Characterizing large river history with shallow geophysics: Middle Yukon River, Yukon Territory and Alaska

Geomorphology 67 (2005) 391 – 406 www.elsevier.com/locate/geomorph

Characterizing large river history with shallow geophysics: Middle Yukon River, Yukon Territory and Alaska Duane G. Froesea,T, Derald G. Smithb, David T. Clementb a

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 b Department of Geography, University of Calgary Calgary, Alberta, Canada T2N 1N4 Received 15 February 2002; received in revised form 29 October 2004; accepted 4 November 2004 Available online 23 March 2005

Abstract Changes in longitudinal valley-fill thickness can provide inferences about fluvial behaviour over millennia. Increased thickness of channel deposits may reveal climate-induced accelerated sediment supply, basin subsidence, tectonic displacements, or rising downstream base level. Decreases in thickness, expressed as terraces, may suggest active uplift, decreased sediment flux, or falling base level within a reach. To date, methods to characterize valley fills have depended largely on borehole data. However, recent developments in shallow geophysics make such investigations possible and provide a practical methodology to investigate alluvial rivers. We carried out a longitudinal geophysical survey, coupled with floodplain chronologies to characterize a 1000 km reach of the middle Yukon River between western Yukon Territory and central Alaska. Ground penetrating radar (GPR) and electrical resistivity ground imaging (ERGI) profiles along the middle Yukon River between central Yukon and Alaska, coupled with channel-bottom echo profiles, demonstrates that there is little variation in thickness of gravel fill and depth of active scour holes. GPR surveys near the White River, the largest sediment source in the upper Yukon, show no significant longitudinal variation in gravel thickness (8–16 m) up to near Circle, Alaska (530 km of wandering channels). Geophysical profiles across the Yukon Flats sedimentary basin (430 km of braided and wandering channels) show an increase in gravel thickness (25–30 m), but this is consistent with compaction of underlying lacustrine sediment and maximum depths of contemporary scour holes. From the western margin of the Yukon Flats to the Dalton Highway bridge (30 km, confined valley with no floodplain or braid bars), borehole data at the bridge indicates 6–10 m of gravel on bedrock beneath 3–6 m of water. Floodplain stratigraphy indicates there is little evidence of fluvial aggradation of the middle Yukon during the Holocene. Radiocarbon ages from the gravel–overbank silt contact near present river level demonstrate a general vertical stability of the river with a period of rapid eolian accumulation during the late Pleistocene and early Holocene. The consistency of gravel-fill and scour hole depth, and lack of Holocene terraces along the 1000 km reach of the Yukon, indicate that the river has been in a state of mass-balance equilibrium of sediment transport (input=output) over the Holocene. Despite massive sediment influxes from the glacial-fed White River and crossing major faults, variation in valley-fill depth is

T Corresponding author. Tel.: +1 780 492 1968; fax: +1 780 492 2030. E-mail address: [email protected] (D.G. Froese). 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.11.011

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not significant. And further, the Yukon Flats reach shows no evidence of bed aggradation during the Pleistocene and has probably existed in equilibrium since the late Pliocene. D 2004 Elsevier B.V. All rights reserved. Keywords: Fluvial; Sediment mass-balance; Grade; Yukon River; Shallow geophysics; GPR; ERGI

1. Introduction Our understanding of large alluvial rivers is mostly a byproduct of river engineering studies driven by concern for navigation, channel stabilization, and flood control. Most of these studies have relied heavily on borehole data to determine subsurface sediment characteristics. Pioneering work by Fisk (1947) on the Mississippi River, and To¨rnqvist (1993) and Berendsen and Stouthamer (2001) on the RhineMeuse River exemplify this approach. However, for most large rivers, or those outside heavily developed regions, this approach is too costly and most drilling equipment too immobile. Alternative approaches to river characterization based on remote sensing have been successful in characterizing river history on decadal-to-century timescales through the development of reach-to-reach sediment budgets on the Amazon (Mertes et al., 1996), Fraser (McLean, 1990) and Ob’ rivers (Smith and Alsdorf, 1998). These studies provide clear inferences on recent alluvial history, but not necessarily whether the observations are representative over millennial time scales at which tectonic and climatic forces may influence a river. The approach used in this study is based on the assumption that persistent tectonic, sediment supply, climatic, or base-level changes influencing a river will result in mappable thickness variations in longitudinal valley fills. In short, these are deviations from longterm mass-balance equilibrium, or the dgradedT profile of the river. We use the definition of grade following Mackin (1948) as a measure of mass-balance equilibrium (input=output) over dgradedT time (100s–1000s years; Schumm, 1977). Once imaged through geophysical investigations these variations in valley-fill depth can help to deduce longer-term processes, or past influences, on an alluvial river. Some examples of the utility of using channel–valley-fill depth variations include: climate and sea-level reconstructions of the lower Mississippi River (Fisk, 1947; Saucier, 1994);

subsidence along the middle Magdalena River, Colombia (Smith, 1986); sea level influence along the Mattole River, California (Merritts et al., 1994); and increased upstream sediment supply of the North Saskatchewan River, Canada (Smith, 1973). In this study we use a shallow geophysical approach, coupled with recent terrace chronologies to characterize a 1000 km reach of the middle Yukon River between central Yukon and Alaska. The objectives of this paper are to (i) introduce an inexpensive, environmentally sensitive and efficient approach to characterize large alluvial rivers using shallow geophysics; (ii) determine whether recent vertical tectonic movements have occurred along the middle Yukon; and (iii) characterize the long-term sediment mass-balance of the middle Yukon River.

2. Middle Yukon River study area 2.1. Physiography Middle Yukon River drainage is generally northwest-trending, reflecting the structural grain of accreted terranes and large transcurrent faults that occupy the northern Cordillera of Yukon and adjacent Alaska (Tempelman-Kluit, 1980). Yukon River drainage begins in southwestern Yukon in the glacierized St. Elias Mountains and northwestern British Columbia (Fig. 1). The St. Elias Mountains, an extension of the Wrangell and Alaska ranges of adjacent Alaska are amongst the highest in North America, with peaks exceeding 6000 m and uplift rates estimated at 0.4 mm/year (O’Sullivan and Currie, 1996). Immediately north of the St. Elias Mountains, the Denali Fault separates the tectonically active southern Yukon Territory from the more stable and much lower interior Yukon plateaus. Yukon River drainage between Whitehorse and the Alaska border is within the Interior Plateau, a region underlain largely by metamorphic and volcanic rocks (Gordey and Make-

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Fig. 1. Yukon River study area and locations mentioned in text. Lithostratigraphic sites (letters in white circles) are shown in Fig. 6.

peace, 1999). Near the Alaska border, the Yukon River intersects the dextral strike-slip Tintina Fault, which marks the boundary between sedimentary rocks accumulated along the ancient North American margin (including the Ogilvie Mountains) and the assemblage of exotic terranes that accreted to the margin of the craton (Gordey and Makepeace, 1999). On the Canadian side, the Tintina is a single linear feature extending from the border with British Columbia to central Alaska where the Tintina separates into two distinct lineaments south of the Yukon River that have been purportedly active in the Quaternary (Barker, 1986; Dover, 1994). Near the village of Eagle, the Yukon River flows north of the Tintina Fault and continues within sedimentary rocks of the western Ogilvie Mountains until it enters the Yukon Flats sedimentary basin. The Yukon Flats is a major physiographic feature in central Alaska consisting of more than 30,000 km2 of marshy lowland between the Tintina Fault and Brooks Range. Gravity modeling of the basin

suggests an extensional graben complex which may contain up to 3 km of fill (Dover, 1994; Kirschner, 1994). Dover (1994) suggested the low topography of the Yukon Flats is evidence of continuing extension and subsidence of the basin. The sedimentary-fill age of the Yukon Flats is known from pollen collected from a borehole at Fort Yukon (Williams, 1962) and from a subsequent 390 m core drilled by the United States Geological Survey in 1994. These cores indicate 3–4 m of overbank and eolian sediment overlying 25 m of gravel and at least 365 m of sands, silts, clays and lignite facies (Phillips, 2002). Interpretation of the core suggests a lacustrine origin for the fine-grained sediments (Phillips, 2002). On the basis of correlation of pollen zones in the core, basal lignites recovered are of mid-Miocene age (ca. 15 Ma) and the uppermost sediments immediately below the gravel are roughly 3 Ma old (T. Ager, United States Geological Survey, written communication, 2000; Froese et al., 2001).

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2.2. Quaternary history Uplands of the Yukon–Tanana Terrane in western Yukon are generally below 1400 m, and the dissecting incised valleys have relief of up to 500 m. The Yukon Plateau is an uplifted erosional or planation surface produced from extensive subaerial exposure in the early–mid Tertiary (Tempelman-Kluit, 1980). Through the late Tertiary, a south-flowing paleoYukon River is hypothesized, with a drainage divide near the Alaska border, connecting the area north of the Tintina Fault zone with the Pacific Ocean (Tempelman-Kluit, 1980; Duk-Rodkin et al., 2001; Froese et al., 2001). This drainage likely extended across the area presently occupied by the St. Elias and Coast mountains, and may have persisted through uplift of the southwestern Yukon ranges during the Pliocene (Tempelman-Kluit, 1980). The Yukon River would have been diverted to the northwest into central Alaska by the growth of late Cenozoic ice sheets by 2.6 Ma (Froese et al., 2000; Duk-Rodkin et al., 2001). This reorganization of Yukon River drainage may be recorded by late Pliocene gravel overlying Miocene– Pliocene lacustrine sediments in the Yukon Flats in central Alaska (Froese et al., 2001). This earliest Cordilleran glaciation, responsible for reorganizing the Yukon River drainage, was the most extensive glaciation in the northern Cordillera (Froese et al., 2000). Subsequent Cordilleran Ice Sheet glaciations have been more restricted through the Pleistocene, leaving the middle Yukon River largely unaffected by direct glaciation. Duk-Rodkin’s (1999) compilation of glacial limits in Yukon Territory indicates that only middle-Pleistocene ice would have intersected the Yukon River from the north between Dawson and Eagle, and the upper Stewart and Yukon rivers above the study reach. Duk-Rodkin (1996) mapped a middle Pleistocene outwash terrace surface down the Yukon River to the Alaska border. Other workers have suggested that outwash gravels of late Pleistocene age intersect the modern floodplain of the Yukon River in central Yukon (Fuller, 1994; Jackson, 2000). Our fieldwork and terrace chronology along the upper Yukon are consistent with these observations. Little work has been done on the history of glaciation in east-central Alaska. Weber (1986) indicated that local alpine glaciers occupied the upper

elevations of the Yukon–Tanana upland south of the Yukon River. These local glaciers were restricted to their local valleys during the last glaciation advancing a few 10s of kilometers from their cirques, and more extensive, earlier glaciers were more than 30 km from the Yukon River. In the Yukon Flats area, a local ice complex extended south from the Brooks Range in the upper Chandalar River (Williams, 1962). However, a survey up the Chandalar River in 1999 indicated that no outwash terrace sequence was present below Venetie, suggesting limited influence by these valley glaciers on the Yukon. The Porcupine River, however, was the site of catastrophic floods when glacial Lake Old Crow, in northern Yukon was rerouted into central Alaska during the last glacial Laurentide maximum (Hughes et al., 1981; Thorson and Dixon, 1983). The effect of these floods on the Yukon Flats is unclear, but the deposition of a lowangle braid fan on the lower Porcupine, northeast of Fort Yukon, has been related to the floods (Thorson, 1989). 2.3. Hydrology and sediment transport The Yukon River, fifth largest in North America and twentieth globally in terms of discharge, has an average discharge N6000 m3/s (Brabets et al., 2000). In the study reach, the Yukon drains an area of about 500,000 km2 upstream of Dalton Highway bridge, AK, with an average discharge of ~3600 m3/s (Brabets et al., 2000). The river is located within the northern boreal forest and zone of discontinuous permafrost (Brown, 1978). Discharge is characterized by a nival regime that usually begins with a mechanical ice break-up in early–mid May followed by a spring freshet in mid–late June. Grain size distribution (D50 mode) along proximal channel bars (upstream end) of the middle Yukon range from coarse-cobble/boulder ( 6 to 10 f) in the White River/Dawson region, fining to pebble-cobble near Circle ( 5 to 3 f), to pebble-gravel ( 2 f) at the distal end of the Yukon Flats (Clement, 1999). River surface slope on the White River is N0.001, b0.0005 on the Yukon near Dawson City, the slope lowers to 0.0004 as the river enters the Yukon Flats, and flattens to 0.00013 at the distal end of the basin (Fig. 2; Clement, 1999).

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Morphologically, the Yukon River is braided between the White River confluence and Dawson (Figs. 2 and 3), changing to dwanderingT, characterized by an irregularly sinuous channel with more permanent islands between Dawson and Circle (cf. Desloges and Church, 1987). Radiocarbon ages from forested islands near Dawson suggest little channel migration in the last 3000 years (Table 1). Where the Yukon River intersects the Yukon Flats sedimentary basin near Circle, the river morphology quickly changes from largely a single channel, wandering with forested islands, to strongly braided with an increase in channel width from 0.5 km above the Yukon Flats to nearly 6 km at Circle (Fig. 4). Near Fort Yukon, below the Porcupine River confluence, channel stability increases and the river morphology again becomes wandering, characterized by one or two meandering channels with two or three highsinuousity slough channels along the margins (Clement, 1999). Channel anabranches decrease downstream, eventually forming a meandering pattern for a few wavelengths above the Dalton reach (Clement, 1999). The Dalton reach (30 km) is largely confined by bedrock and lacking a significant floodplain. A cobble bed is indicated from borehole logs in the Dalton reach (Clement, 1999); and further the river is still gravel-bedded below the confluence with the Tanana River downstream (Froese and Smith, unpublished data). The White River, the largest sediment source in the upper Yukon basin, originates in the glacierized St.

Fig. 3. Yukon River morphology photos. (A) Looking upstream on the braided White River. (B) Photo of stable forested islands of the confined reach near Dawson. Village of Moosehide in center of photo.

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Table 1 14 C Ages from the middle Yukon River Site Lab # A B B F H K M M O O P Q Q

None givena WAT4190 WAT4191 BETA1287572b GX-27742 WAT4157 WAT4158 WAT4159 WAT4100c WAT4160 WAT4161 AA38537 AA38538

14

Context

Material

C Age F1r (YBP)

Buried Soil Overbank Overbank In gravel Top of loess Base of sandsheet Overbank sed’s Base of sandsheet Base of sandsheet Base of sandsheet Base of sandsheet Buried Soil Buried Soil

Wood Wood Wood Bone Wood Wood Wood Wood Wood Wood Wood Twig Twig

9560F80 2910F60 2680F60 12,270F50 5360F90 10,250F70 10,300F90 10,300F80 11,550F70 11,500F100 10,500F80 9806F62 9321F50

Site letters found in Figs. 1 and 6. All other ages this study. a Age from Friele et al. (2001). b Age from Saleeby and Sattler (1999). c Age from Clement (1999).

Elias Range in southwestern Yukon Territory. Suspended sediment load near the Dalton Highway bridge is estimated at 30 million t/year (Brabets et al., 2000), of which nearly 90% is derived from the White River, which is b10% of the drainage basin area. No bed load measurements have been made on the middle Yukon; but using the ratio of suspended to bed load derived from data on the physiographically

similar and nearby Tanana River by Burrows et al. (1979) of 1–3%, we estimate annual bed load transport of 0.3–0.9 million t/year. In contrast to that of most large rivers in North America, the Yukon discharge and sediment yield has negligible human influence. In the headwaters of Yukon Territory and northwestern British Columbia most of the coarse sediment load is captured within lake deltas.

3. Methodology The investigation of the middle Yukon consists of: (i) a longitudinal shallow geophysical survey using ground penetrating radar (GPR) and electrical resistivity ground imaging (ERGI) along channel bars and islands at 20–40 km intervals over a 1000 km distance; (ii) geophysical bottom profiling (echo sounding) of active scour hole depths along the study reach; and (iii) stratigraphic and sedimentologic description of natural exposures and collection of organic material for radiocarbon dating. 3.1. Shallow geophysics A pulseEKKO IV ground penetrating radar (GPR) system in reflection mode with antennae frequencies of 12.5 and 25 MHz, separated by 8 and 4 m, respectively, was used with a 1000-V transmitter.

Fig. 4. Yukon River in the Yukon Flats immediately upstream of the village of Fort Yukon, view looking upstream.

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Profiles 100–500 m long were shot at mid-channel and side-channel gravel bars within 0.5 m of the water table. Each shot (1 m spacing, single vertical wiggle trace on profiles) was vertically stacked 64 times with a sampling rate of 1600 ps. Profiles were processed and plotted using pulseEKKO IV Vers. 4.2 software with several algorithms, but are presented with constant gain control. An average near-surface, twoway travel time of 0.08 m/ns was determined by common midpoint surveys conducted at profile sites. GPR works similar to seismic methods with the major difference being that it uses electromagnetic (EM) rather than an acoustic energy source. In GPR surveys, short bursts of high frequency EM energy (generally 10–1000 MHz range) are transmitted into the ground. Where there are changes in electrical properties of sediments, reflections of energy are returned which are detected at the surface by a receiver. Shot at 1 m intervals for hundreds of meters,

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reflections are recorded from which stratigraphy and gross lithology can be interpreted. Variations in the dielectric properties of sediments results in signal returns that are proportional to the dielectric contrast, allowing the contact between the alluvial gravel and bedrock or lacustrine sediments to be well defined (Fig. 5). In resistive, coarse-grained sediment (e.g. coarse gravel free of silt or clay), depths of 70 m at low frequencies (12.5 MHz: Smith and Jol, 1995) can be detected. A number of detailed accounts provide a review of the theory and methodology behind GPR that is beyond the scope of this work (Ulriksen, 1982; Davis and Annan, 1989). Electrical resistivity ground imaging (ERGI-AGI system) uses an array of multiple electrodes (typically 14–56), a resistivity meter, a switching device, and a laptop computer. A current powered by a 12 V battery is passed from a pair of electrodes to form an electrical field in the subsurface, and a second pair of electrodes

Fig. 5. Typical bedrock profiles imaged with GPR along the Yukon River in Reach 1. (A) 25 MHz survey along mid-channel bar near km 195. Profile shows prominent reflector (bedrock at 350 ns, 11–12 m depth). (B) 25 MHz profile immediately upstream from the Tintina Fault zone. Profile shows a prominent reflector at 225–250 ns, ~10 m depth. (C) 25 MHz survey along mid-channel bar downstream from the Tintina Fault near the Alaska border. Profile shows prominent reflector at 200–225 ns, with refracted reflections below ~7–8 m depth.

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measures the potential drop of the electrical field (ground resistivity). Multiple measurements of the ground’s resistivity are made by switching electrode pairs along the electrode layout (spread). Measurements made with electrodes spaced farther apart record resistivity from deeper zones in the subsurface. For example, a 10 m spacing for 56 electrodes would image to a depth of 120 m; in a general sense penetration is about 20% of the length of electrode spread (Loke, 2000b). More detailed descriptions of the resistivity method are described elsewhere (Reynolds, 1997; Loke, 2000a,b; Baines et al., 2002). Once the data for a profile is collected, it is fit into a block model of the suburface to differentiate lithology using RES2DINV software (Loke, 2000b). Further computational steps remove noise and resolve each block’s contribution to the subsurface model. This method has an advantage over GPR in that it works well in conductive environments where EM energy (as in GPR) is quickly attenuated. We use this method as a compliment to the GPR data because borehole measurements are not feasible in the Yukon Flats National Wildlife Preserve. Notably, however, the vertical resolution of ERGI is dependent on electrode spacing and array size. With the 10–12 m spacing used in this study, we estimate vertical resolution is about 5 m, and place more emphasis on the GPR depths of the subsurface fill that are direct measurements of two-way travel time from a reflector. In this way, we use the ERGI data to confirm the two-layer model indicated by the GPR data. 3.2. Stratigraphic and chronologic approach Natural exposures were cleaned with shovels and trowels, then photographed, documented, and sampled. Detailed vertical lithostratigraphic logs were measured on a bed-by-bed basis; and where exposures allowed, horizontal logs were collected noting lateral variation and facies changes within units. Sections were initially subdivided into stratigraphic units on the basis of sediment type and general sedimentologic features. Most large exposures along the middle Yukon were described, but most extended between the middle-Pleistocene and Pliocene and are not relevant to the theme of this paper. Included are representative late Pleistocene and Holocene stratigraphic sections with chronologic control. In this

paper the gravel-sand/silt (channel/overbank) contact is used throughout the study reach as a datum for vertical changes of channel position over time. Organic material for radiocarbon dating was collected to determine long-term vertical stability along this contact. Chronology of sections is provided from radiocarbon dating of organic material by either AMS or conventional methods or by the presence of the late Holocene (1500–2000 14C YBP) northern lobe of the White River tephra (Lerbekmo et al., 1975).

4. Results and interpretation The presentation of results is divided into three physiographic reaches between the Yukon–White rivers confluence and the Dalton Highway bridge (Fig. 1). Each reach description includes its physiographic character, results of geophysical investigation, and floodplain stratigraphy. 4.1. Reach 1: White River to Circle Reach one begins at the White River confluence with the Yukon River and extends to Circle, AK, a distance of 530 km. From the confluence to the mouth of the Klondike River at Dawson City, a distance of 120 km, the Yukon River shows a changing surface morphology with distance from the White River. The upstream part of the reach consists of unstable gravel braid bars, a channel pattern that changes downstream to one of more stable forest-covered islands with rare channel bars (wandering). Valley floor widths (channel and floodplain) range from 0.5 to 3.5 km. Between the White River and Eagle, the Yukon is incised into high-grade metamorphic rocks of Yukon Tanana terrane; the narrowest valley widths are associated with this reach. Between Eagle and Circle, the Yukon River is within sedimentary rocks of the Ogilvie Mountains, which are more easily erodible, allowing a wider valley. Twenty-nine GPR profiles were shot on channel bars at ~20 km intervals along the river. Three 100 m subsets of typical profiles collected with 25 MHz antennae are shown in Fig. 5. In each profile, bedrock can be reliably interpreted from a high amplitude, continuous reflection occurring between 225 and 425 ns, corresponding to 8–16 m depths relative to the

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water surface in July 1999. For the 530 km from the White River, we interpret gravel resting on bedrock with depths comparable to that of scour holes recorded by echo-sounding. At Dawson City, several boreholes indicate a fluvial gravel thickness of 10–12 m over bedrock confirming the GPR results (McKinney, 1974). Between Fortymile and Eagle, several unusually deep scour holes occur where the Yukon River crosses the Tintina Fault. At two locations, scour holes 22 to 25 m deep were recorded at points where the river crosses the fault, which we interpret as deeper scour zones made possible by altered and brecciated rocks at the fault contact. Floodplain chronology in Reach 1 is provided by five radiocarbon dates (Table 1) and multiple occurrences of the northern lobe of White River tephra (1500–2000 14C YBP) (Lerbekmo et al., 1975) (Fig. 6). All chronologic samples are located above the channel bar overbank or eolian sediment contact, providing a minimum age of the river elevation. Site A (Fig. 6), adapted from Friele et al. (2001), includes a radiocarbon dated sample that indicates river level was near its present elevation at 9500 14C YBP. An additional age of 12,270F50 14C YBP at Site F confirms the vertical stability of the Yukon through this reach for the Holocene (Saleeby and Sattler, 1999). At Site B, part of an island exposure (Figs. 1 and 6), two radiocarbon ages from 2910F60 14C YBP at the overbank contact and 2550F60 14C YBP near the present surface, indicate vertical and lateral stability through the reach. As well, an age of 5360F90 14C YBP at Site H from organic silt overlying 3 m of loess as well as multiple occurrences of the White River tephra (Fig. 6) are consistent with the long-term vertical stability of Reach 1. 4.2. Reach 2: Yukon Flats sedimentary basin Reach 2 covers 430 km of the Yukon Flats, between Circle and 30 km upstream of the Dalton bridge (Fig. 1). Where the Yukon River enters the Yukon Flats, it abruptly spreads laterally from a 500m wide channel with limited floodplain in Reach 1 to a 4–6 km wide braided channel belt with scrollpatterned floodplain up to 10 km wide (Figs. 2 and 4). At the confluence with the Porcupine River at Fort Yukon, the fluvial style changes from braiding to a

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transitional wandering gravel-bed river (Clement, 1999). Increased multi-channel meandering (wandering) with lateral accretion characterizes the reach between Beaver and Stevens Village (Figs. 1 and 2). From Stevens Village to the distal basin, the reach meanders in a single channel (Fig. 2). Seventeen GPR profiles and six ERGI profiles were shot in the Yukon Flats (430 km). Three typical ERGI profiles collected with 10 and 12 m electrode spacing are shown in Fig. 7. As well, 12.5 MHz antennae were used in the Yukon Flats since the 25 MHz antennae used for Reach 1 were unable to confidently detect and delineate the gravel-fill thickness. In each profile radar reflections up to 800 ns are present, but below that depth signal return is abruptly attenuated (Fig. 8). ERGI profiles indicate a two-layer lithostratigraphy with a highly resistive strata (gravel) in the upper 30 m overlying a more conductive strata (fine-grained sediments) (Fig. 8). Both the GPR and ERGI images (Figs. 7–9) are consistent with borehole data at Fort Yukon that indicates 30 m of gravel overlying fine-grained sediments (Phillips, 2002). Floodplain chronology through Reach 2 is provided by eight radiocarbon ages (Table 1) collected at the gravel-sand/silt contact or within overlying eolian sediments, providing a minimum age on the river elevation during the late glacial and Holocene. At Site K (Figs. 1 and 6) wood at the overbank contact was dated at 10,250F70 14C YBP. Twenty kilometers upriver of Fort Yukon, wood at the overbank contact was dated at 10,300F80 14C YBP. Between Fort Yukon and Beaver, at Sites 0 and P, wood is dated at 11,550F70 14C YBP and 11,500F100 14C YBP, respectively. Further, at Site Q, near Stevens Village, ages of 9800F60 14C YBP and 9321F50 14C YBP were determined from organics within loess 7 and 9 m above the river, respectively. 4.3. Reach 3: Dalton bridge reach Reach 3 covers the final 30 km between the end of the Yukon Flats and Dalton Highway bridge. Here, the confined river is flanked by steep bedrock and loess cliffs. No gravel bars or islands within this reach on which to carry out GPR and ERGI surveys. However, in lieu of geophysical opportunities, 10 borehole logs along a cross-sectional profile at the Dalton bridge

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Fig. 6. Lithostratigraphic logs along the middle Yukon River. Letters refer to locations in Fig. 1. Typical latest Pleistocene–early Holocene exposures consist of fluvial gravel up to 4 m, overlain by fine-grained overbank and eolian sediment, with negligible mid–late Holocene accumulation.

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Fig. 7. Typical ERGI images obtained from Dipole-Dipole surveys in the Yukon Flats: (A) km 575; (B) km 630; and (C) km 690. Each image shows a two-layer model of high resistivity (gravel) overlying lower resistivity sediments (sands-silts from borehole data at Fort Yukon).

indicates 6–8 m depth of channel-bed gravel resting on bedrock in water depths (summer stage) of 3–6 m (Alyeska Pipeline Services, 1972). These combined gravel and water depths are approximately equivalent to the gravel depths in Reach 1 determined by geophysics and borehole data.

5. Discussion This study is the first attempt to use shallow geophysics to indirectly examine long-term sediment mass balance of a large river system. The geophysical data indicate little longitudinal variation in thickness of fluvial fill. Reaches 1 and 3 valley-fill depths range between 8 and 16 m, approximately the depths of local modern scour holes. In Reach 2, the Yukon Flats shows a greater depth of the valley-fill gravel, 25–30 m, which we think can be accounted

for by moderate compaction (15–20%) of the upper 100 m of the N350 m of fine-grained sediments below the gravel known to be present in the Yukon Flats at Fort Yukon (Phillips, 2002). We base this estimate on the calculated differences in sedimentation rates of the upper 30 m (silt/clay) of the Magdalena River sedimentary basin of Colombia (Smith, 1986). This data suggests ~20% compaction in that basin during the Holocene. Minor compaction of the upper 100 m of underlying lacustrine sediment in Yukon Flats could easily accommodate an additional 10–15 m of basin-fill gravel thickness relative to the upstream and downstream reaches. The abrupt thinning of the gravel fill from 25 m at the distal end of the Yukon Flats to 12- to 14-m thick (including river depth) in the Dalton reach further supports compaction as the primary cause for thickening of the valley-fill. In addition, borehole data at Fort Yukon (Phillips, 2002) indicate basin subsidence was

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A

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Fig. 8. Typical GPR profiles collected with 12.5 MHz antenna in the Yukon Flats. Each profile shows a resistive unit of gravel overlying conductive sediments that quickly attenuate the signal. These depths vary slightly across the basin but were less than 35 m at all sites imaged. (A) km 600; (B) km 625; and (C) km 640.

active in the Miocene through mid-Pliocene (Froese et al., 2001). The uniform thickness of gravel fill across the Yukon Flats suggests no differential compaction (at least at the sites that we imaged), and indicates basin subsidence was probably not active during the Quaternary. In the final reach of the study area, the Dalton bridge section, borehole data indicates 6–8 m of fluvial gravel-fill beneath the 3–6 m present water depth, suggesting an overall geomorphic history similar to that of Reach 1. Thus, it suggests that if it were not for the change in

substrate between Reaches 1 and 3 (on bedrock) and Reach 2 (Yukon Flats—with its compaction) the valley-fill gravel depth of the middle Yukon River over the 1000 km investigated would probably be nearly equivalent between 8 and 16 m. The lack of increased depth of fluvial gravel in the Yukon Flats downstream suggests that the basin is no longer subsiding. If the distal end of the basin were subsiding, an increase in valley-fill gravel depth would be expected; however, imaged profiles in the basin indicate a gravel thickness of 30 m, which is

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Depths of valley fill from ERGI, GPR and borehole data

Fig. 9. Compilation of shallow geophysical determined depths and borehole data along the middle Yukon River. The data indicates that the confined reach, above Circle, AK, ranges from 8 to 16 m in depth, approximately the same as the present-day scour depths in adjacent reaches. In the Yukon Flats area, channel-fill depth abruptly increases to 25–35 m as a result of basin subsidence (which likely ended in the late Pliocene). Borehole data at Fort Yukon (km 600) indicate that mid–late Pliocene lacustrine sediments are overlain by fluvial gravel and that compaction of the fine-grained sediments likely accounts for the increased depth through the reach. Channel-fill depth decreases in the Canyon/ Dalton Highway Reach (km 1000), indicating that if it were not for the change in substrate, the middle Yukon River would have a near-uniform valley-fill depth through the study reach.

consistent with borehole data at Fort Yukon. And further, the borehole data at Fort Yukon indicates an upper age on the core of ca. 3 Ma at the top of the lacustrine sediments (Froese et al., 2001), providing a maximum age for the overlying gravel. The presence of unusually deep scour holes (up to 25 m deep) between Fortymile River and Eagle at sites where the Yukon River crosses the Tintina Fault suggests that a fault-fractured and brecciated zone is more deeply eroded by fluvial scour. The lack of variation in valley-fill thickness revealed by GPR across the Tintina Fault suggests no significant recent vertical movement along the fault zone. This observation is consistent with a lack of disturbance of PlioPleistocene strata exposed in the Tintina Trench northwest of Dawson City (Duk-Rodkin et al., 2001) and local faulting east of Dawson which only displaces middle Pleistocene outwash (Hughes et al., 1972; Froese, 2004). Floodplain chronology in the Yukon Flats indicates a lack of fluvial aggradation since the late Pleistocene (Fig. 9). The Pleistocene surface identified by Williams (1962) that the Yukon River has exposed at Sites M, O, P and Q (Figs. 1 and 6) indicates that the surface consists of a thin gravel unit (at present river level) overlain by 4–14 m of eolian sediment. Radiocarbon ages indicate the fluvial–eolian contact ranges from 9.8 to 11.5 ka 14C YBP (Table 1). Thus, we conclude, based on the late Pliocene age of the upper lacustrine

sediments and the late Pleistocene age of the fluvial– eolian contact in river cutbanks, that the gravels in the Yukon Flats represent a composite unit of fluvial deposits from the late Pliocene to the present. The long-term mass-balance equilibrium of the Yukon Flats and upstream reach of the Yukon River indicated by the geophysical data and floodplain chronology is inconsistent with short-term sediment yield and transport data presented in Brabets et al. (2000). Estimated suspended-sediment load at Dawson and Eagle are roughly 27 million t/year. Despite the addition of an estimated 7 million t/year from the Porcupine River at Fort Yukon and 1 million t/year from the Chandalar within the Yukon Flats, the downstream estimate below Yukon Flats at Ramparts is only 27 million t/year. This would suggest a net deposition in the Yukon Flats of 8 million t/year. However, the Ramparts data is based on 17 samples measured between 1962 and 1967 (T. Brabets, United States Geological Survey, personal communication, 2001), suggesting that the sediment transport numbers are at best approximations as those authors suggest (Brabets et al., 2000). Hydrometric data and our field experience indicate the occurrence of bankfull discharge or greater events are rare, about once in 20 years, of which all in the Yukon Flats were ice jam events (Clement, 1999). The largest recorded summer discharge (1992), only achieved bankfull discharge at Beaver. This suggests that annual net storage of

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overbank sediment in the Flats is minimal, and the only long-term storage occurs within abandoned slough channels, and along channel and bar margins. The geophysical data and radiocarbon ages, with their crude resolution, suggest net deposition is negligible over the last several thousand years. The Yukon Flats is an excellent geologic example of limited accommodation space given the absence of Quaternary subsidence. The only significant accommodation space created along the 1000 km of the middle Yukon was provided in the Yukon Flats from compaction of underlying fine-grained sediments. This probably occurred in the late Pliocene or early Pleistocene associated with the initial deposition of gravel in the basin. The lesson demonstrated by the Yukon data is that this river is an efficient sediment transporting system in a state of long-term massbalance equilibrium. In addition, this study shows some general features of large rivers that have not been well documented. Previous studies that have examined the influence of climate on large rivers (see review by Blum and To¨rnqvist, 2000) largely outline the competing forces of baselevel change and upstream sediment supply (from ice sheets during the Pleistocene) and downstream controls from sea-level change. The rapid responses that have been documented largely reflect these competing processes. In contrast, the middle reach of the Yukon, in its interior continental setting far removed from late Pleistocene Ice Sheets, was too distal to aggrade from oversupply of glaciofluvial sediment. And further, the middle Yukon setting is too far above the Yukon River mouth (N1000 km) to be influenced by sea-level variations. As a result, the primary changes on the middle Yukon were associated with eolian processes on the floodplain rather than rapid aggradation or incision of its channel.

6. Conclusions GPR and ERGI provide indirect methods to characterize long-term (millennial) sediment budgets of alluvial rivers that otherwise would require spatially and temporally intensive sampling programs at great expense. This approach is useful in determining the impact of tectonic and climatic influences on long-term sediment budgets.

The consistency of gravel valley fill along the 1000 km of the middle Yukon River, coupled with similar depths of contemporary scour holes and a lack of recent terraces, indicates the Yukon has been in a state of mass-balance equilibrium of sediment transport during the Holocene. Additionally, the Yukon Flats sedimentary basin reach may have been in a state of mass-balance equilibrium since the late Pliocene. Our geophysical data along the Yukon Flats indicates no evidence of subsidence since the late Pliocene. Additionally, along the Tintina Fault we found no evidence of recent vertical movement despite crossing the fault zone several times. Despite the considerable time (since the late Pliocene) represented by the Yukon River within the Yukon Flats, little sediment volume is preserved; and therefore, we conclude the system is little more than a giant conveyor belt-like system, effectively transporting its sediment load through the middle Yukon River reach.

Acknowledgements The Natural Sciences and Engineering Research Council (NSERC) supported purchase of geophysical equipment and financed most of the field costs. Additional support for field research was provided by the Geological Society of America grants-in-aid of research (Roscoe Jackson III Research Fund), the Northern Science Training Program from the Department of Indian Affairs and Northern Development and the Yukon Geology Program. Field assistance was provided by John Laughton, Gary Parkstrom, and Nadine Raynolds. We acknowledge the support for research permits provided for this work by the National Park Service (Yukon-Charley Rivers National Preserve), Bureau of Land Management for work in the Yukon Flats area, and the Yukon Heritage Branch in Yukon Territory. Useful reviews were provided by A. Gupta, R. Giardino, Y. Martin, G. Osborn, and N. Rutter.

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