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<br />FINAL REPORT, November 2003 <br />High-jlow Requirements for the Duchesne River <br /> <br />Changes in Stream Flow in the Study Area <br />Total annual runoff and flood magnitudes have been highly variable over the period of <br />record. The years with total annual runoff in the upper quartile of the measured record averaged <br />5.8 times the mean annual flow of the years with total annual runoff in the lowest quartile of the <br />distribution. The magnitudes of 1.5-year, 2-year, and 5-year floods, and the flows exceeded 10, <br />50, and 90 percent of the time are all at least five times greater in upper-quartile years than in <br />lower-quartile years. Wetter and drier years appear to cluster in the latter half of the 20th century, <br />when three relatively dry periods lasting roughly a decade were separated by two relatively wet <br />periods, each lasting about 5 years. <br />Mean annual runoff for water years 1972 through 2000, after the Bonneville Unit of the <br />Central Utah Project (CUP) began operations, declined by approximately 10 percent from the <br />mean annual runoff for the pre-project water years of 1943 through 1971. The impact ofthe <br />CUP is most pronounced during years with total annual flow in the lowest quartile of the record. <br />For these years, mean annual flow is 33 percent less since 1972 than for before 1972. The <br />frequencies and magnitudes of large discharges decreased less than the frequencies and <br />magnitudes of moderate discharges. The magnitudes of 1.5-year and 2-year floods and of the <br />flow exceeded 50 percent of the time decreased by more than 25 percent after 1972, while the <br />magnitude of the 5-year flood decreased by only about 5 percent. <br />A significant decline in stream flow through the study area occurred in the mid-1920s, as <br />determined from statistical extension of the hydrologic record. The total annual flow, magnitude <br />of floods, and the magnitude of flow of various durations all were much greater for the period <br />between 1912 and 1924 than for the period between 1925 and 2000. <br /> <br />Discharges Necessary to Access High Bars and Secondary Channels <br />One-dimensional hydraulic models developed at three detailed study sites indicate that <br />floods ranging from the 2-year to the 2.6-year events are required to initiate significant flow onto <br />high bar surfaces. The average discharge that inundates these surfaces is 4,000 ft3 Is. Somewhat <br />smaller magnitude events with discharges of approximately 3,000 fe/s and a recurrence interval <br />of 1.7 years are sufficient to produce flow into the main chute channels. Flows capable of <br />inundating the floodplains and higher bar ridges are larger than the 3.2-year flood and may be <br />approximated by the 6-year event. <br /> <br />xi <br />