Stream-terrace genesis: implications for soil development

Geomorphology, 3 (1990) 351-367 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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Stream-terrace genesis: implications for soil development W i l l i a m B. B u l l

GeosciencesDepartment, Universityof Arizona, Tucson, AZ 85721 (U.S.A.) (Received November 15, 1989; accepted after revision April 19, 1990)

ABSTRACT Bull, W.B., 1990. Stream-terrace genesis: implications for soil development. In: P.L.K. Knuepfer and L.D. McFadden (Editor), Soils and Landscape Evolution. Geomorphology, 3:351-367. Genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting, base level of erosion, complex response, threshold of critical power, diachronous and synchronous response times, and static and dynamic equilibrium. Climatic and tectonic stream terraces are major terraces below which flights of minor complex-response degradation terraces can form. These three types of terraces can be summarized by describing a downcutting-aggradation-renewed downcutting sequence for streams with gravelly bedload. By tectonically induced downcutting, streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion. Lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream-terrace landform. Aggradation events record brief reversals of long-term tectonically induced downcutting because they raise active channels. They may be considered as major (the result of climatic perturbations) or minor (the result of complex-response model types of perturbations). Climatically controlled aggradation followed by degradation leaves an aggradation surface; this type of fill-terrace tread is the fundamental climatic stream-terrace landform. Aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces. Raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form; first in alluvial fill and then in underlying bedrock along tectonically active streams. These are complex-response terraces because they result from interactions of dependent variables within a given fluvial system. Pauses in degradation to a new base level of erosion, and/or minor episodes of backfilling, lead to formation of complex-response fill-cut and strath, or of fill terraces. Fill-cut terraces are formed in alluvium; they are complex-response terraces because they are higher than the base level of erosion. Good exposures and dating are needed to distinguish static equilibrium complex-response minor strath terraces from dynamic equilibrium tectonic (major) straths. Strath terraces may be regarded as complex-response terraces where degradation rates between times of terrace-tread formation exceed the long-term uplift rate for the reach based on ages and positions of tectonic terraces. Late Quaternary global climatic changes control aggradation events and even the times of cutting of major (tectonic) straths, because the base level of erosion can not be attained during times of climatically driven aggradation-degradation events.

Most terrace soils form on treads of climatic and complex-response terraces. Aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence. Each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads. Multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces. Pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors. Climatic terraces of adjacent watersheds also should be roughly synchronous (correlatable) - variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds. Such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times, and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations. Soils on climatic terraces of distant watersheds will not be synchronous if their respective aggradation events occur during fun-glacial times and interglacial times. Soils on some complex-response terraces may be diachronous within a given fluvial system, and typically are diachronous between watersheds.

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Introduction The subjects of soils science and fluvial geomorphology meet on flights of stream terraces whose treads rise like stair steps above active channels. Soil chronosequences are crucial to the geomorphologist who seeks to understand times and causes of terrace-tread formation. Terrace treads are landforms with temporal significance that document important transitions in types or rates of fluvial process. For the soil scientist terrace treads are much more than gently sloping planar surfaces that provide stable sites for pedogenesis and approximately uniform gravelly parent material. Ages and heights of terrace soils above active channels are in large part controlled by regional and local rates of uplift, and by strengths of late Quaternary climatic perturbations. Streamchannel downcutting converts active channels and floodplains into terrace treads, thus initiating sites with minimal erosion or deposition where soils of great age may form. In studies of soil-landscape interrelations, one should be curious as to whether terrace-tread incision is cuthe result of uplift, climatic changes, or is due to internal adjustments within the fluvial system. Whether a given soil profile remains as a relict surface or becomes buried is largely a matter of magnitudes of climate-change induced aggradation and of rates of tectonically induced downcutting. Soil-landscape interrelations are best understood when we decipher the genesis of both stream terraces and their soil profiles. Climatic and tectonic controls on streamterrace formation were de-emphasized with the introduction of the highly useful complex-response model for stream-terrace formation (Schumm, 1973, 1977; S c h u m m and Parker, 1973 ). This new approach has led to intriguing statements such as for Douglas Creek in northwestern Colorado. "It is not possible to

WILLIAM B. BULL

correlate the terraces purely on the basis of elevation nor to relate the terrace remnants throughout the valley to specific stimuli such as climatic variations or base-level change" y d ( S c h u m m et al., 1987, p. 124). One may wonder "do tectonic and climatic terraces even exist?, and is it possible to separate them from complex-response terraces?" All three types of terraces can be recognized along many streams. Climatic aggradation surfaces and tectonic straths are major terraces below which flights of minorcomplex-response degradation (fillcut and strath ) and fill terraces can form. My intent is to summarize conceptual models for the formation of tectonic, climatic, and complex-response stream terraces, and to provide diagnostic criteria for field identification of these three fundamental classes of terraces. Discussion will focus on aggradation and degradation events, which represent departures from equilibrium conditions in fluvial systems, and will conclude that formation of complex-response terraces is initiated by climatic and tectonic perturbations. Of course stream terraces from in other ways such as impoundment of alluvium behind landslide dams, stream piracy in certain piedmont settings, in response to late Quaternary sea-level changes, and responses to a great variety of h u m a n impacts on the environment. Such special situations are beyond the scope of this paper, which concentrates on c o m m o n causes of Quaternary terrace formation in arid and h u m i d regions for fluvial systems with sources of gravelly bedload. Thus for many earth scientists, this general essay is about the types of stream terraces that your soils are forming on. Aggradation and degradation are the processes by which streams depart from equilibrium conditions and thereby initiate processes that lead to the creation of stream terraces. Some of the complexities of stream terraces can be understood through application of the con-

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STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

cepts of tectonically induced downcutting, base level of erosion, complex response, threshold of critical power, diachronous and synchronous response times, and static and dynamic equilibrium. With neither climatic or tectonic perturbations streams would downcut to a single base level of erosion and few, if any, terraces would form. Changes in climatic and tectonic controls make studies of streams far more interesting. Changes in these independent variables are responsible for paired (having continuity along a valley) climatic and tectonic stream terraces, whose formation sets the stage for degradation during which flights of paired or unpaired complex-response terraces are formed. Conceptual models for studies of stream terraces

This section provides a brief but essential summary of background information about conceptual models and types of stream equilibrium and terraces.

ThreshoM of critical power in streams Formation of stream terraces involves changes in the behavior of a fluvial system. Modes of degradation and aggradation for streams are separated by the threshold of critical power (Bull, 1979, in press), defined as a ratio equal to one where the numerator (stream power) consists of those variables that if increased favor degradation, and the denominator (resisting power) consists of those variables that if increased favor aggradation: stream power (driving factors ) 1.0 resisting power (resisting factors ) Aggradation of bedload occurs when resisting power exceeds stream power. Thus the deposits of typical fill terraces consist of bedload materials, as the suspended load is washed downstream. Both tectonic and climatic changes may in-

fluence the components of the threshold of critical power. Uplift tends to increase slopes, and cooler and/or wetter climates generally increase stream discharge; either change will increase stream power. Changes in the amount and size of sediment yielded from hillslopes affects resisting power, which also varies with changes in hydraulic roughness.

Tectonically induced downcutting Increase in fluvial-landscape relief emanates from rising geologic structures through the process of tectonically induced downcutting by streams. Each stream acts as a connecting link between all parts of a watershed. Amounts and rates of tectonicaUy induced downcutting are functions of local and regional uplift rates, stream gradient and discharge, and resistance of earth materials to degradation during peak flows. Intervals of net deposition along streams - aggradation events - record departures from the long-term trend of tectonically induced downcutting. Aggradation events separate intervals of tectonicaUy induced downcutting and thus create opportunities to describe much more detail about a history of stream behavior.

Longitudinal profiles and the base level of erosion The base level of erosion concept integrates the system, equilibrium, and base-level concepts (Figs. 1, 3, and 5 ). It is the equilibrium (graded) longitudinal profile below which a stream can not degrade, and at which neither net erosion nor deposition occurs (Powell, 1875; Barrell, 1917). The process of tectonically induced downcutting continues until a stream has the minimum gradient needed to transport its sediment load with the prevailing streamflow characteristics. This concept pertains to all reaches of a fluvial drainage net. A reach of a stream at the base level of erosion has attained a time-independent configuration of its longitudinal profile that is maintained as

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long as the controlling variables do not change. This highly useful concept considers a longitudinal profile as being an infinite spatial sequence of adjacent base levels (Gilbert, 1879 ).Changes in any of the stream power and resisting power variables may alter the gradient, longitudinal profile, a n d / o r altitude of a particular reach, thereby causing adjacent reaches to make concurrent adjustments. In a temporal sense the base level of erosion may be re-established at multiple positions within the landscape. Altitudinal separations of these positions may be on the order of 10 to 100 m in rapidly rising terrains where climatic perturbations cause aggradation that prevents streams from accomplishing net downcutting much of the time. Conversely, in slowly rising reaches subject to minimal climatic effects, altitudinal separations of different positions of the base level of erosion may be less than 1 m as the reach cuts down at the same rate that it is being raised. Aggradation events in tectonically stable areas may cause temporary departures from the base level of erosion, but subsequent degradation events return the stream to the same level (Haynes, 1987; Waters, 1988). Haynes' welldated Curry Draw site (Fig. 1 ) clearly shows three episodes of Holocene stream-channel downcutting to a c o m m o n base level of erosion, which is the same as the level of a broad

WILLIAM B. BULL

late Pleistocene valley floor. The Qmi, Qso, and Qco and underlying units were deposited before the Holocene. Erosion between 13 and 12 ka* formed a broad shallow channel that was backfilled with the Graveyard sand (Qgr) by 10 ka. The 11 _+0.1 ka basal contact of the Qcl bed is present at many other sites in the southwestern North America; thus is appears to be a stratigraphic unit with climatic significance. In contrast the uppermost Qtv unit is a gravelly sand produced by discontinuous initiation of modern arroyo cutting in a reach 1 km upstream; it is an example of a complex-response fill terrace. Similar depths of paleochannels imply similarity of net effects of interacting watershed variables for different episodes of channel entrenchment. Soil profiles on former terrace treads tend to be buried in such tectonically stable sites, and the Quaternary earth scientist works mainly with a stratigraphic record of climatic changes and complex responses rather than with flights of stream terraces. The opposite effect occurs at sites where uplift is rapid (Figs. 3,5 ) and induces streams to degrade over the long term, because increase of relief and stream gradient increase the *l ky=1000 years; 1 k a = l ky before present. (North American Commission on Stratigraphic Nomenclature, 1983, Article 13).

/

Qtv

Fig. 1. General cross-section of the stratigraphy at Curry Draw in southeastern Arizona showing multiple eposides of arroyo downcuttingand backfillingsince the termination of deposition of the Coro Marl (Qco) at 13 ka. Note the similar depths of modern and Holocenearroyos. (From Haynes, 1987.)

STREAM-TERRACE GENESIS: IMPLICATIONSFOR SOIL DEVELOPMENT

power of a stream to do work. Treads of late Quaternary stream terraces may differ in altitude by > 50 m where uplift is rapid, but strength of soil-profile development may be similar because terrace formation is closely spaced in time. In contrast, soils on tectonically inactive pedimented terrains may differ in age > 100 ky even though they may be separated in altitude by < 5 m.

Types of equilibrium in streams Before discussing types of equilibrium in streams, we need to classify stream terraces. Terrace treads record former levels of streams and may be classed as fill, fill-cut, and strath (rock-cut) terraces (Howard, 1959; Ritter, 1986); each type of terrace may be paired or unpaired. Unpaired terraces (Davis, 1902) typically occur on the insides of meander bends of a stream that is steadily downcutting, but may even be isolated straths where a stream impinges on bedrock hills. Paired terraces are remnants of a formerly continuous level of a stream, and as such may be regarded as synchronous or diachronous time lines. A paired terrace commonly occurs on both sides of a valley, but more importantly it has continuity along the valley. A fill terrace is formed by aggradation and subsequent channel incision into the alluvium that leaves remnants of the former valley floor as the tread of a paired or unpaired fill terrace. The tread represents a time of crossing of the critical-power threshold as the mode of stream operation changes from aggradation (or equilibrium) to degradation. The base-level-of-erosion concept describes reaches of streams that clearly have achieved one of two types of equilibrium - static equilibrium and type 1 dynamic equilibrium (Bull, in press). Static equilibrium is characterized by a lack of either aggradation or degradation of the streambed (the graded-stream definition of Leopold and Bull, 1979 ). Both types of equilibrium, and the crossing of the erosional-

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depositional threshold in streams - the threshold of critical power - can be recognized by the presence of distinctive landsforms. Periods of static equilibrium are common during intermittent degradation of gravelly valley fills. Fill-cut terraces form by lateral stream erosion into alluvium during brief periods of static equilibrium that are followed by renewed stream-channel downcutting that isolates the terrace tread. Fill-cut terraces are genetically the same as minor straths. Fill-cut surfaces are beveled in alluvium, and strath surfaces are beveled in bedrock, where bedrock is best considered as either rock or old alluvium (gravels deposited during preceding aggradation events). The concept of dynamic equilibrium, which Hack ( 1960, 1965 ) applied to mountains, can be applied to reaches of streams where two categories may be defined in terms of relative attainment of the base level of erosion (Bull, in press). Type 1 dynamic equilibrium is present when rates of tectonically induced downcutting equal rates of uplift, thus allowing the longitudinal profile of the stream to attain a succession of base levels of erosion in a tectonically active reach. Diagnostic landforms include major straths and a valley floor that is sufficiently wide for the formation and preservation of strath terraces. Type 2 dynamic equilibrium is present in streams that have a strong tendency towards the base level of erosion but may not have attained it. Diagnostic landforms include narrow valley floors without strath terraces, and longitudinal profiles that plot as concave lines on arithmetic graphs and as straight lines on semi-logarithmic graphs. Type 2 reaches are regarded as being close to the base level of erosion because of straight semi-logarithmic plots of longitudinal profiles and stream-gradient indices Hack ( 1973, 1982 ) that are similar to those for type l reaches (Fig. 2 ). Disequilibrium is characteristic of degrading streams upstream from most type 2 reaches, or in reaches affected by active faulting (Fig.

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W I L L | A M B. B U L L

20

"D 40 E o 60

~'o,,,x~- 280 320

~6

/-260

"~o.~2%20

~ 80

/-420 "~510 1:~790

o n

100

10

100 Percent of stream length from headwaters

Fig. 2. Dimensionless semi-logarithmic profile of the Right Branch of the Charwell River, New Zealand based on a map with a 33 m contour interval. Numbers are streamgradient indices in gradient meters, which are similar for reaches characterized by type 1 dynamic equilibrium (broad strath and stream terraces) and type 2 dynamic equilibrium (rapid downcutting along a narrow valley floor in rugged mountains). The disequilibrium reach reflects lack of adjustment to late Quaternary surface ruptures along the Hope fault, in part because deposition of alluvium kept nickpoints in bedrock from being exposed to streamflow between 26 _+2 and 8 + 3 ka. (Modified from Bull and Knuepfer, 1987, fig. 4c.)

2). By definition, such streams have not attained time-independent longitudinal profiles of the base level of erosion. Like degrading hillslopes, these downcutting streams also reflect orderly interactions between variables (Bull, 1975) which may represent conditions far removed from equilibrium situations described by the base level of erosion. Diagnostic landforms include highly convex valley sideslopes in V-shaped canyons, and longitudinal profiles that plot as convex lines on arithmetic and semi-logarithmic graphs. Erosional widening of valley floors in bedrock is the main fluvial process during periods of static (minor strath formation) or type 1 dynamic equilibrium (major strath formation ). Strath and fill-cut terraces differ from fill terraces in that only thin layers of stream gravels remain on surfaces of fluvial truncation. These thin gravels may be regarded as lag deposits or the cutting tools that would have been

entrained during the next flood discharge if the active channel had remained at the same level. Thick alluvial fills of the next climatically induced aggradation event may be deposited on major strath surfaces formed during times of type 1 dynamic equilibrium. Paired major strath terraces are common along reaches of streams that are being tectonically elevated. A direct analogy can be made with flights of marine terraces. Shore platforms at altitudes that exceed the altitude of the +6_+ 2m sea-level highstand attained at about 125 ka are clear evidence of a rising landmass between times of sea-level highstands. Major strath surfaces that are above the strath beneath an active channel are clear evidence of tectonically induced downcutting. The advances in dating of landforms, and of topographic measurement and analysis, that have been made during the past few decades, together with a better understanding of rates of geomorphic processes, permits application of the above classification of equilibrium and disequilibrium conditions. Where workers prefer only a single term for the graded stream, I suggest "equilibrium" be used in a general sense to identify reaches of streams that clearly have attained the base level of erosion - either static equilibrium or type 1 dynamic equilibrium. Genesis of stream terraces Tectonic terraces

Streams may be portrayed as passing through a simple sequence of stages in response to tectonic and isostatic, or to climatic perturbations. Where stream power, uplift rates, and erodibility of materials permit, streams degrade to achieve and maintain type 1 dynamic equilibrium. Lateral erosion of bedrock is the dominant process and may continue long enough to create beveled surfaces beneath active channels; with sufficient time pediments may form. Such major straths are the funda-

STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

mental tectonic stream-terrace landform. In contrast, minor straths and fill-cut surfaces form as a result of complex response model types of perturbations. A stream that maintains type 1 dynamic equilibrium in a rising reach attains many base levels of erosion. Ideally, no terraces would form, but in the real world slight variations in short term uplift and stream-channel downcutting rates commonly result in several low ( 1 to 4 m high) strath terraces. These low terraces may be considered as tectonic terraces where heights and ages of strath surfaces indicate a mean rate of tectonically induced downcutting that equals the long term uplift rate. Conditions at each site will dictate whether such terraces should be grouped as a single irregular major strath, or whether they should be split out as separate small tectonic terraces. Beveling of the surface continues until climatic or tectonic perturbations upset the prevailing type 1 dynamic equilibrium conditions. For example, should local faulting or folding substantially raise and disrupt the equilibrium of a reach, accelerated streamchannel downcutting will continue upstream from the base-level fall until a new base level of erosion is established. The former base level of erosion is preserved as a major strath terrace whose remnants are mantled with a thin blanket of gravelly cutting tools in systems with sources of coarse bedload. Initiation of a climate-change induced aggradation event may also terminate major strath formation, but in this case the tectonic landform of a major strath surface may be buried beneath thick alluvium. Climatic terraces

Aggradation events represent brief reversals of long term trends of tectonically induced downcutting; they may be considered as major (the result of climatic perturbations) or minor (the result of complex-response model types of perturbations). Aggradation generally is caused by inability of a stream to transport all of its

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bedload due to decrease in stream discharge, a n d / o r increase in amount and size ofbedload (Bull, in press). Thicknesses of different ages of valley-floor alluvium in rising reaches of streams provide relative measures of the strengths of successive climatic perturbations, or for the same major aggradation event when comparisons are made between adjacent drainage basins. Such geomorphic responses to climatic changes are modulated by the sensitivity of the rock types that underlie the source watersheds (Bull, in press, ch. 3 ).Aggradation of climate-change induced deposition of valley-floor fills typically accelerates and then decelerates before ceasing when the threshold of critical power is crossed (Bull and Knuepfer, 1987, fig. 6). Initiation of degradation leaves an aggradation surface; this type of fill-terrace tread is the fundamental climatic stream-terrace landform. Aggradation surfaces may be buried by subsequent eposides of deposition (Fig. 1 ) unless intervening tectonically induced downcutting lowers the active channel sufficiently that younger aggradation surfaces form below older surfaces (Fig. 3 ). Flights of such fill terraces commonly have parallel treads. Such parallelism of longitudinal profiles is suggestive of similar hydraulic conditions during times of maximum valley-floor aggradation. The combinations of interacting variables may have been different, but each parallel tread records a similar net effect of behavior at the times of crossing of the criticalpower threshold. Late Quaternary climatic changes have resulted in both backfilling of valleys with gravelly deposits and with subsequent degradation of the valley fill. Although the overall process of degradation to a new base level of erosion may be considered as a situation of resisting power exceeding stream power, pauses or even brief reversals in the pattern of stream-channel downcutting may occur. Such minor aggradation events generally are the result of complex responses of drainage systems.

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WILLIAM B. BULL

Fig. 3. Diagrammatic sketch of tectonic, climatic, and complex-response terraces in a setting of 1 m / k y regional uplift and strong climatically induced aggradation events that temporarily reverse the trend of tectonically induced downcutting. Late Quaternary geomorphic responses to climatic changes and tectonic perturbations, together with complex-response adjustments, are recorded as stream terraces (instead of as a stratigraphic record) because of the large vertical space provided by long-term tectonic and episodic depositional elevation of the stream channel. Each tectonic strath forms during a period of dynamic equilibrium after tectonically induced downcutting has caused the stream to degrade to a new base level of erosion, each aggradation surface is the tread of a major fill terrace whose tread records the end of a climatically induced aggradation event. Complex responses result from internal adjustments within the fluvial system. They are preserved here as flights of minor strath, fill-cut, and fill terraces that record pauses in degradation form the level of an aggradation surface to the subsequent tectonic strath terrace.

Complex-response terraces Climatic and tectonic stream terraces are the primary responses to changes in independent variables of fluvial systems of sufficient magnitude to cause thresholds between modes of operation to be crossed. These major terraces provide the framework on which complex-response, or minor terraces form. These complex-response terraces may be young or old, low or high, and limited or extensive. They result from internal adjustments (Schumm, 1973, 1977; Schumm and Parker, t973) as dependent variables interact in fluvial systems that have been subjected to climate and tectonic perturbations. Flights of complex-response terraces may be regarded as secondary becausethe vertical space in which they form is created either by climatically induced aggradation or by uplift. Both local base-level processes raise active channels and set the stage for the formation of a flight of degradation terraces. Streams commonly degrade intermittently to a new base level of erosion (Figs. 1, 3, 5 ). Pauses

in degradation, and/or brief episodes of backfilling, are the times of formation of complexresponse fill-cut and strath or fill terraces. The piedmont reach of the Charwell River (Figs. 3, 5 ) provides an example of how selfarresting feedback mechanisms can cause temporary attainment of static equilibrium in a stream that is entrenching into bouldery valley fill or the underlying bedrock (Bull and Knuepfer, 1987 ). During degradation below an aggradation surface, selective entrainment and transport of bedload (Gomez, 1983; Brayshaw, 1985 ) by the river caused winnowing of the valley fill and progressive accumulation of cobbles and boulders on the streambed. This bouldery lag deposit armored and protected the streambed from further degradation by increasing shear stresses needed to set the bed in motion and by increasing hydraulic roughness. Armoring of streambeds arrests stream-channel downcutting. Then lateral erosion predominates where the stream impinges on the valley sides. Such brief episodes of static equilibrium may

STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

end abruptly when a perturbation such as a 1 ky flood event moves and breaks the bouldery lag, thereby reducing hydraulic roughness and resisting shear stresses. Renewed degradation can then form a lag gravel in a new streambed at a lower altitude. Remnants of the former streambed are preserved as terrace treads. Thus, without invoking either secular climatic or tectonic perturbations, a self-arresting feedback mechanism can occur repeatedly to form a flight of degradation terraces. Each Charwell River terrace remnant has a well sorted gravel cap that is more bouldery than the underlying massive silty aggradation-event gravels into which the terrace is cut. Such degradation terrace treads record pauses in valleyfloor degradation - static equilibrium - when adjustments between all variables interacted to transport bedload with neither aggradation or degradation of the streambed (Leopold and Bull, 1979 ). Complex-response unpaired fill terraces may form when tectonic base-level fall creates the vertical space for minor aggradation to occur in the reach immediately downstream from an active fault. It was sand-box modeling of this sequence of events that led Stan Schumm to propose the idea of complex response of drainage systems. Local vertical tectonic deformation causes tectonically induced downcutting in the form of headcuts that migrate upstream from the active fault. The surface-rupture event is responsible for an increase in sediment yield in the reach upstream from the fault and in valley-floor deposition in the reach downstream from the fault. Timing of the tectonically induced minor aggradation event is in part dependent on the rate of headcutting, which accelerates footslope erosion rates as the tectonic perturbation migrates upstream along trunk and tributary streams at ever decreasing rates (Schumm et al., 1987). Although the vertical space for the minor fill terrace was the result of a tectonic base-level fall, the rate, timing, and height of aggradation above the active channel are complex functions of adjusting

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variables in both the hillslope and stream subsystems. Vertical displacement rates across many range-bounding faults are rapid enough to favor deposition of alluvial fans instead of valley fills. In such cases complex-response variations of sediment-yield increase results in variations in rates of fan accumulation. Plentiful good exposures and opportunities for dating help distinguish static equilibrium complex-response (minor) terraces from type 1 dynamic equilibrium tectonic (major) straths. Equilibrium (fill-cut) terraces formed in alluvium deposited on top of the most recent tectonic strath clearly are complex-response terraces, because such terraces lie above the altitude of the bedrock beneath the stream channel at the time of initiation of the most recent aggradation event in either type 1 or type 2 reaches. Strath terraces above a modern tectonic strath most likely are complex-response strath terraces if they are of limited areal extent, but additional information about rates of processes is needed in order to be sure. Rates of degradation determined by ages and altitudinal separations of degradation terrace treads are especially useful. Strath terraces may be regarded as complex-response terraces where rates of degradation between times of terracetread formation (Bull and Knuepfer, 1987, fig. 6) exceed the long term uplift rate for the reach based on ages and positions of tectonic terraces. In such cases type 1 dynamic equilibrium had yet to be attained at the time of formation of the minor strath terraces. Diachronous and synchronous stream terraces

Stream terraces sweep through mountain canyons and across piedmonts in humid and arid climatic settings. Smooth longitudinal profiles of terrace treads appear to be profiles of former single valley floors; they strike the casual observer as being time lines passing through the erosional landscapes that surround them. Soil scientists would like to use

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stream-terrace treads as planar reference surfaces formed at a specific time in order to decipher altitudinal climatic controls on soilprofile formation. But are terrace treads or straths sufficiently synchronous to be regarded as time lines? Commonly the answer lies in the hands of the soil scientist who can describe and compare strengths of soil-profile development for pedons along a given terrace tread in different reaches of a fluvial system. Analyses of morphological and chemical characteristics of soil profiles can be used to evaluate whether differences between pedons, if present, are attributable to different times of formation, altitudinal changes in soil microclimate, or to differences in parent materials including variable input of atmospheric dust. Tectonic strath and climatic aggradation surfaces may differ in their degree of synchroneity. The present happens to be a time of attainment of the base level of erosion for many powerful streams, especially those flowing on soft materials. Modern straths that extend for many kilometers beneath stream-channel gravels of powerful rivers are clear evidence for straths being a synchronous fluvial landform. Thus major straths are synchronous landforms regardless of whether or not strath genesis was diachronous. As an equilibrium tectonic landform, synchronous major straths in coastal areas may be graded to marine terrace shore platforms formed during the sea-level highstand of the past 6 ka. Strath formation may end with the beginning of an aggradation event that buries the beveled bedrock surface or with stream-channel incisement into the strath. Synchronous straths may become synchronous or diachronous strath terraces. Strath terraces are synchronous only if an entire strath is buried or incised at the same time. Although it may be difficult to date times of initiation of entrenchment of strath and fill-cut terraces, it seems reasonable that increases of stream power caused by increases in stream discharge throughout a drainage basin should affect the entire strath reach at the same time. Cataclys-

WILLIAM B. BULL

mic flows can be important in terminating equilibrium conditions or in the crossing of thresholds (Bull, 1988 ). Diachronous strath terraces form where incisement is the result of faulting that increases stream power in the upstream reach by increasing gradient. Such strath incisement is time dependent because it migrates upstream from the tectonic base-level fall as headcuts and rapids. Rates of nickpoint migration are slow for ephemeral streams flowing on resistant rock and rapid for large perennial streams flowing on weak materials. Fill-terrace treads mark times of switching from aggradational to degradational modes of operation, a change that commonly occurs at different times along a stream. Aggrading valley floors may extend for many kilometers, but unlike strath genesis, the level of streamflow is rising rather than remaining at about the same level. Rates of aggradation typically increase and then decrease as the threshold of critical power is approached (Bull, in press, fig. 4.29 ). The decrease in some cases may be interpreted as the result of changes in variables during times of climatic transition that affect threshold of critical power. Longitudinal profiles of fill terraces generally do not record attainment of equilibrium conditions. Instead, initiation of stream-channel incisement, for a given reach, may be considered as a momentary transition between two disequilibrium modes of streamflow - aggradation switching to degradation - rather than the end of a protracted time span as in the case of strath-terrace formation. Along small streams this switch generally is not readily discernible in soil profiles, which implies synchroneity. Aggradation surfaces of climatic terraces commonly are diachronous for large rivers of humid regions that may need several thousand years for a slug of bedload created by a climatic change to shift long distances downstream (Jackson et al., 1982; Fisk, 1944). Aggradation surfaces are more likely to be

STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

synchronous for small powerful streams in humid regions where abundant stream power favors brief reaction times and short response times to climatic perturbations such as the Pleistocene-Holocene climatic change. In arid and semiarid regions available stream power typically is small and amounts of bedload are large. Such controls severely limit the rates at which streams can respond to either climatic or tectonic perturbations, and large spatial variations in times of initiation of aggradation can occur. An example is Cajon Creek in the Transverse Ranges of southern California; this fluvial system was the subject of an excellent study by Weldon ( 1986 ). Cajon Creek had a diachronous reaction time to latest Pleistocene-Holocene climatic changes, so the initiation of the aggradation event was progressively later with increasing distance upstream from the basin mouth. Aggradation began about 17 ka at the mouth and then migrated upstream, reaching the headwaters at about 6 ka. Aggradation had ended by 10 ka for the 15 km reach upstream from the mouth but had yet to start in the headwaters reach! One cross-check for Weldon's model of diachronous stream-terrace formation is to compare soil profiles on the 12 to 6 ka terrace tread. Unfortunately, soil-forming factors other than time vary spatially in the drainage basin. The downstream (17 to 10 ka) reach has a subhumid climate and schist gravel parent material. The upstream (10 to 6 ka) reach has a semiarid climate and sandy parent materials with substantial atmospheric dust derived from the adjacent Mojave Desert. Bruce Harrison of the University of New Mexico is presently studying these complex soils; he has described 75 pedons and is emphasizing colluvial-wedge catenas and toposequences (Harrison et al., 1990). It is important to make the distinction between synchroneity of time of formation of a terrace landform within a single fluvial system and synchroneity of stream terraces on a regional basis. Regional synchroneity involves

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the important topic of terrace correlations. Tectonic stream terraces would seem to have the best regional synchroneity, especially where graded to marine terraces. This is because there appear to be times (like the present) that favour attainment of the base level of erosion. Climatic stream terraces - aggradation surfaces - may vary greatly in their degree of regional synchroneity. One should expect diachronous terraces in regions characterized by strongly seasonal arid to semiarid climate, and where drainage basin area and relief range from small to large and rock types are different. In a general sense all the diverse basins in such a region will respond to a given climatic perturbation, but times of formation of aggradation surfaces between watersheds may vary by 3 to > 10 ky. Conversely, fluvial systems with similar geomorphic properties and processes will tend to have synchronous climatic terraces. An example of synchronous aggradation surfaces in adjacent drainage basins are the climatic terraces of the Seaward Kaikoura Range of the South Island of New Zealand. My work in progress indicates that climatically induced aggradation events are synchronous for drainage basins underlain by fractured greywacke that have reliefs of 1500 to 2500 m and areas of 30 to 100 km 2. For nine fluvial systems reaction and response times appear to be synchronous. The last two climatically induced aggradation events occurred between 38 and 31 k a _ lka, and between 26 and 14 ka_+2 ka. Such terraces seem to be synchronous in regard to their times of genesis, both within a given drainage basin and in other drainage basins of the climatic-lithologic-topographic province. Fill-cut and minor strath terraces can be synchronous or diachronous. For example, an ~ 11 ka fill-cut terrace along the Charwell River has radiocarbon-calibrated greywacke cobble weathering rind ages that vary by _+ 1 ka, which is within the analytical uncertainty of the dating method. The same terrace level in valleys that are tributary to the Charwell River has ages

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as young as 5.3 _+0.6 ka, which is clearly diachronous compared to the same terrace along the trunk stream. Terrace-tread formation and incision along the trunk valley is subject to the synchronous action of a powerful stream, but tributary streams degrade slowly and only after erosional base-level falls have occurred in the trunk valley. Complex-response terraces tend to be regionally diachronous because they result from local adjustments within individual fluvial systems. Examples of geomorphic processes that may operate on local instead of regional scales to create regionally diachronous complex-response terraces include ( 1 ) local rainfall events that disrupt stream channel lag gravels and renew degradation that leads to the creation of fill-cut or strath terraces, (2) local faulting that initiates incisement of straths or of aggrading reaches that are close to the threshold of critical power, or initiates minor pulses of aggradation within selected reaches as headcutting migrates upstream. The astronomical clock as applied to climatic and tectonic terraces

Ultimately, the earth's climate is a function of solar insolation. The combined precise cycles of variations in the earth's orbital parameters not only may be a fundamental cause of late Quaternary climatic changes, but may also provide the ultimate geochronological tool - an astronomical clock. Geomorphic and pedogenic utilization of the astronomical clock is beset by the need to understand the time lags of response to astronomical perturbations that are associated with diachronous or synchronous behavior of fluvial systems. The clearly similar times of solar insolation maxima in the Northern Hemisphere and the K / A r ages of corals that provide ages for sea-level highstands (Fig. 4 ) underscores the relevance of the astonomical clock for the sea-level change system which appears to behave synchronously with minimal response times to changes in so-

WILLIAM B. BULL

lar insolation (Chappell and Shackleton, 1986). Such a global temporal tool may be useful for comparing times of formation of climatic stream terraces and for tectonic stream terraces where times of attainment of the base level of erosion have been modulated by climate changes. The timing of geomorphic responses to climatic changes as a result of Pleistocene-Holocene climatic change in even a few study areas reveals profoundly different numbers and times of geomorphic events. Aggradation events occur during full-glacial times in humid, mesic climates (Sierra Nevada, Rocky Mountains in the western United States, Southern Alps of New Zealand), and during interglacial times in arid-thermic climates (Mojave Desert). Numbers of aggradation events vary greatly from region to region, perhaps reflecting climatic and lithologic controls of the times needed to replenish hillslope sediment reservoirs so the next stripping event can send a sufficiently large surge of detritus through the fluvial system to be recorded as an aggradation surface. Only three aggradation events appear to have occurred in the hot deserts of southwestern North America during the past 150 ka, but at least five aggradation events appear to have occurred along the humid, mesic streams of the Seaward Kaikoura Range during the same interval (Bull, in press, ch. 5 ). At Curry Draw (Fig. 1 ) the "single" aggradation event associated with the PleistoceneHolocene climatic change has distinct phases of aggradation with well-defined intervening periods of stream-channel downcutting. This record in part may be attributable to complex responses. The record of tectonic straths is more likely to be complete in humid than in arid regions, especially where soft rocks allow powerful streams to attain new base levels of erosion between closely spaced episodes of aggradation (Fig. 5 ). The timing of global climatic changes is a major control on the times of cutting of major straths - the fundamental tectonic sur-

STREAM-TERRACEGENESIS:IMPLICATIONSFOR SOILDEVELOPMENT

363

-

150

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Charwell

River major strath cutting ..,,

0 m -

lU

.................

120 1M124 118

........

...........................

< -5O -100 -150 0

25

50

75

100

125

150

Age, ka Fig. 4. Comparisons of times of insolation maxima (bold numbers) during the past 150 ky, and K / A r ages (open numbers) for sea-level highstands from the data of Chappell and Shackleton (1986), with the times of aggradation events in the Mojave Desert, and times of major strath cutting along the Charwell River, New Zealand. Times of straths younger than 55 ka are dated; older than 40 ka are estimated from calibrated rates of tectonically induced river downcutting.

I

I

I

I

I

I

i

I

Times of aggmdatlon of valley flu Degradation of valley fill with pauses at times of complex-response terraces

12o .m_E

80 "~ o D.

40

0

10

20 Time,

30

40

in ka

Fig. 5. Summary of combined influence of tectonic and climatic controls on the late Quaternary behavior of the Charwell River, New Zealand as reflected by changes in streambed altitude. This threshold-equilibrium plot is for the reach downstream from the range-bounding Hope fault; it shows times of aggradation, degradation, and attainment of the base level of erosion. T marks times of crossing of the threshold of critical power. Complex-response terraces were formed during pauses in degradation and are times of static equilibrium. Type 1 dynamic equilibrium occurred during three periods, such as 4 to 0 ka, when degradation by the river had attained the base level of erosion and maintained equilibrium conditions by continuing to degrade at a rate equal to the concurrent uplift rate.

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face - because the base level of erosion can not be attained during times of climatically driven aggradation-degradation events. Tectonism is a local process as compared to global climatic change, so it is fortuitous that climatic changes modulate tectonic terrace formation so as to be in agreement with the astronomical clock. Tectonic terraces can not form if a stream never attains the base level of erosion, and only one base level of erosion will be present if a stream never departs from type 1 dynamic equilibrium conditions. The Charwell River in Fig. 5 describes a reach of a fluvial system that spends most of its time either aggrading in response to climatic changes or degrading in attempts to catch up with new base levels of erosion. Because of a combination of rapid uplift and large amounts of intermittent aggradation, it barely has enough time after attaining the base level of erosion to bevel a new tectonic strath. Thus small time windows of attainment of equilibrium provide an opportunity to check for agreement with the times of insolation maxima as described by the astronomical clock. The agreement would not be nearly as certain if the stream remained at the base level of erosion for two-thirds of the time. Discussion

Summary of Charwell River observations The stream terraces of the 40 km 2 Charwell River basin summarize my present thoughts about types of equilibrium and modes of terrace formation (Fig. 5 ). This fluvial system is sensitive to tectonic and climatic perturbations and has numerous complex-response terraces. Ages of stream-terrace landforms are based on radiocarbon analyses, one tephra age, calibrated cobble weathering rind analyses of greywacke cobbles on terrace treads, lichenomerry, and uniform rates of lateral displacement (33 ___3 m / k y ) of terraces cut by the Hope fault. The reach downstream from the Hope fault is rising at 1.3 m / k y but frequent large

WILLIAM B. BULL

aggradation events tend to prevent prolonged attainment of the base level of erosion. Termination of each episode of valley floor alluvial backfilling created a climatic stream terrace an aggradation surface. In order to reach a new base level of erosion the stream had to degrade through the valley fill, and then through an increment of bedrock equal to the amount of uplift since the last time that stream had attained the base level of erosion. The two most recent climatic terraces and the three recent tectonic terraces have a timing that agrees with the astronomical clock (Fig. 4). Aggradation events occurred during times of full-glacial climate, and aggradation surfaces were formed when the climate changed to an interglacial mode. Although stream-channel uplift of the reach is best considered as a local process, the times of formation of tectonic terraces were modulated by the profound influence of climate-change induced variations in bedload and water yielded from the watershed. The sedimentology, stratigraphy, diagenesis, and plant-fossil record of the two most recent aggradation events - Flax Hills and Stone Jug - are greatly different (Bull, in press ch. 5 ), and clearly indicate a relatively colder and dryer climate during Stone Jug time. The earlier Flax Hills event was a product of larger streamflows and more voluminous debris from hillslopes subject to periglacial processes. Figure 5 shows heights, not volumes of deposits, which vary considerably along the 13 km of the Charwell River between the Hope fault and the junction of the Charwell and Conway Rivers. It is only near the mountain front of the Seaward Kaikoura Range that the spatially limited Stone Jug aggradation rose high enough along some streams to spread as alluvial fans over Flax Hills surfaces. Flights of complex-response degradation terraces formed after initial incisement of each aggradation surface. The degradation part of the Flax Hills event was so short that only two or three fill-cut terraces were formed; in Fig. 5 these are inferred from downstream reaches.

STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

The degradation part of the Stone Jug event continued for 10 ky, which allowed time for a dozen fill-cut and strath terraces to form. Only the treads dated by Knuepfer (1988) are shown. The types, ages, heights of complex-response terraces do not appear to correlate with flights of degradation terraces in nearby watersheds and are thus are unrelated to the astronomical clock, except for the generality that they formed during transition times between glacial and the present interglacial climates. Terrace soils Gently sloping planar treads of stream terraces are preferred sites for many soils studies because of their erosional stability and because tread formation at each pedon occurred at a point in time. Sequences of pedons along the longitudinal profiles of stream terraces may be synchronous or diachronous depending on the type of terrace. The above discussions provide food for thought regarding ( 1 ) the conceptual framework for the stream-terraces on which soils form, and (2) terrace features that affect soils genesis. Most terrace soils are on surfaces controlled by Quaternary global climatic changes and by internal adjustments within fluvial systems that produce complex-response terraces. The climatic landform of aggradation surfaces may provide an ideal flight of surfaces on which to study a soils chronosequence. Each aggradation event is recorded by a single relict soil where tectonically induced donwcutting is sufficient to provide clear altitudinal separation of the terrace treads. Multiple paleosols are typical of tectonically stable regions where younger aggradation events have had sufficient strength to spread alluvium over the treads of older climatic terraces. Pedons on a given climatic terrace in small fluvial systems commonly are roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic fac-

365

tors in different parts of the watershed. Climatic terraces of adjacent watersheds also should be roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds. Such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times, and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations that are obvious in their soil profiles. Furthermore, soils on climatic terraces of distant watersheds will not be synchronous if aggradation events occur during full-glacial times in one area and during interglacial times in the other area. The common case of several levels of major straths along a valley reflects a tectonic origin. Such beveled bedrock surfaces and their thin gravel caps are not sites of soil-profile formation where buried by deposits of subsequent climate-change induced aggradation events. Local faulting may cause incision of major straths in some tectonically active settings and thereby allow soils to develop on treads of tectonic stream terraces, but this situation is far less common than soil-profile formation on treads of climatic terraces. Some complex-response terraces tend to be diachronous within a given fluvial system. Fill terraces of complex-response origin have times of tread formation that by definition have spatial variability. Soils for a spatial sequence of pedons will appear different unless the time span of formation for the entire terrace forming event is brief. Correlations of complex-response terraces between fluvial systems are likely to be diachronous. Because these flights of degradation terraces result primarily from interactions between dependent variables in individual systems, similarities of soil profiles on stream terraces of different valleys may be nothing more than coincidence.

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Specific soil properties can also be affected by types of stream terraces. Soils developed in thin gravel caps of strath terraces of humid regions are likely to be gleyed, whereas welldrained soils are more likely on thick gravels. Relief is less and terrace treads are wider in many tectonically inactive settings than along streams with rapid rates of tectonically induced downcutting. Development of exceptionally old and well preserved soil profiles thus is more likely in some tectonically inactive settings. Acknowledgements Review comments by Vance Haynes, Ed Keller, Pete Knuepfer, Craig Kochel, Les McFadden, and Phil Tonkin resulted in a more complete and better expressed text. This work has resulted from financial sponsorship of many field studies in western North America and in New Zealand by the National Science Foundation and U.S. Geological Survey. References Barrell, J., 1917. Rhythms and measurement of geologic time. Geol. Soc. Am. Bull., 28: 745-904. Brayshaw, A.C., 1985. Bed microtopography and entrainment thresholds in gravel-bed streams. Geol. Soc. Am. Bull., 96: 218-223. Bull, W.B., 1975. Allometric change of landforms. Geol. Soc. Am. Bull., 86: 1489-1498. Bull, W.B., 1979. Threshold of critical power in streams. Geol. Soc. Am. Bull., 90: 453-464. Bull, W.B., t988. Floods - degradation and aggradation. In: V.R. Baker, R.C. Kochel and P.C. Patton (Editors), Flood Geomorphology. Wiley, New York, pp. 157-165. Bull, W.B., in press. Geomorphic Responses to Climatic Change. Oxford University Press, New York, 746 manuscript pages, to the published. Bull, W.B. and Knuepfer, P.L.K., 1987. Stream adjustments to uplift and climatic change, Charwell River, New Zealand. Geomorphology, 1:15-32. Chappell, J. and Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature, 324:137-140. Davis, W.M., 1902, Base-level, grade, and peneplain. J. Geol., 10:77-111. Fisk, H.N., 1944, Geological investigation of the alluvial

WILLIAM B. BULL

valley of the lower Mississippi River. Mississippi River Commission, Vicksburg, Mississippi. Gilbert, G.K., 1879, Geology of the Henry Mountains (Utah). U.S. Geographical and Geological Survey of the Rocky Mountain Region, U.S. Government Printing Office, Washington, D.C., 170 pp. Gomez, B., 1983. Temporal variations in bedload transport rates, the effect of progressive amoring. Earth Surf. Proc. Landforms, 8: 41-54. Hack, J.T., 1960. Interpretation of erosional topography in humid temperate regions. American Journal of Science (Bradley Volume), 258-A, pp. 80-97. Hack, J.T., 1965. Geomorphology of the Shenandoah Valley, Virginia and West Virginia, and origin of the residual ore deposits. U.S. Geological Survey Prof. Pap. 484, 84 pp. Hack, J.T., 1973. Stream-profile analysis and stream gradient index. U.S. Geol. Surv. J. Res., l: 421-429. Hack, J.T., 1982. Physiographic divisions and differential uplift in the piedmont and Blue Ridge. U.S. Geological Survey Prof. Pap. 1265, 49 pp. Harrison, J.B.J., McFadden, L.D. and Weldon III, R.J., 1990. Spatial soil variability in the Cajon Pass chromosequence: implications for the use of soils as a geochronological tool. In: P.L.K. Knuepfer and L.D. McFadden (Editors), Soils and Landscape Evolution. Geomorphology, 3: 399-416, this issue. Haynes, C.V., Jr., 1987. Curry Cochise, County, Arizona, A late Quaternary stratigraphic record of Pleistocene extinction and paleo-Indian activities. Geological Society of America Centennial Field Guide, Cordilleran Section, pp. 23-28. Howard, A.D., 1959. Numerical systems of terrace nomenclature; a critique. J. Geol., 67: 239-243. Jackson Jr., L.E., McDonald, G.M. and Wilson, M.C.. 1982. Paraglacial origin for terraced river sediments in Bow Valley, Alberta. Can. J. Earth Sci., 19: 2219-2231. Knuepfer, P.L.K., 1988. Estimating ages of late Quaternary stream terraces from analysis of weathering rinds and soils. Geol. Soc. Am. Bull., 100: 1224-1236. Leopold, L.B. and Bull, W.B., 1979. Base level, aggradation, and grade. Proc., Am. Philos. Soc., 123:168-202. Powell, J.W., 1875. Exploration of the Colorado River of the west (1869-72). Smithsonian Institute, Washington D.C. Ritter, D.F., 1986. Process Geomorphology. Brown, Dubuque, Iowa, 603 pp. Schumm, S.A., 1973. Geomorphic thresholds and complex response of drainage systems. In" M. Morisawa (editor), Fluvial Geomorphology. Binghamton, S.U.N.Y., Publications in Geomorphology, 4th Annual Meeting, pp. 299-310. Schumm, S.A., 1977. The Fluvial System. Wiley, New York, 338 pp. Schumm, S.A. and Parker, R.S., 1973. Implications of complex response of drainage systems for Quaternary alluvial stratigraphy. Nature, 243: 99- 100. Schumm, S.A., Mosley, M.P. and Weaver, W.E., 1987.

STREAM-TERRACE GENESIS: IMPLICATIONS FOR SOIL DEVELOPMENT

Experimental Fluvial Geomorphology. Wiley, New York, 413 pp. Waters, M.J., 1988. Holocene alluvial geology and geoarchaeology of the San Xavier reach of the Santa Cruz River, Arizona. Geol. Soc. Am. Bull., 100: 479-491.

367 Weldon, R.J., 1986. Late Cenozoic geology of Cajon Pass; implications for tectonics and sedimentation along the San Andreas fault. California Institute of Technology, Ph.D. thesis, 400 pp.