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<br />FINAL REPORT, November 2003 <br />High-jlow Requirements for the Duchesne River <br /> <br />Discharges Necessary to Entrain Gravel <br />The discharges necessary to entrain gravel were estimated by comparing values of average <br />boundary shear stress derived from our hydraulic models with the critical shear stress needed to <br />mobilize gravel particles of the sizes found on the surface of riffles at detailed study sites. <br />Results indicate that reach-averaged shear stress approaches the threshold of entrainment over <br />riffles and runs at discharges of approximately 4,000 ft3 Is. These results are consistent with field <br />observation of gravel entrainment during the spring 2001 peak, which briefly reached 2,900 ft3/s. <br />Little gravel entrainment was observed to occur as a result of this discharge. <br /> <br />Channel- forming Discharges <br />Our gravel entrainment and inundation analyses converge on a threshold discharge of <br />about 4,000 fe/s for mobilizing a significant portion ofthe bed and inundating some overbank <br />surfaces. This discharge level has a long-term daily mean recurrence interval of about 2.4 years <br />and is exceeded about 1.6 percent of the time. Comparison of the history of channel changes and <br />activity rates with the historic hydrology demonstrate that the level of channel activity is low <br />when flows above this critical discharge threshold are less frequent. Two channel-forming flow <br />hydro graphs are suggested. It is proposed that a moderate channel-forming hydrograph be <br />implemented in 30 percent of all years, and a larger channel-forming hydrograph be <br />implemented in 10 percent of all years. <br /> <br />Discharges Necessary to Flush Fine Sediment <br />Loss of fine sediment transport capacity has historically resulted in the loss of habitat in <br />the lower Duchesne River. Fine sediment transport capacity adequate to prevent further habitat <br />loss can be maintained by a combination of the proposed channel-forming flow regime and <br />additional flushing flows. The proposed flushing flows consist of 7 days with discharges greater <br />than 3,000 ft3 Is in 30 percent of all years, and 7 days with discharges greater than 2,500 ft3 Is in <br />and additional 30 percent of all years. <br /> <br />xii <br />