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<br />FINAL REPORT, November 2003 <br />High-jlow Requirements for the Duchesne River <br /> <br />INTRODUCTION <br /> <br />The objectives of this study were to (1) characterize the geomorphic attributes of the <br />present channel and alluvial valley ofthe lower Duchesne River between the mouth of the Uinta <br />River and the Green River; (2) evaluate the flows, sediment transport, and channel processes that <br />formed and maintained the present channel/valley system; (3) evaluate the degree to which the <br />present channel/valley system is in a state of dynamic equilibrium with recent stream flows; and <br />(4) determine the discharges necessary to insure that existing geomorphic and habitat conditions <br />can be maintained in the future. This study complements biological studies being conducted by <br />the Ute Tribe, U.S. Fish and Wildlife Service, and state of Utah concerning the distribution and <br />life history of two species of endemic endangered fish, the Colorado Pikeminnow and the <br />Razorback Sucker, that occupy the lower Duchesne River. The collective results of these studies <br />will be used in development of a comprehensive recommendation for the minimum flows <br />necessary to maintain the role of the lower Duchesne River as habitat for endangered fish. <br />The research described in this report utilizes a strategy for determining channel- <br />maintenance flow requirements employing an historical analysis of geomorphic change in the <br />study area, in addition to more traditional methods of characterizing modem fluvial geomorphic <br />processes. We determine the nature of past responses to changes in water and sediment <br />discharge on a decadal time scale, and thereby place present channel processes and conditions <br />within an historical context. <br /> <br />High Flows for Channel and Habitat Maintenance <br />Stream channel morphology is a function of water discharge, the type and amount of <br />sediment being transported, and the character of the materials making up the channel bed and <br />banks. Changes in any of these variables may cause the stream channel to reconfigure its <br />geometry or plan form so that the imposed water and sediment loads can be transported through <br />the system (Schumm 1969). Decreases in water discharge or increases in sediment load can <br />potentially cause channel narrowing or bed aggradation, both of which reduce the capacity of a <br />channel to carry peak flows (Warner 1994). Large flows during infrequent wet years then have <br />an increased potential to result in destructive flooding or trigger complex channel adjustments <br />that can take decades to complete. It is therefore critical for channel maintenance that the <br /> <br />1 <br />