Journal of Sedimentary Research, 2005, v. 75, 844–860 DOI: 10.2110/jsr.2005.066 RADAR ARCHITECTURE AND EVOLUTION OF CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS: FRASER AND SQUAMISH RIVERS, BRITISH COLUMBIA, CANADA C.L. WOOLDRIDGE* AND E.J. HICKIN Department of Geography and Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada e-mail: [email protected] ABSTRACT: The alluvial architecture and evolution of two kilometer-scale compound bars in the wandering gravel-bed Fraser and Squamish rivers, British Columbia, Canada, are described. Integrating ground-penetrating radar, bathymetry, and aerial photographs enables the internal architecture to be linked to the evolution of the gravelly barforms over the previous 50 years. These linkages reveal the sedimentary mechanisms that formed the various architectural packages within compound bars and unit bars. Growth of compound bars is controlled by the accretion of unit bars onto discrete segments along their gravelly edges. The attachment of unit bars deflects the thalweg to impinge on and erode specific portions of bars and channel banks. Bar growth leads to the stabilization of bars, vegetation colonization of bar interiors, and island formation. It is the formation of islands, along with channel avulsions, that maintains channel division in wandering rivers. Seven styles of gravelly deposition are imaged in the alluvial architecture. Vertical-accretion deposits formed from the deposition of gravelly bedload sheets are the most common strata. A moderate abundance of slipface deposits preserves high-relief bar margins. Lateral accretion dominates point-bar deposits. Downstream-accretion deposits govern some phases of down-bar growth. Partial and complete channel-fill and chute-fill deposits are eroded into underlying sediments, as are scour-and-fill deposits. Upstream-accretion deposits are uncommon. A depositional model of gravelly channel bars in the two rivers is presented and reveals that the architecture is made up of depositional styles similar to those observed in braiding-river successions, although the sedimentary packages are preserved in different proportions. Differences in braiding-river and wandering-river sedimentology largely reflect the relatively frequent migration of channels and bars in braiding rivers, which preserve high proportions of channel fill and chute fill, and confluence scourand-fill deposits. Conversely, the moderately stable island and channel network in wandering rivers limits channel shifting and consequently preserves a low number of channel fills. Moderate proportions of slipface strata and coherent patterns of sand deposition along bar tops provide evidence of comparatively uniform flow patterns in independent channel segments divided by islands. These patterns of bar sedimentation and channel shifting preserved in the alluvial architecture appear to be signature characteristics of wandering rivers. The occurrence of similar architecture in both the Fraser and Squamish rivers suggests that the model likely applies to most wandering gravel-bed rivers. INTRODUCTION The processes and dynamics of river behavior in wandering gravel-bed rivers have become better understood in recent years, but the subsurface sedimentary architecture (from channel base to bar-top deposits) has not * Present address: EnCana Corporation, 421 7th Avenue SW, P.O. Box 2850, Calgary, Alberta, T2P 2S5, Canada Copyright E 2005, SEPM (Society for Sedimentary Geology) been directly linked to formative flow processes and patterns of sediment erosion and deposition. Work by Roberts et al. (1997) using groundpenetrating radar (GPR) to image gravelly channel and floodplain subsurface structures in a modern wandering river, as well as Passmore and Macklin’s (2000) description of the alluvial architecture exposed in cut banks of late Holocene wandering-river deposits, has given some insights into the architecture of wandering gravel-bed rivers. In both cases, however, patterns of sediment transport and bar evolution were inferred and not directly known. For the purposes of this study, alluvial architecture is the geometry, proportion, and spatial distribution of fluvial sediments within a storey (or storeys). A storey is a deposit of a single channel bar and adjacent channel fill (Bridge and Mackey 1993). Although the genetic classification of channel bars is still unresolved (e.g., Best et al. 2003), compound bars are complex sedimentary bodies that have been erosionally reworked and are made up of amalgamated unit bars and/or compound bars, whereas unit bars are simple depositional barforms that internally show unmodified depositional sedimentary structures (N.D. Smith 1974). This study links the alluvial architecture of channel bars to patterns of sediment transport and bar evolution over the previous 50 years in the Fraser and Squamish rivers, British Columbia, Canada. Radar facies and subsurface structures were imaged with GPR, and the migration history and bed topography of barforms and channels were mapped and interpreted from aerial photographs and bathymetry. Recent studies in sand-bed and gravel-bed rivers have used similar methodologies to elucidate patterns of channel migration and the deposits of braid bars, which have informed and refined models of bar sedimentation in braiding rivers (Best et al. 2003; Lunt et al. 2004). Differences in patterns of bar evolution, bar sedimentation, and proportions of strata preserved in braiding-river and wandering-river alluvial architecture led to the development of a model of gravelly sedimentation on channel bars. Our model provides the complement to sandy facies models in wandering gravel-bed rivers that describe the sedimentology and facies assemblages of sandy deposits preserved in floodplain and channel bar-top sediments (Desloges and Church 1987; Brierley 1989, 1991a, 1991b). STUDY AREA AND METHODS Study Sites The Fraser and Squamish rivers flow through the Coast Mountain Range as wandering gravel-bed rivers for a portion of their length (Table 1) prior to making an abrupt transition to meandering rivers before they discharge into the Strait of Georgia and Howe Sound in southwest British Columbia (Figs. 1, 2A). Two compound bars situated in these wandering gravel-bed reaches are the study sites. In the Squamish River, bank-attached TFLENT Bar (Tree Farm License Entrance; Fig. 2A, B), 1.1 km long and up to 400 m wide, is located 2.1 km upstream of the Ashlu River confluence at 34 to 35 m above sea level. In the Fraser River, Wellington Bar (Fig. 2C, D) is a mid-channel island 1527-1404/05/075-844/$03.00 JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS TABLE 1.— Fraser and Squamish River statistics. River Drainage Area (km2) Mean Annual Flow (m3 s21) Flood of Record (m3 s21) Wandering Reach (km) Mean Width (m) Mean Depth (m) Slope Surface D16 (mm) Surface D50 (mm) Surface D84 (mm) Fraser Squamish 250 000 3 4101 , 17 2002 55 5123 6.63 0.000184 5–84 11–184 17–294 3 600 2395 2 6106 10.5 , 707 , 2.57 0.00158 25–388 55–808 93–1388 1 at Mission (Water Survey of Canada Station 08MH024); 229 May 1894; 3at Agassiz (McLean et al. 1999); 4at Wellington Bar (McLean et al. 1999); 5at Brackendale (Water Survey of Canada Station 08GA022); 68 October 1984; 7at Ashlu Bridge; 8at TFLENT Bar (Brierley and Hickin 1985). about 1.1 km wide and 3.2 km long, positioned 6.6 km downstream of the Harrison River confluence at 7 to 8 m above sea level. Aerial Photographs Channel bars, vegetation, and channel boundaries were mapped from rectified aerial photographs to identify patterns of channel and bar change from 1943 to 2001 using ArcView software. The photographs were rectified digitally using ER Mapper software (Leys and Werritty 1999). Analyzed photographs were taken at unevenly spaced intervals of ten years or less in which discharge was relatively low and bars were emergent. Channel-Bar Topography and Bathymetry Bar-top surfaces were surveyed at each site using a level and stadia rod to topographically correct the GPR profiles. The topographic surveys were tied to UTM coordinates through GPS surveys, to align the GPR grids with the bathymetry, and to position the grids on rectified aerial photographs. Channel-bottom surveys of the Squamish River were completed in 2001 using a Lowrance X-16 chart recorder mounted on a 3-m-long inflatable boat. Channel-bottom surveys of the Fraser River were completed in 1952, 1984, and 1999 (McLean 1990; McLean and Church 1999). 845 Ground-Penetrating Radar Ground-penetrating radar enabled the examination of the entire bar thickness, from channel base to bar top. In contrast, trenching provided incomplete data because high water tables limited sedimentary analysis to bar-top deposits, which are a small proportion of the bar thickness. A pulseEKKO IV GPR system was used with a 400 V transmitter. Three antenna frequencies (200, 100, and 50 MHz) were used at each bar to vary the depth of penetration and image resolution of subsurface structures. The 100 and 50 MHz antennas imaged sedimentary structures to depths of about 15 and 25 m, respectively, whereas 200 MHz radar profiles were limited to 10 m. Approximately 1 km of ground was profiled in rectilinear grids (, 200 m 3 , 200 m) on each bar in October 2000. The antennas were oriented perpendicular to the direction of travel at fixed trace spacings with constant separation between antennas (Table 2). An average velocity of 0.085 m ns21 was calculated from common-midpoint surveys at each site. The radar profiles were processed for: time-zero adjustment, dewow to correct for signal saturation, 7-point down-trace and 2 trace-to-trace averaging to improve the signal-to-noise ratio, and automatic gain control to compensate for signal attenuation with depth. All traces were stacked 128 times. Radar Interpretation Radar facies and radar elements were identified using the principles of radar stratigraphy (Beres and Haeni 1991; Jol and Smith 1991) derived from seismic stratigraphy (Mitchum et al. 1977). Radar facies are mappable, 3D units composed of reflections whose internal reflection configuration (e.g., shape, orientation), continuity, amplitude, polarity, spacing, and external 3D geometry differ from adjacent units. Radar elements combine closely related facies into facies associations, and are characterized by the facies assemblage, their external form, and internal geometry (cf. Allen’s (1983) architectural elements). To avoid frequency-dependent radar interpretations (Woodward et al. 2003), we imaged the subsurface with three antenna frequencies to allow GPR profiles of the same strata to be interpreted at three different scales. This process of interpretive redundancy aids in identifying facies and the subsurface architecture. Genetic interpretations of radar facies and elements are derived from observations of bedform and barform morphology, geometry, sediment organization seen in cut-bank exposures and trenched sections, patterns FIG. 1.— Location of TFLENT Bar (TB) and Wellington Bar (WB) sites in the Squamish and Fraser rivers, British Columbia. 846 C.L. WOOLDRIDGE AND E.J. HICKIN JSR FIG. 2.—Aerial views of the channel bar sites. A) TFLENT Bar (TB) in the wandering reach upstream of the abrupt transition to a meandering planform, Squamish River. B) GPR grid imaged on TFLENT Bar. The thicker line segments show the locations of the GPR profiles shown in Figures 6 and 7. C) Wellington Bar (WB) and other islands dividing flow in the Fraser River. D) GPR grid imaged on Wellington Bar. The thicker line segments show the locations of the GPR profiles shown in Figures 10, 11, and 12. British Columbia Government photographs (A) 5012:244, (B) 96036:19, and (C) 99001:22,29. Photograph (D) SRS6164:90 courtesy of the Department of Geography, UBC. of bar change mapped from aerial photographs, scour-and-fill patterns, and channel-bed topography mapped from bathymetry. Reflection orientations are stated in reference to mean flow directions with respect to the orientation of the channel belt (river-scale flow, not local flow). All dip angles reported are apparent dip angles. MORPHOLOGY AND SEDIMENTATION IN WANDERING GRAVEL-BED RIVERS Wandering rivers have a morphology that is intermediate between braiding and meandering with multiple channels separated by islands (islands are vegetated bars; Neill 1973; Lewis and McDonald 1973; Kellerhals et al. 1976) rather than channel division by bars common to TABLE 2.— GPR parameters. Frequency, f (MHz) Trace Spacing (m) Antenna Separation (m) Sampling Rate (ps) 200 100 50 0.10 0.25 0.50 0.5 1.0 2.0 800 800 1600 braiding rivers. Wandering rivers have an irregularly sinuous, single dominant channel with one or two secondary channels that have reduced velocities and carry reduced flow volumes. This channel arrangement, along with moderate bedload transport rates, leads to the stabilization of mid-channel bars and their subsequent evolution to islands (Morningstar 1987; Desloges and Church 1989). The planform of wandering gravel-bed rivers contrasts with that of braiding rivers, where pervasive channel splitting leads to individual threads of flow shifting frequently as bar deposition and erosion occur (Ashmore 1991). In wandering rivers portions of compound bars are actively reshaped every year, but numerous bars and islands have persisted for more than 50 years and are deemed to be stable channel elements (with many of them formally named in both rivers). The reoccupation and excavation of preexisting channels is likely more frequent than the development of new channels in wandering rivers (Gottesfeld and Johnson Gottesfeld 1990). Wandering rivers tend to develop laterally unstable ‘‘sedimentation zones’’ in which channel-bar growth, bank erosion, and bar shifting occurs (Church 1983). Eroded sediment is transported through stable channel zones and deposited downstream in another sedimentation zone, initiating channel instabilities and bar growth (Desloges and Church 1987; McLean and Church 1999; McLean et al. 1999). JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 847 FIG. 3.— Typical barforms, bedforms, and surface features of wandering gravel-bed Fraser and Squamish rivers. A) Definitions of barforms and channel types in wandering rivers, Squamish River. B) Slipface margin (1 m thick) of a kilometer-scale unit bar, Wellington Bar, Fraser River. Note sandy dunes and ripples in foreground. C) Bedload sheet (0.10 to 0.15 m thick) prograding over an older, armored bar surface, Fraser River. Hand trowel (0.15 m high) is circled for scale. Photograph by D.G. Ham. D) Island cut-bank exposure with gravels (deposited by bedload sheets) encased in horizontally bedded and structureless bar-top sands overlying crudely, horizontally bedded sandy gravels, Fraser River. 0.3 m ruler is circled for scale. Photograph by M.C. Roberts. E) Logjam instigating scour-hole development, TFLENT Bar, Squamish River. The magnitude of channel-forming discharge is important in governing bar formation because gravel transport appears to increase exponentially above the critical threshold of motion for gravel (McLean et al. 1999). The two rivers have different hydrologic regimes, and in consequence, contrasting bed-material transport regimes. In the Squamish River, sediment is episodically mobilized by high-magnitude, short-duration, autumn rains (events that last on the order of days). In the Fraser River, the annual, early summer snowmelt flood transports bedload for a number of weeks. Figure 3A shows morphological features common to wandering rivers including unit bars (Fig. 3B), and gravelly bedload sheets on bar surfaces (Fig. 3C) and encased in bar-top sands (Fig. 3D). Bar surfaces are usually armored with a layer of open-framework gravels (Fig. 3C), but subsurface deposits are matrix-filled sandy gravels (Fig. 3D). The surface and 848 C.L. WOOLDRIDGE AND E.J. HICKIN subsurface gravel clasts are similar in size (McLean et al. 1999). Channel sands (1 to 3 m thick) overlie the gravelly deposits (Fig. 3D) on the highest portion of the bars and are colonized by alder and cottonwood (Boniface 1985). Surface deposits of sand (dunes and ripples, Fig. 3B) are also found on lower bar areas, such as on the downstream margins of bars, in cross-bar chutes, along secondary channels, and in sloughs. Chutes are shallow channels (up to 3 m deep) that cut across bar interiors, and sloughs are abandoned channels that receive moderate- and high-stage flows (Fig. 3A). Secondary channels are either former main channels that are still active or chutes that were excavated but never occupied by the main channel (Fig. 3A). Logjams (Fig. 3E) affect sediment transfer and influence bar growth in the Squamish River (Hickin 1984), but they are not as important in governing bar growth in the much larger Fraser River. EVOLUTION AND STRATIGRAPHY OF TFLENT BAR, SQUAMISH RIVER Evolution of TFLENT Bar TFLENT Bar evolved from an assemblage of small mid-channel islands and bars separated by multiple channels in 1951, to a large bankattached compound bar in a single-thread channel (with subordinate sloughs) by the 1970s (Fig. 4). In 1984, the flood of record eroded islands in the reach and locally deposited gravel onto the bar head but did not immediately reorganize the reach (Hickin and Sichingabula 1988). Subsequently, however, the enlarged bar head forced the thalweg against the western bank, instigating bank retreat over the next decade (Fig. 4). During this time, two unit bars (labeled 1 and 2 in the 1996 panel of Fig. 4) accreted to the western edge of TFLENT Bar. Unit bar 1 was deposited by 1994, and between 1994 and 1996 unit bar 2 (the site of GPR profiling) aggraded vertically to become a prominent barform. After 1996, the growth of unit bar 2 deflected formative flows away from unit bar 1, enabling a dense cover of 3-m-high willow to become established on unit bar 1 by 2000. Willow was beginning to colonize the surface of unit bar 2 by 2000 as well. Radar Facies and Elements of TFLENT Bar The GPR grid imaged on TFLENT Bar is shown in Figure 2B, with two GPR lines displayed in Figures 6 and 7. The interpretation of the stratigraphy is uniquely derived from the grid of GPR profiles, bathymetry (Fig. 5), and aerial photographs (Fig. 4). Radar Facies 1: Subhorizontal, Continuous, Subparallel Reflections Radar facies 1 is characterized by stacked (1 to 5 m thick), horizontal to subhorizontal, continuous (20 to 150 m long), parallel to subparallel reflections (Figs. 6, 7). The facies is preserved at all levels within the stratigraphy, and in some cases its ubiquitous extent dominates the succession. Reflections can be traced in both flow-parallel (Fig. 6) and flow-normal (Fig. 7) GPR profiles because their subhorizontal, subparallel character is three-dimensional. Interpretation: Vertical-Accretion Deposits.—Radar facies 1 is interpreted as vertically accreted gravelly sheets deposited from bedload sheets migrating across bar surfaces and channel floors. Bedload sheets (Whiting et al. 1988; Livesey et al. 1998), formerly described as diffuse gravel sheets by Hein and Walker (1977), are low-amplitude, relatively planar features, up to 0.2 m thick (Fig. 3C), composed of ‘‘normally loose’’ sediment (Church 1978). Individual sheets may be imaged in 200 MHz profiles (Fig. 7) because their thickness is roughly equivalent to the vertical JSR resolution of the reflections. The subparallel nature of the facies is probably due to the intermittent nature of bedload transport whereby bedload sheets overtake and bury other stalled sheets (Ashmore 1991), creating subdued topographic relief. Sheets are commonly preserved on bar-top surfaces (Ferguson and Werritty 1983) because, during falling stages as discharge drops below the threshold of motion for gravel, flows are not competent to rework the sediments, stranding the sheets on bar tops. The facies occurs at all stratigraphic levels because it likely represents the dominant process of deposition in these and other wandering gravel-bed rivers. Radar Facies 3: Low-Angle (2 to 8u ), Downstream-Dipping, Subparallel Reflections Radar facies 3 is characterized by low-angle (2 to 8u), offlapping, divergent to subparallel reflections (1 to 7 m thick, 25 to . 200 m long) that dip downflow (Fig. 6). Extensive sets of downlapping tangential reflections occur at all stratigraphic levels. The flow-normal radar image is made up of subhorizontal and undulose reflections (Fig. 7). Interpretation: Downstream-Accretion Deposits.—Radar facies 3 is interpreted as downstream-accretion deposits in which bedload sheets are deposited on the downstream margins of bars (Fig. 6). The facies documents the migration of gravelly bedload sheets over bar tops or along channel floors onto downstream bar margins, causing barforms to aggrade and translate downstream. The subhorizontal and undulose flownormal reflections indicate that sedimentation is influenced by mean flow conditions depositing extensive sheet-like strata, as well as by more variable local flow conditions depositing larger low-relief bedforms (Livesey et al. 1998). Radar Facies 5: Small- to Medium-Scale (0.5 to 3 m), Steeply Inclined (13 to 26u ), Oblique Reflections Radar facies 5 is characterized by continuous sets (10 to . 90 m long) of small- to medium-scale (0.5 to 3 m thick), roughly parallel, steeply inclined (13 to 26u ), oblique reflections that dip downflow (Fig. 6) and normal to flow (Fig. 7). The facies is typically found in packages that are bounded above and below by radar facies 1 and in some instances the packages thicken downflow. The facies is found at all stratigraphic levels, but it rarely dominates the entire succession. Interpretation: Bar-Margin Slipface Deposits.—Radar facies 5 is interpreted as bar-margin slipface deposits indicative of gravelly bedload sheets periodically avalanching and passing over high-relief bar margins into deeper water at high-stage flows. Bar margins are typically oriented downflow, but in many cases flow crosses the bar top obliquely, causing the barform to prograde cross-flow and downstream. Changes in flow competency during bar formation cause the angle of the slipfaces to vary, such that the angle typically declines downstream along a set of slipfaces. Reactivation and erosion of bar margins during falling stages reduces the angle of the slipface, as does sand deposition on the slipface and at its toe when discharge falls below the threshold of motion for gravel. Radar Facies 6: Large-Scale (4 to . 10 m), Steeply Inclined (11 to 28u ), Oblique Reflections Radar facies 6 is characterized by large-scale (4 to . 10 m thick), parallel, steeply inclined (11 to 28u ), oblique reflections that dip downvalley and in some cases can be traced up-dip into horizontal R FIG. 4.—Evolution (1951 to 1996) of TFLENT Bar, Squamish River. Changes in channel position and bar morphology superimposed on GPR grid (dashed box) and 1996 British Columbia Government photograph 96036:19, Q (discharge) 5 364 cumecs (m3 s21). Flow is to the bottom of the page. JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 849 850 C.L. WOOLDRIDGE AND E.J. HICKIN JSR FIG. 5.—Bathymetry across TFLENT Bar, Squamish River. Sounding locations are shown in the photograph. A) Sounding across a riffle in a straight reach adjacent to GPR line (Fig. 7) along the bar top. B) Cross-channel sounding across a subaqueous mid-channel bar. C) Sounding along a steeply dipping (19u) 4-m-deep scour hole. D) Cross-channel sounding across the scour (dipping 10 to 13.5u). 0.0 m depth is the water level at low stage flow (discharge < 50 m3 s21). FIG. 6.—GPR profile (100 MHz), TFLENT Bar, Squamish River. A) Flow-parallel profile. B) Interpreted profile. Radar facies 1 (vertical accretion), 3 (downstream accretion), 5 (slipface accretion), 6 (delta foresets and topsets), and radar elements Ia (channel deposits) and I(3) (chute filled with downstream-accretion deposits) are delineated. Storeys 1 and 2 record the thickness of the Holocene channel-belt deposits. Intersection with Figure 7 is shown. JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 851 FIG. 7.— GPR profile (200 MHz), TFLENT Bar, Squamish River. A) Flow-normal profile. B) Interpreted profile. Radar facies 1 (vertical accretion), 5 (slipface accretion), 6 (delta foresets and topsets), and radar elements I (chute deposits) and I(3) (chute filled with downstream-accretion deposits) are delineated. Intersection with Figure 6 is shown. reflections (Fig. 6). The flow-normal radar image is composed of subhorizontal reflections (Fig. 7). Interpretation: Delta Foreset (and Topset) Deposits.—Radar facies 6 is interpreted as delta foresets in which the inclined reflections dip close to the 25u angle of repose of gravel found on gravelly delta fronts (D.G. Smith 1991). The large-scale character of the deposit, both its thickness and widespread extent, suggests that local channel scouring did not produce the foresets. Also, the stratigraphic thickness of the facies varies from 4 to 10 m, but the top of the facies always occurs at , 200 ns (, 8.5 m depth), indicating progradation into a standing body of water. Further, some of the steeply dipping reflections can be traced up-dip into horizontal reflections suggestive of depositional topset strata (Jol and Smith 1991). Radar Element I: 2D, Bounding, Concave-Up Reflections Radar element I is distinguished by a 2D, basal, concave-up reflection that truncates adjacent reflections and is filled with a mix of subhorizontal and/or steeply inclined reflections (Figs. 6, 7). The element extends 30 to 45 m laterally, with depths approaching 4 m, and the concave-up edges dip 6 to 11u into the center of the form. There are few preserved forms, and they occur within the middle and upper portions of the bar stratigraphy. Interpretation: Channel and Chute Deposits.—Radar element I is interpreted as main-channel and chute-channel fills bounded by erosional concave-up channel edges. The chutes and main channels were filled by a number of depositional events during abandonment. Linking Radar Stratigraphy to the Evolution of TFLENT Bar A prominent undulating reflection , 9 m below the surface of the bar partitions the stratigraphic succession into two sedimentary bodies (Figs. 6, 7). Below 9 m, the radar stratigraphy images facies 6 with large-scale, steeply inclined reflections dipping downvalley that trace delta-front progradation into a flooded Squamish River valley in the late Pleistocene (Friele et al. 1999). The delta foresets are overlain by subhorizontal reflections recording the deposition of topset beds before relative sea level dropped and alluvial sedimentation commenced. Above 9 m, the radar stratigraphy images the alluvial fill, which we interpret to be made up of two storeys whose thicknesses are roughly equivalent to the depth of formative flows in the modern channel. The storeys are separated by bounding surfaces defined by systematic reflection terminations such as erosional truncation of the chute edges in storey 1 (radar element I(3) in Fig. 7), and the depositional onlap of slipface deposits between 120 and 140 m in storey 2 (radar facies 5 in Fig. 6). The pervasive extent of these bounding surfaces differentiates them from internal scour surfaces within a storey such as the erosional surface bounding the chute channel (radar element I(3) in Fig. 7) which locally scoured into the bar strata. Further evidence of this bipartite division is the preservation of the chute element I(3) in storey 1 and its absence in the photographic record (Fig. 4). The 3D geometry of the fill shows that the chute was filled with downstream-accretion deposits (radar facies 3), yet there is no evidence of a chute directing flow to the southwest between 1951 and 1996. Moreover, the floodplain occupied that location prior to 1951, indicating that storey 1 represents Holocene deposits laid down before 1951. In contrast, the GPR-imaged stratigraphy in storey 2 matches the depositional history traced in the photographic record and preserves the style of unit-bar sedimentation and history of compound-bar development. The channel form , 7 m below the bar surface (radar element Ia in Fig. 6) is the subsurface expression of the channel locally scouring into storey 1 in response to bar growth, which constricted the channel after the flood of 1984. The position of the buried paleochannel coincides with the western edge of the former floodplain up to 1967 and records a flow direction that matches the main channel across the site up to 1984. Subsequent channel shifting, bank erosion, and bar deposition after 1984 displaced the channel farther westward, forcing flow away from the site. The growth of unit bar 2 after 1984 (the uppermost 3 m of strata in storey 2) was achieved by bedload sheets migrating along the surface of the bar, which caused it to aggrade vertically and form high-relief bar margins. Sediment avalanched over and filled the lee of the bar margins, forcing the barform to prograde downstream and override the paleochannel. The contrasting modes of sedimentation within the storey illustrate the multidirectional growth of the larger compound bar. 852 C.L. WOOLDRIDGE AND E.J. HICKIN JSR JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 853 EVOLUTION AND STRATIGRAPHY OF WELLINGTON BAR, FRASER RIVER Evolution of Wellington Bar In 1943, Wellington Bar was a large, bank-attached, unvegetated bar (Fig. 8). A 100 year flood in 1948 eroded a chute channel across the bar head adjacent to the southeastern channel margin. By 1952 the chute had incised into the bar (evident in the 1952 bathymetry in Fig. 9A) and detached it from the bank, forcing it to a mid-channel position. During the next decade, high-stage flows excavated the chute and shifted the thalweg to the southeast channel. The avulsion reduced flow volumes and velocities in the former main channel, leading to the episodic deposition and erosion of several mid-channel bars and the accretion of sediment to the northwest margin of Wellington Bar by 1979. Continued bar growth from 1967 to 1993 broadened the bar head (the site of GPR profiling) and enabled vegetation to colonize the higher (more stable), central portion of Wellington Bar by 1986. The bar head vertically aggraded 2 m (between the 1984 and 1999 bathymetry in Fig. 9A), causing the tip of the bar to migrate upstream until 1999, where it remained relatively unchanged through 2001. Radar Facies and Elements of Wellington Bar The GPR grid imaged on Wellington Bar is shown in Figure 2D with three GPR lines displayed in Figures 10, 11, and 12. Radar Facies 2: Low-Angle (1 to 6u ), Cross-Stream Dipping, Subparallel Reflections Radar facies 2 is characterized by continuous sets (55 to . 150 m long, 1 to 8 m thick) of stacked, low angle (1 to 6u, typically 5 to 6u), parallel to subparallel reflections that dip normal to flow (Fig. 10). The facies makes up a minor portion of the alluvium on Wellington Bar, but on point bars it dominates the stratigraphy from channel base to bar top (Wooldridge 2002). The flow-parallel radar expression is made up of continuous, parallel, subhorizontal to horizontal reflections (Fig. 12). Interpretation: Lateral-Accretion Deposits.—Radar facies 2 is interpreted as lateral-accretion deposits that record the progressive onlap of gravelly bedload sheets onto bar surfaces, causing barforms to build out laterally into the channel. This interpretation is supported by the flownormal apparent dip angle of the reflections matching, and being coincident with the dip angle of the upper bar surface. Stalled bedload sheets on bar surfaces and the flow-parallel, subhorizontal reflections document the continuous nature of sheet-like sedimentation. Radar Facies 4: Low-Angle (, 1u ), Upstream-Dipping, Parallel Reflections Radar facies 4 is characterized by stacked (. 6 m thick), continuous (. 200 m long), low-angle (, 0.5u), parallel reflections that dip upstream (Fig. 11). The facies is found across the bar head, but it is limited to the upper portion of the stratigraphy. The reflections onlap an upstreamdipping lower bounding surface and hence the thickness of the radar facies increases upflow. The flow-normal radar profiles show subhorizontal reflections (Figs. 10, 12). Interpretation: Upstream-Accretion Deposits.—Radar facies 4 is interpreted as upstream-accretion deposits. The migration of gravelly bedload sheets onto an upstream-dipping bar surface causes the strata to dip upstream and the bar to accrete upstream. The apparent dip angle of the reflections is coincident with the , 1u upstream dip of the bar surface. FIG. 9.— Bathymetry across Wellington Bar, Fraser River. A) 1999, 1984, and 1952 soundings show typical wandering river cross-channel geometries. 0 m elevation is mean sea level. 1999 water level is at low stage flow (discharge 5 677 m3 s21). Extent of bar surface profiled with GPR is shown. Bathymetry courtesy of the Department of Geography, UBC. B) 1952 (British Columbia Government photograph 1622:24 taken 2 October 1952, discharge 5 2350 m3 s21), and C) 1999 sounding locations (NNNN) (British Columbia Government photographs 99001:22, 29 taken 20 March 1999, discharge 5 701 m3 s21). r FIG. 8.—Evolution (1943 to 1999) of Wellington Bar, Fraser River. Changes in channel position and bar morphology superimposed on GPR grid (dashed box) and 1999 British Columbia Government photograph 99001:29 taken 20 March 1999, Q (discharge) 5 701 cumecs (m3 s21). Flow is from top right to bottom left. 854 C.L. WOOLDRIDGE AND E.J. HICKIN JSR FIG. 10.— GPR profile (50 MHz), Wellington Bar, Fraser River. A) Flow-normal profile. B) Interpreted profile. Radar facies 1 (vertical accretion), 2 (lateral accretion), 4 (upstream accretion), 5 (slipface accretion), 6 (delta foresets), and radar elements II and IIa (scour and fill) are delineated. Storeys 1 and 2 record the thickness of the Holocene channel-belt deposits. Intersection with Figure 11 is shown. Radar Element II: 3D, Steep-Sided (5 to 19u), Scallop-Shaped Reflections Radar element II is bounded by 3D, scallop-shaped reflections. The steep-sided reflections dip , 7u downstream (Figs. 11, 12) and 5 to 18u cross-flow (Fig. 10) into the center of the element, which is up to 16 m thick, . 150 m long (parallel to flow), and . 180 m wide (normal to flow). The radar stratigraphy records two sets of reflection packages with the smaller set (radar element IIa, up to 6.5 m thick, 30 m long parallel to flow, and 50 m wide normal to flow) locally entrenched within the larger set (radar element II; Figs. 10, 12). Radar element IIa shows flow-parallel, scallop-shaped reflections dipping downstream between 13 and 19u (Fig. 12). The reflections truncate each other in a downstream direction at similar depths (, 13.5 m below bar surface) and are filled with reflections showing variable dip. Flow-normal profiles record symmetrical, troughshaped bounding reflections that dip , 18u into the center of the element and are filled with a set of steeply dipping reflections (Fig. 10). Interpretation: Scour-and-Fill Deposits.—Radar elements II and IIa are interpreted as scour-and-fill structures. The continuous, highamplitude bounding reflections are erosional scours that have truncated the adjacent strata. This interpretation is contingent on the threedimensional scallop-shaped bounding form of the element (Figs. 12, 5C, D), because in two dimensions the troughs appear as channel forms (Figs. 10, 5D). The scours are filled by strata that show changes in dip angle, a consequence of the unsteady process of sediment avalanching into the scours from both the upstream edge and the sides of the scours. FIG. 11.— GPR profile (50 MHz), Wellington Bar, Fraser River. A) Flow-parallel profile. B) Interpreted profile. Radar facies 1 (vertical accretion), 3 (downstream accretion), 4 (upstream accretion), 5 (slipface accretion), 6 (delta foresets), and radar element II (scour and fill) are delineated. Intersection with Figure 10 is shown. JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 855 FIG. 12.—Three-dimensional GPR profile (50 MHz), Wellington Bar, Fraser River. A) 3D profile. B) Interpreted profile. Radar facies 1 (vertical accretion), 2 (lateral accretion), 3 (downstream accretion), 4 (upstream accretion), 5 (slipface accretion), 6 (delta foresets), and radar elements II and IIa (scour and fill) are delineated. Linking Radar Stratigraphy to the Evolution of Wellington Bar A wavy reflection , 17 m below the surface of the bar separates deeper, low-amplitude radar reflections from high-amplitude reflections in the Wellington Bar stratigraphy (Figs. 10, 11, 12). Similarly to TFLENT Bar, the deeper stratigraphy shows steeply inclined foresets (radar facies 6) dipping downvalley, tracing the progradation of a delta front into a flooded Fraser Lowland in the late Pleistocene during deglaciation (Clague et al. 1982). As alluvial sedimentation commenced, the Fraser River incised into and partly eroded the delta foresets. The foresets were likely preserved here because of their mid-valley position where the river is unconfined (the channel belt is not impinging on hard points) and scour depths are limited. The upper 17 m of the stratigraphy is split into two storeys by a subhorizontal bounding surface about 8 m below the bar surface. The bounding surface erosionally truncates the scour-and-fill structures and corresponds very closely to the depth of the channel floor mapped in the 1952, 1984, and 1999 bathymetry (Fig. 9A). This suggests that the architecture of storey 2 (above 8 m depth) should match the evolution of the bar recorded in the photographic record (Fig. 8). The deposits in storey 2 are the product of flow divergence across the mid-channel bar head with gravelly bedload sheets laterally accreting to the northwest bar margin, depositing low-angle strata after the thalweg had switched to the southeast channel by 1967. As flow diverged across the southeast portion of the bar, it cut a high-relief bar margin over which sediments avalanched to form slipface deposits. Subsequent sheet-like deposition associated with an extensive (, 1 km wide) gravelly unit bar prograding over the bar between 1984 and 1999 caused the barform to grow upstream and laterally. Neither storey images the architecture of Wellington Bar when it was a bank-attached barform in 1943 (Fig. 8), so it is not possible to reconstruct the transition of the bank-attached bar to a mid-channel bar. Storey 1 is made up of nested scour-and-fill structures (radar elements II and IIa) eroded into downstream-accretion and slipface-accretion deposits. The smaller scours are locally entrenched into the fill of the larger scour and were likely formed in response to barforms migrating through and constricting a secondary channel, causing it to overdeepen. In contrast, the larger scour undoubtedly records the constriction of the main channel because it scales to modern scours that erode up to 16 m and extend across most of the width of the main channel. A MODEL OF GRAVELLY CHANNEL-BAR DEPOSITION IN WANDERING GRAVEL-BED RIVERS Figure 13 summarizes the geometries of the six radar facies and two radar elements identified in the TFLENT and Wellington Bar stratigraphies. These same facies and elements were observed at four other sites that we analyzed on two compound bars in the Fraser River, bankattached Queens Bar and a point bar, Calamity Bar (Wooldridge 2002). This suggests that the results presented here are representative of bars typically found in wandering rivers. The geometry, proportion, and spatial distribution of the facies and elements are used to construct a model of gravelly channel-bar deposition in the two wandering gravelbed rivers (Fig. 14). The model also depicts the hierarchical arrangement of bedload sheets, unit bars, and compound bars, along with morphologic features such as logjams, hard points (bedrock), chutes, sloughs, 856 C.L. WOOLDRIDGE AND E.J. HICKIN JSR JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS 857 FIG. 14.— A model of gravelly channel-bar deposition in the Fraser and Squamish wandering gravel-bed rivers based on the integration of GPR, bathymetry, and aerial photograph data. secondary and main channels, and bar-top sands colonized by vegetation common to wandering gravel-bed rivers. Sandy deposits were not examined or included in the model because channel-sand facies have already been described on TFLENT Bar, and in a wandering gravel-bed river facies model (Brierley 1989, 1991b). The model reflects our observation that vertical-accretion deposits constitute , 40% of channel-bar strata and thus dominate the architecture of wandering rivers. This pattern is also the most prevalent in the Roberts et al. (1997) GPR images of floodplain and channel-bar sediments in a wandering river. Observations of massive or crudely horizontally bedded cobble and gravelly strata in wandering-river cutbank exposures further suggest that vertical accretion is the prevailing mode of sediment deposition (Ferguson and Werritty 1983; Desloges and Church 1987; Passmore and Macklin 2000). The model shows that slipface deposits make up , 20% of bar deposits. Ferguson and Werritty (1983) note that well defined foresets are rare in cut-bank exposures, but Roberts et al. (1997) also imaged slipface deposits, indicating that high-relief bar margins occur on many bars but are not present on all bars. Lateral-accretion deposits constitute , 70% of point-bar deposits but only , 10% of the alluvial architecture. Downstream-accretion deposits make up the bulk of distal unit-bar sediments (, 60%), but only , 10% of the architecture. Roberts et al. (1997) also imaged thick sets of lateralaccretion and downstream-accretion deposits, although low-angle inclined strata (either lateral or downstream) are not commonly observed in cut-bank exposures. Partial and complete channel-fill deposits are preserved in the architecture (, 7%), and similar deposits have also been imaged in floodplain sediments (Roberts et al. 1997). The model synthesizes the first descriptions of upstream-accretion deposits, and scour-and-fill structures in wandering rivers. Upstreamaccretion deposits make up the smallest proportion of the architecture (, 4%), suggesting that this style of sedimentation is not a primary mechanism of bar growth. Scour-fill deposits are more commonly preserved in the architecture (, 7%), but the process of scouring does not appear to be widespread in wandering rivers. DISCUSSION The integration of GPR, bathymetry, and aerial photographs links largescale depositional processes and subsurface alluvial architecture for the first time in wandering gravel-bed rivers. Yet, questions remain regarding the uniqueness of wandering-river style. The multiple-channel character of wandering rivers identifies them as braiding rivers (Galay et al. 1998), although Nanson and Knighton (1996) refer to them as a type of anabranching river. Braiding rivers are defined by multichannel flow separated by bars, whereas anabranching rivers (of which wandering rivers are a specific type) are characterized by multiple channels divided by stable islands (up to bankfull stage). The division of flow around islands creates independent flow patterns in adjacent channels, unlike those in braiding rivers (Bridge 1993). The maintenance of stable islands reduces the frequency of flow bifurcation and convergence and preserves different proportions of sedimentary packages in braiding and wandering rivers. The nature of frequent channel shifting in braiding rivers preserves moderate amounts of complete and truncated, partial channel fills in the sedimentary record (Ramos and Sopena 1983; Vandenberghe and van Overmeeren 1999; Ekes 2000). This contrasts with wandering rivers, where the infrequent occurrence of channel avulsions does not permit many channels to be abandoned, filled, and potentially preserved within the alluvial architecture. Similarly, the excavation and reoccupation of abandoned channels also limits opportunities to preserve channel fills. The deepest scours in wandering rivers (excluding scours caused by bedrock impingements) occur adjacent to recently accreted unit bars that have constricted the channel. Scouring deepens channel floors, before r FIG. 13.— Radar configurations in the wandering gravel-bed Squamish and Fraser rivers. Radar facies 1 to 5 and radar elements I and II are typical channel-bar reflection configurations. Radar facies 6 images deltaic sedimentation. The facies are depicted at different antenna frequencies because these reflection profiles provide the clearest images of the structures. 858 JSR C.L. WOOLDRIDGE AND E.J. HICKIN flow laterally erodes opposing channel banks or island margins. Scour deposits make up a smaller proportion of the architecture, because of the infrequent accretion of unit bars. Confluence scours are not a significant depositional mechanism, because island stability maintains established channel networks and limits channel avulsions and bar migration. In braiding rivers, however, rapid bar migration and frequent channel shifting form numerous confluence scours across the channel belt (Best 1987; Bridge 1993). The scour-fill deposits are preserved in the architecture (Siegenthaler and Huggenberger 1993), unlike other facies that are reworked as barforms are mobilized (Salter 1993; Best and Ashworth 1997). Lateral accretion is locally significant in directing point-bar growth and the enlargement of mid-channel bars in both wandering and braiding rivers (N.D. Smith 1974; Ori 1982; Ramos and Sopena 1983; Ashmore 1991). It is a significant and widespread depositional style in braiding rivers with slowly migrating bars (Lunt et al. 2004). The accretion of sediment onto gently dipping bar-tail margins is important in directing down-bar growth during some phases of bar migration (S.A. Smith 1990; Lunt et al. 2004). Of less importance in these rivers is the case where flow expansion across a bar head enables unit bars to accrete onto the bar head, which in turn forces the upstream translation of the compound bar. This style of bar formation is not commonly preserved in wandering-river or braiding-river successions (Lunt and Bridge 2004). The majority of bar deposits in wandering river successions are made up of horizontal, parallel, continuous strata (vertical-accretion deposits). This has also been observed in gravelly deposits of ancient and modern braiding rivers, indicating that gravels are generally transported as individual bedload sheets in these rivers (N.D. Smith 1974; Hein and Walker 1977; Ramos and Sopena 1983; S.A. Smith 1990). Trough crossstrata deposited by gravel dunes typically constitute a very small proportion of braiding-river architecture (Morison and Hein 1987; S.A. Smith 1990), although the work of Lunt et al. (2004) provides an exception; they suggest a model of braid-bar sedimentation in which trough cross-strata make up a high proportion of the architecture, with planar strata rarely preserved. The moderate abundance of continuous sets of slipface deposits in wandering rivers is much greater than the limited sets typically observed in braiding-river deposits (Eynon and Walker 1974; N.D. Smith 1974; Steel and Thompson 1983; Ramos and Sopena 1983; Morison and Hein 1987; S.A. Smith 1990; Lunt et al. 2004). Extensive sets of continuous slipfaces are not formed in braiding rivers because frequently changing flow and sediment transport in developing flow expansions interrupts the unidirectional growth of bars (Ashmore 1991). In contrast, stable bars and islands in wandering rivers hinder flow expansion and maintain the directionality of flow and routing of sediment to bar edges and slipfaces. The spatial distribution of bar-top sand deposits described by Brierley (1991a, 1991b) also reveals this differential pattern of sediment deposition. In braiding reaches, sandy facies assemblages are complexly arranged across bar tops in highly discontinuous patterns. Chutes and channels adjusting their orientation at different flow stages deposit variable thicknesses of sand atop dissected and irregular gravel-bar surfaces. The same sandy facies assemblages are observed in wandering reaches, except that the sands are deposited in consistent down-bar trends but irregular across-bar trends. The relative stability of the channels directs more uniform flow orientations depositing sands atop gravelbar surfaces that have not been dissected or incised into by chute channels. These patterns of bar sedimentation demonstrate that compound bars in braiding rivers evolve and respond to changes in local flow conditions, while compound bars in wandering rivers react to decadal changes in river-scale flow and/or sediment input (Church and Jones 1982; Church 1983). Correspondingly, large floods affect the stability of bars and channels in wandering rivers, but floods do not produce immediate largescale changes such as channel avulsions. Rather, unit bars attached to compound bars deflect flood-stage flows onto the edges of bars and banks and erode small portions of the bars and banks, creating small-scale channel changes. These changes are amplified by successive flows transporting pulses of eroded sediment onto well-defined depositional zones (sites of unit-bar attachment), which ultimately cause channel shifting and bar growth. While our model points to key differences between braiding and wandering rivers, it also illustrates that concepts proven in braiding rivers are also present in wandering ones. The idea that styles of bar sedimentation occur independently of river size (scale independence) has recently been demonstrated in the architecture of sand-bed and gravel-bed braiding rivers (Best et al. 2003; Lunt et al. 2004). Scale independence is also observed in these wandering rivers, in which the same depositional styles are preserved in the Squamish River and in the much larger Fraser River. Further, the geometry and type of facies preserved in the lower storeys match those observed in the upper storeys, indicating that the style of bar sedimentation and channel depths have not changed in either river during the deposition of the two storeys. The unequal size of the two wandering rivers is evident in the higher stacking density of facies in the Squamish River architecture. Shorter sets of facies indicate narrower channel and channel-belt widths, while thinner facies point to shallower channels and lesser scour depths in the Squamish River (, 7 m) than the Fraser River (. 16 m). The depth of scour has also been found to control the geometry and stacking of channel and bar deposits in the architecture of gravelly braiding rivers (Ashworth et al. 1999). CONCLUSIONS This study has presented detailed descriptions of all of the radar facies and elements imaged in the alluvial architecture of the Fraser and Squamish rivers. For the first time the internal structure of gravelly sediments in wandering gravel-bed rivers has been linked to patterns of deposition that control the evolution of barforms. It has been shown that the evolutionary history of bars is fully revealed only if their study integrates GPR profiles, bathymetry, and aerial photographs in order to synthesize complete descriptions of bar architecture. A depositional model of gravelly channel bars in the two rivers is presented and reveals that the architecture is made up of depositional styles similar to those observed in braiding-river successions, although there are some important differences as well. In both river types, upstream-accretion deposits are uncommon, lateral-accretion dominates point-bar deposits, downstreamaccretion deposits govern some phases of down-bar growth, and verticalaccretion deposits formed by the deposition of gravelly bedload sheets are the most prevalent strata. Differences in the sedimentology of braiding rivers and wandering rivers largely reflect the relatively frequent migration of channels and bars in braiding rivers, which preserve high proportions of channel and chute fill, and confluence scour-and-fill deposits. The network of moderately stable islands and channels in wandering rivers, on the other hand, limits channel shifting and consequently preserves a low number of channel fills. Moderate proportions of slipface strata and coherent patterns of sand deposition along bar tops in wandering rivers provide evidence of comparatively uniform flow orientations directing the attachment of gravelly unit bars onto the margins of compound bars. These patterns of bar sedimentation and channel shifting preserved in the alluvial architecture appear to be signature characteristics of wandering rivers. The occurrence of similar architecture in both the Fraser and Squamish rivers suggests that the model likely applies to most wandering gravel-bed rivers, but further testing is needed to confirm this wider application. JSR CHANNEL BARS IN WANDERING GRAVEL-BED RIVERS ACKNOWLEDGMENTS We thank the reviewers for their constructive critiques. Sean Todd, Joel Epp, Sarah Baines, Solvej Patschke, Scott Babakaiff, Matt Cleary, Tim Johnsen, Quinn Jordan-Knox, Channa Pelpola, Crystal Huscroft, and Greg Matsuo are gratefully thanked for providing assistance in the field. Fraser River bathymetry was graciously supplied by Dr. M. Church from the Department of Geography, University of British Columbia. The project was partly funded by the Natural Sciences and Engineering Research Council of Canada and the Association of Professional Engineers and Geoscientists of British Columbia. 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