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8 BIOLOGICAL REPORT 24 <br /> <br />•ti:ti <br />r. <br />OOOOOW <br />z 4#1 Z:.* AI <br />Island Bar <br />Alluvium L_ I Water <br />i Bedrock 1-1 <br />rp Coarse Sediment Fine Sediment <br />-? Surface Flow -01 ? Sub-surface Flow <br />Fig. 2. Schematic representation of geomorphic processes that form low velocity habitats in constrained (canyon, <br />top panel) and unconstrained alluvial (floodplain, bottom panel) reaches of the Upper Colorado River Basin where <br />endangered fishes are routinely found. In both panels the current condition is baseflow. In the top panel a <br />wall-based channel formed during a higher flow period, creating an eddy that persists and causes deposition of <br />fine sediment in the backwater at the downstream end of the channel. Declining flows from the preceding high <br />discharge period also increased the velocity of water draining across the point bar, thereby leaving clean, coarse <br />cobble. In the bottom panel a midchannel or island bar and a back-bar channel were built during high flow, <br />allowing low velocity habitats to form on the downstream ends. Chute channels of clean cobble formed on the <br />steep, downstream edge of the island bar, as velocity increased with declining volume of flow over the bar. At <br />baseflows, fine sediments are deposited on the aggraded portion of the bar front in relation to river stage. The <br />back-bar channel and point bar function similarly to the wall-based channel. In all cases river water penetrates <br />the alluvium at the upstream end of the bar creating interstitial, subsurface flow that discharges into the low <br />velocity environments and the river as change in elevations reverses the piezometric (downward) gradient to <br />the water table. Hence, habitats used by endangered fishes are dynamic in time and space and are controlled <br />by sediment supply and size, channel morphometry (especially slope and relative constraint by bedrock), and <br />the volume and duration of the previous peak flow events (developed from Tyus 1984, Harvey et al. 1993, and <br />discussions with Jack Schmidt, Utah State University, Logan).