have a tendency to be channelized. This proposition could reasonably be tested using imagery that is now widely available. The advent of LiDAR has revolutionized our ability to resolve surface topography (Figure 15), including slopes (Figure 16) and channels (Figure 17). Staley and Wasklewicz (2006) and Frankel and Dolan (2007) used LiDAR

450

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

1000

Meters

Figure 15 Illustration of the improvement in topographic resolution between 10-m USGS DEM, and 1-m LiDAR DEM of Globe fan, Providence Mts, CA, USA. Note the absence of contour-line ghosts in the LiDAR, and its ability to portray anthropogenic features, like the road and railroad. Although we will increasingly be able to extract accurate slope and channel dimensions from such data, particularly when merged with imagery, it is not yet clear what hypotheses we will test.

Figure 16 Hill-shaded 1-m LiDAR for Globe fan, Mojave. Image illustrates the dramatic contrast between the slopes of transport processes on the fan and in valleys, and erosional process on the hillslope sediment source area. Contours shown every 10 m. Note the decline in surface slopes from 8% to 12% at fanheads to less than 2% at the distal fan.

Waters Divided: A History of Alluvial Fan Research and a View of Its Future

451

Providence Mts, CA, 1-m DEM

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Figure 17 (a) Valley network extracted from a regridded 2-m LiDAR using a threshold channel area. Valley slopes decline systematically toward the 1–2% values of the axial wash, but the railroad berm significantly deflects the network. (b) Image showing the 1-m valley network (blue) over the actual channel network. Note truck for scale. The 1-m LiDAR data resolve valleys with too much sinuosity, effectively biasing channel slopes downward. Photography courtesy of Sarah Robinson.

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topography to examine to surface morphology of fans in Death Valley, and proposed that local relief could be used to discriminate between different depositional process. Stock and others (2007) used LiDAR to characterize the slopes of abandoned surfaces, arguing that their higher curvature was consistent with B8% increase in past bedload supply. Miller and others (2009) have used LiDAR topography from Globe and the Avawatz Mts to help map older fan surfaces. As of this writing, however, it remains to be seen how we will use LiDAR to test new hypotheses for alluvial fan evolution. Perhaps its most utilitarian application will be a base upon which surficial geology and hydraulic modeling can be combined to yield a new generation of flood hazard maps. Perhaps some of the most active alluvial fans of the future will be found at the margins of the Earth’s melting valley glaciers and ice sheets. Work on these fans could provide field tests of the insights evolving out of the laboratory, and from past fieldwork on less active fans. For instance, use of ADCP and bedload traps in these settings could help clarify how distributary networks divide up their water and sediment, the fundamental process that separates fans from other fluvial networks. Such work could clarify the mechanics of overbank deposition of bedload, and provide a theory for long profiles of alluvial fans that depend on the rate of this process down fan. Repeat ground-based and airborne LiDAR and imagery in such settings could clarify network changes. In the 130-plus years following Drew’s recognition of alluvial fans, we have just begun to make progress on understanding the dynamics of how water and sediment are divided down these distributary systems. Progress has been slow in part because flow in these systems is so intermittent that we rarely have the opportunity to measure it directly. As a consequence, our understanding of the processes that divide up water and sediment downfans remains rudimentary. A large body of experimental work has arisen to address this gap in field observations, and this work has begun to give us testable hypotheses for fieldwork. New technologies promise to take some of the problems we have previously resigned to these laboratory settings, and liberate us to pursue them on alluvial fans in the field. Such progress would be timely. The hazards from flooding and sedimentation on alluvial fans remain a threat to both developed and developing countries in a warming world.

Acknowledgments I would like to thank Ellen Wohl for her patience and wise guidance, without which this chapter would not have occurred. I benefited from discussions and reviews by Doug Jerolmack, whose insights on experimental work greatly improved that section. My colleagues at the USGS, Matt Larsen, Andy Cyr, David Bedford, Jane Ciener, and David Miller corrected many of my errors with their kind reviews. Those that remain are my own fault. I would like to thank the USGS Global Climate Change Program and Landslide Hazards Program for providing salary to write this chapter, and the USGS Mendenhall Program for providing the funding for my work on alluvial fans.

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Biographical Sketch Dr. Jonathan Stock holds degrees from U.C. Santa Cruz, University of Washington, and University of California, Berkeley. He uses field observations, mapping, monitoring, and modeling to understand the evolution of steep landscapes by geomorphic processes. His previous work has been directed at constraining the paleorelief of mountain ranges using detrital minerals, numerical calibration of long-term river incision laws, and the mechanics and topographic signature of debris-flow incision. He is currently working on the mechanics of erosion on Pacific high islands, near real-time monitoring and historical mapping of shallow landslides and debris flows, the geologic record of large storms, and the evolution of alluvial fans. He is a research geologist at the U.S. Geological Survey in Menlo Park, CA, USA.