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BACKGROUND The Green River and Flaming Gorge Dam The Green River originates in western Wyoming, flows south through Wyoming, Utah, and Colorado and merges with the Colorado River in southeastern Utah (Figure 1). The Green River provides recreation, hydropower, irrigation water, and habitat for a number of fish species, including the Federally endangered Colorado squawfish (Prychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha) and bonytail (Gila elegans). Since October of 1962, flows in the Green River have been regulated by Flaming Gorge Dam, located near the border of Utah and Wyoming. Releases from the dam are determined by a number of factors, including flood control, water use, and hydropower. Compared to pre-dam historical records,. Flaming Gorge Dam has reduced the magnitude of peak spring flows and has increased the magnitude of flows in the fall and winter. This can be seen in Figure 2, where the average flow at the Jensen gaging station for each day of the year for the period 1946-1962 (pre-dam period of record) is compared to the average flow for each day of the year for the period 1963-1997 (post- dam). Use of Flaming Gorge Dam as a peaking hydropower facility has also led to increased daily fluctuations in flow at downstream locations during many seasons of the year. Winter fluctuations in flow have been observed to increase the activity of some Green River endangered fishes especially during ice breakup and formation of ice jams (Valdez and Masslich 1989, Valdez 1994). Shoreline scouring resulting from ice breakup and movement has also been suggested to be undesirable for overwintering endangered fish (USFWS 1992). Although, the degree to which fluctuating flows in winter, ice breakup, and shoreline scouring affect endangered fish is not well understood, it appears that it is desirable to avoid repeatedly or prematurely breaking up ice covers that form on the river in areas used by overwintering endangered fish. Consequently, information about the effects of fluctuating flows due to hydropower or maintenance operations at Flaming Gorge Dam. on ice processes in the Green River are needed. This need for additional information about the effects of winter flows on conditions in overwintering areas for endangered fishes was also identified in the Biological Opinion on the Operation of Flaming Gorge Dam (USFWS 1992). The Study Reach The study reach of the Green River extended from River Mile 248 to about River Mile 316 (Figure 3). Flaming Gorge Dam is located upstream of the study reach at River Mile 410. As presented in Valdez and Masslich (1989), the study reach can be effectively divided into two regions. The reach from Split Mountain Canyon to the Jensen Bridge comprises the upper region. It has a moderate gradient with gravel-cobble substrate upstream and sand substrate downstream. The lower region extends from the Jensen Bridge to Ouray Bridge and is generally a low-gradient, meandering river with a substrate consisting primarily of sand. Upstream of the study reach, where the Green River passes through Split Mountain Canyon, the river has a high gradient and numerous rapids. 2 1 1 1 1