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<br />FINAL REPORT, November 2003 <br />High-jlow Requirements for the Duchesne River <br /> <br />EXECUTIVE SUMMARY <br /> <br />The objectives ofthis study were to (1) characterize the geomorphic attributes ofthe <br />present channel and alluvial valley of the 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 channeVvalley 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. <br />Our strategy for meeting these objectives employed an historical analysis of geomorphic <br />change in the study area, in addition to more traditional methods of characterizing modern fluvial <br />geomorphic processes. Analysis of historical aerial photography in a geographic information <br />system (GIS) was used to quantify past channel responses, and the record of channel change was <br />compared to changes in water and sediment discharge. This comparison allowed us to determine <br />the role of decade-scale periods of high and low flow in forming and maintaining the channel <br />and valley of the Duchesne River. The form and functioning ofthe present river was quantified <br />through extensive field measurements of channel and valley characteristics, and through the use <br />of computational methods. Main findings from this work are: <br />1. The lower Duchesne River consists of four distinct zones with differing morphologies and <br />histories. Channel morphology and response to flow varies in time and between zones. <br />2. Channel-forming discharge on the lower Duchesne River is about 4,000 ~/s. Gravel <br />mobilization and inundation of high bar surfaces occur at this discharge. <br />3. Little channel activity occurs during periods when the volume of stream flow in excess of <br />the channel-forming discharge of 4,000 ft3/S is less than 7,000 ft3 Is-days per year. Physical <br />habitat is created and maintained during decades when the volume of stream flow in excess <br />of 4,000 ft3/S is greater than 7,000 ft3/s-days per year. A channel-forming flow regime to <br />maintain channel activity at rates sufficient to support all habitat components is proposed. <br />4. The recurrence of daily mean discharges of 4,000 fe /s is about 2.2 years for the period from <br />1943 through 1971. The recurrence of this discharge has increased to 3 years since <br />completion of the Bonneville Unit of the CUP in water year 1972. <br /> <br />IX <br />