8188 - Laserfiche WebLink

Home Browse Search

Final Report 3-3 September 2000 Climate varies considerably across the Green River basin. In the semiarid rangelands, which make up most of the basin's area, annual precipitation is generally less than 25 em per year. In contrast, many of the mountainous areas that rim the upper portion of the basin receive, on average, more than 100 em of precipitation per year. Most of the total annual stream flow in the Green River basin is provided by snowmelt. Because of this, natural flow is very high in late spring and early summer and diminishes rapidly in midsummer. Although flows in late summer through autumn can increase following rain events, natural flow in late summer through winter is generally low. Water and sediment inputs to the Green River and its tributaries are not uniformly distributed across the basin. The principal water sources are high-elevation areas, especially in the northeast portion of the basin. Conversely, the semiarid parts of the basin at lower elevations contribute most of the sediment. Ioms et al. (1965) estimated the annual suspended sediment discharge of the Green River basin to be 25,340,910 metric tons prior to regulation. About 13% of the suspended load of the entire Green River basin was found to originate in the Green River basin upstream of the Yampa River confluence. About 6% originates from the Yampa River basin, about 26% originates from the Green River basin between the Yampa and White Rivers, and about 54% originates from the basin downstream of the White River. Dams and reservoirs have been constructed in the basin mainly to supply water for irrigated agriculture (Table 3.1), and these facilities have resulted in reductions in Green River flow. Table 3.2 lists estimated depletions for 1998 due to water development in the basin. The largest depletion in the Green River basin occurs in the Duchesne River basin. In addition to depleting flow volume, reservoirs modify the pattern of flow in the Green River to meet demands of irrigation, power generation, recreation, and other uses. Generally, the larger the reservoir is in relation to its watershed, the greater is its potential to modify the natural flow pattern. Of the reservoirs in the basin, Flaming Gorge, which is capable of storing approximately twice the annual inflow, has the largest effect on Green River flow patterns. Flaming Gorge Dam has reduced the sediment load in the river downstream. This reduction results primarily from the presence of the dam (rather than operations), which traps sediment. Following completion of the dam, Andrews (1986) estimated that mean annual sediment discharge at the USGS gage near Jensen, Utah, decreased by 54% when compared with the average annual pre- dam suspended sediment load. Similarly, the decrease in mean-annual sediment load at the USGS gage near Green Ri ver, Utah, was estimated to be 48 % following completion of Flaming Gorge Dam (Andrews 1986). Andrews noted that the decrease in mean annual suspended sediment load at Jensen is approximately equal to the incoming sediment load to Flaming Gorge Reservoir. At Green River, Andrews noted that the decrease in suspended sediment load following reservoir closure greatly exceeded the amount of sediment trapped in the reservoir. He concluded that sediment inflow to the Green River downstream from the Duchesne River exceeds the transport of sediment out of Reach 3.