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D Bedrock o Alluvium m Water flm ~ Coarse Sediment II" i .., F ne Sediment --. Surface Flow - - ~ Sub-surface Flow Figure 2. Schematic representation of geomorphic processes that form low velocity habitats in constrained (canyon, top panel) and unconstrained alluvial (floodplain, bottom panel) reaches of the Upper Colorado River Basin where endangered fishes are routinely found. In both panels the current condition is baseflow. In the top panel a wall-based channel formed during a higher flow period, creating an eddy which persists and causes deposition of fine sediment in the backwater at the downstream end of the channel. Declining flows from the preceding high discharge period also increased the velocity of water draining across the point bar thereby leaving clean, coarse cobble. In the bottom panel a midchannel or island bar and a back-bar channel were built during high flow, allowing low velocity habitats to form on the downstream ends. Chute channels of clean cobble formed on the steep, downstream edge of the island bar, as velocity increased with declining volume of flow over the bar. At baseflows, fine sediments are deposited on the aggraded portion of the bar front in relation to river stage. The back-bar channel and point bar function similarly to the wall- based channel. In all cases river water penetrates the alluvium at the upstream end of the bar creating interstitial, subsurface flow that discharges into the low velocity environments and the river as change in elevations reverses the piezometric (downward) gradient to the water table. Hence, habitats used by endangered fishes are dynamic in time and space and are controlled by sediment supply and size, channel morphometry (especially slope and relative constraint by bedrock) and the volume and duration of the previous peak flow events (developed from Tyus 1984, Harvey et al. in press and discussions with Jack Schmidt, Utah State University, Logan, UT). 11