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Final Report 3-39 September 2000 shoreward to streamward side at the downstream end of the bar. The topography of a bar is more complex where there are more chute channels. At some sites and in some years, secondary bars become attached to the shoreward margins of these compound bars. At the downstream end of most compound bars, chute channels may converge into one persistent and deep secondary channel that separates the downstream end of the compound bar from the floodplain. The remainder of the bars are composed of broad, level platforms and linear ridges that may be partly vegetated. As flow recedes from the annual peak discharge, higher-elevation parts of the bar platform are exposed and small areas of separated flow develop in the lee of these islands. At these discharges, chute channels actively transport sediment. Upon further recession of flow, chute channels at the upstream end of the compound bar become exposed, and flow in the secondary channel ceases. Thereafter, the secondary channel becomes an area of mostly stagnant water. These low-velocity areas (backwaters) provide important nursery habitats for larval fish, especially the Colorado pikeminnow (Section 4.2). 3.6.1.2 Canyon Reaches with Abundant Debris Fans Canyons consist of relati vely straight sections of river with resistant geology on both sides of the river. Debris fans are areas of coarse sediment deposits at the mouths of tributaries; these sediments are delivered to the main channel during high-flow events in tributaries. In canyons, debris fans form a sequence of conditions including: (1) a slack-water area upstream from the debris fan, (2) a channel constriction at the debris fan, (3) an eddy or eddies and associated bars in the expansion area downstream from the fan, and (4) a downstream gravel bar (Figure 3.15; Schmidt and Rubin 1995). These "debris fan-eddy complexes" exist at the mouths of nearly all debris-flow-generating tributaries. Downstream of Flaming Gorge Dam, canyons with abundant debris fans include Lodore Canyon (Reach 1), Whirlpool and Split Mountain Canyons (Reach 2), and Gray and Desolation Canyons (Reach 3). Longitudinal profile, channel geometry, and the occurrence of rapids within canyons are strongly influenced by tributary-fan frequency. The bankfull channel width-to-depth ratio is smaller and the gradient is steepest in reaches with the highest fan frequency; all rapids are caused by debris fans or the gravel bars below debris fans that are composed of reworked debris-fan material. Expansion gravel bars are the other element of coarse-grained alluvial deposits in debris-fan dominated canyons. These bars are located in the flow-expansion zone downstream from debris-fan eddies where wider channel conditions resume (Grams and Schmidt 1999). Debris fans in Desolation Canyon (Reach 3) are large and at low elevation. Only the small active portion of the fan delivers sediment that restricts flow and causes rapids and eddies in the modern channel, whereas the main portion of the debris fan is so large that it acts more like a meander bend as the river flows around the fan (Orchard and Schmidt 2000).