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C7 DRAFT February25,1998 Table 3. Availability of Temperature Data for the Omen and Yampa Rivers. Site Dates of S=Mre Source Omen River at'ria Juana Bottom (RK 389) 1987-1988 U.S. Fish and Wildlife Service Green River at Ouray Refuge (RK 413) 1987-1997 U.S. Fish and Wildlife Service Omen River at Green River, Utah (RK 192) 1987-1995 U.S. Fish and Wildlife Service Grew River at Jensen, Utah (RK 482 and 485) 1987-1990 (RK 492),1991-1997 (RK 485) U.S. Fish and Wildlife Service Green River at Browns Park (RK 587) 1987-1997 U.S. Fish and Wildlife Service 19941995 Colorado State University reen River at Mitten Park (RK 549) 1987-1997 U.S. Fish and Wildlife Service L Reach 1- Flaming Gorge Temperature Releases Typically, I'laming Gorge reservoir has a winter and summer operation. The winter operation calls for water to be released near the bottom of the dam where the temperature is in the range of 4° C to 5° C. In spring, water temperatures are raised to 13° C by using the selective withdrawal system. Reservoir operators adjust the withdrawal system to find a layer of water with a temperature of 13° C throughout the summer, so that a constant temperature of release water is maintained until mid-October when the temperature of the release is lowered to mimic historic winter conditions. This regulation is designed to maintain a trout fishery below the dam. The area of colder temperatures acceptable to rainbow trout extends from Flaming Gorge downstream to near Red Creek, where water temperature begin to warm by 1° to 2° C. Through Browns Park, the river widens, temperature increases, and the fish composition includes species that tolerate slightly warmer temperatures. From Browns Park the river enters Lodore Canyon, which has a north-south orientation which limits direct solar radiation contact with the water. Water temperature in Lodore to the confluence with the Yampa River typically increase by about 2° C as the rock mass of the Canyon radiates solar energy to the air and water. Figure 4 compares temperatures at several locations on four selected dates during 1996 and displays typical effects of Flaming Gorge and environmental factors on the water temperature of the Green River. Reach 2 - Yampa River Influence Thermal mixing at the Green/Yampa River confluence is dynamic. During the winter months, the Green River is warmer than the Yampa River; as the Yampa River begins to warm up in the spring, the temperatures in the Green River are maintained artificially low and stable by Flaming Gorge dam releases. From the beginning of spring runoff through mid- summer, the temperature of the Green River below Echo Park is controlled by the Yampa River. During late summer, the i 12