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e CHAPTER ill. RIVER HYDROLOGY The hydrology of the 44,700 mi2 Green River drainage is dependent on the spring snowmelt in the Rocky Mountains of Wyoming, Colorado and Utah. The principal tributaries include the Little Snake, Yampa and White Rivers in Colorado, the Blacks Fork and Little Snake Rivers in Wyoming, and the Duchesne, Price, and San Rafael Rivers in Utah. Flow in the main stem Green River has been regulated by Flaming Gorge Dam near the Utah-Wyoming border since October, 1962. Additional water storage projects exist throughout the basin which have altered the Green River seasonal hydro graph as measured at the principal gaging stations at Jensen and Green River, Utah. These two gages define the flows in the study reaches. These projects include Fontenelle Reservoir on the Green River upstream of Flaming Gorge, Taylor Draw Dam and Avery Lake on the White River, Strawberry Reservoir on the Duchesne River, Yamcolo Reservoir, Stagecoach Reservoir, Catamount Lake, Steamboat Lake and Elkhead Reservoir on the Yampa River, transmountain diversion on the Little Snake River, and other small water resource projects throughout the basin. e The source areas of water and sediment in the Green River watershed are not distributed equally in the basins. The primary water source areas are located in the headwaters of the various tributaries throughout the Rocky Mountains rimming the basin. These areas have very little sediment yield. On the other hand, sediment is delivered from the semi-arid regions in the middle to lower portions of the watershed as in the Little Snake basin (Andrews, 1986). These regions contribute only a small fraction of the total flow in the Green River. The analysis presented in this chapter is based primarily on two Green River gages: Green River near Jensen, which is located upstream of the Ouray study reach below the mouth of Split Mountain Canyon and at the Green River at Green River1 located upstream of the Canyonlands study reach near the old U.S. Highway 6 bridge. The period of record for the near Jensen gage is water year 1947 through the present. (Two discontinuous records are available for 1904 and 1906.) The period of record at the Green River gage is longer, extending from water years 1895 to 1899 and 1905 to the present. Peak Flows, Mean Annual Flows and Base Flows e The Green River generally peaks in the late spring with the period from about September 1 to March 1 corresponding to base flow. The mean annual discharge is the arithmetic mean of all the mean daily discharges for the entire period of record. The mean annual discharge is generally equalled or exceeded on less than half the number of days in the year. The mean annual discharge for the Green River at the Jensen gage was about 4,360 cfs prior to 1963 and 4,210 cfs after 1963. At the Green River gage the mean flow was 7,960 cfs before 1963 and 5,600 cfs after 1963. The pre-1963 and post-1963 mean annual discharges at Jensen are nearly equal, whereas the decrease from the pre-1963 to the post-1963 mean annual discharge for the 29