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<br />9 <br /> <br />THE GREEN RIVER IN UTAH / <br />THE STUDY AREA <br /> <br />The Green River is the longest tributary of the Colorado River, draining <br />approximately 115,772 km2 (44,850 mi2) of Wyoming, Colorado, and Utah <br /> <br />(Andrews, 1986). The river flows alternately through bedrock canyons and <br /> <br />alluvial valleys. Typical downstream hydraulic geometry relations (Leopold and <br /> <br />Maddock, 1953) do not apply to large sections of the river due in part to the <br /> <br />varying erosional resistance of the local geology (Grams, 1997). Most of the <br /> <br />Green's annual water discharge originates as snowmelt runoff from high <br /> <br />mountains, whereas most of its sediment load is derived from low-elevation <br /> <br />semiarid sections (Andrews, 1990). Flows of the Green River have been <br /> <br />regulated by Flaming Gorge Dam since October 1962. <br /> <br />Streamflow at Green River, Utah, has been affected by climate change, <br /> <br />dam construction, and trans-basin diversions. On the basis of dendro- <br /> <br />chronology, Stockton and Jacoby (1976) showed that the two periods of <br /> <br />greatest annual runoff occurred between approximately 1600 and 1650, and <br /> <br /> <br />between 1900 and 1930. Streamflow gaging began in 1894, and these records <br /> <br /> <br />also indicate that the magnitude of flood flows decreased greatly after 1930 <br /> <br />(Fig. 3). The magnitude of the average flood decreased again after closure of <br /> <br />Flaming Gorge Dam in 1962. The 2-yr recurrence flood was calculated for <br /> <br /> <br />three time periods selected by visual assessment of the data. The 2-yr flood <br /> <br /> <br />decreased from 1190 m3/s for the period of record prior to 1930, to 800 m3/s <br />