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<br />280 <br /> <br />Y.K. CONVERSE ET AL. <br /> <br />In this study, the association with cover is stronger than those with water velocity or depth in channel <br />margins. However, addressing the biological basis for the strong association with cover was beyond the <br />scope of this study. We suspect that the presence of cover affects local conditions of water depth and <br />velocity in channel margins. However, the distance of 2.5 m used in this study as a boundary for habitat <br />measurements along channel margins may have been too broad to detect associations between fish and <br />specific conditions of depth and velocity. Small fish are probably responding to conditions within 0.5-1 <br />m of the water's edge. If so, a stronger association between fish and velocity or depth may have been <br />masked by main channel hydraulic conditions. A better approach would be to analyze the physical <br />conditions only within the channel margin area that subadult humpback chub use, which may vary among <br />shoreline types, rather than a specific distance from shore. <br /> <br />Historic and present habitat condition <br /> <br />This study also demonstrated that mean depth, velocity and cover of shorelines vary with discharge <br />during base-flow conditions. Consequently, higher base flows, which occur a greater proportion of the <br />time in the current flow regime, may reduce subadult humpback chub habitat quality in natural habitats <br />compared with the same during pre-dam conditions in the Colorado River through Grand Canyon. <br />Reduced habitat quality may partially explain why subadults now use naturalized habitats, like vegetated <br />shorelines, which did not exist historically more than they use natural habitats like talus and debris fan <br />shorelines. <br />Vegetated shoreline habitat. Shoreline vegetation in Grand Canyon consists mainly of overhanging <br />tamarisk (Tamarix chinensis) that has stabilized sand deposits. Tamarisk is an exotic riparian plant that <br />has been present in the Colorado River since the early part of this century but was not able to stabilize <br />sand at the water's edge until the onset of flow regulation in 1962. Before that time, annual floods scoured <br />shorelines of any perennial vegetation, leaving extensive sand beaches (Turner and Karpiscak, 1980). <br />Relatively high use of vegetated shorelines by subadult humpback chub implies that subadults may <br />prefer these new areas over natural habitats, like talus or debris fans. Naturalized vegetation may be used <br />more than natural habitats for three reasons: (1) original shoreline conditions may have been modified by <br />flow regulation such that they currently provide only marginally acceptable conditions; (2) previously <br />important shoreline types are no longer present; or (3) vegetated shorelines simply provide better habitat <br />conditions than what naturally exist. Another possibility is that a combination of these conditions exists. <br />Modified shoreline conditions. Flow regulation may alter a suite of physical or biological shoreline <br />conditions, thereby limiting subadult humpback chub habitat quality. Changes in discharge cause basic <br />hydraulic changes in the river (Leopold et al., 1964). Before dam operations, base flows of the Colorado <br />River were two to five times less than the current average base flow. These results suggest that higher base <br />flows in the current flow regime increase mean depth and velocity and decrease cover. At-a-station <br />hydraulic geometry predicts that depth and velocity increase as discharge increases (Richards, 1977); <br />however, it is unclear why the occurrence of cover decreases with increasing discharge. <br />An examination of the cover/discharge relationship for individual shoreline types partially explains this <br />phenomenon. The presence of cover is the result of structural heterogeneity along the wetted perimeter of <br />the channel. The structure of bedrock, debris fan and talus shoreline types depends on morphology of <br />local geology and the shoreline angle of repose along the water's edge. These are geologically dependent <br />shoreline types. As basic geomorphology predicts (Leopold et al., 1964; Ritter, 1978), the angle of repose <br />decreases as colluvium accumulates at the toe of the slope. At higher discharges, the channel margins may <br />encounter more uniform or massive colluvium and higher angles of repose, or shoreline availability may <br />shift to more bedrock and sheer walls; consequently, structural heterogeneity would decrease with <br />increasing discharge among these shoreline types. <br />In contrast, cobble, sand and vegetation shoreline types are formed from main channel activity at high <br />discharges and are exposed as water levels recede. The structure of these shorelines is not as dependent <br />on local lithology; therefore, these shorelines are geologically independent. Because these shorelines are <br />formed by alluvial deposits, their structure and angle of repose at the water's edge is more homogeneous <br /> <br />@ 1998 John Wiley & Sons, Ltd. <br /> <br />Regul. Rivers: Res. Mgmt. 14: 267-284 (1998) <br /> <br /><. <br /> <br />~ <br /> <br />.. <br />