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Final Report 3-32 September 2000 3.5.1 Summer Water Temperatures Summer water temperature is important to the endangered fishes because temperature affects the productivity of the aquatic food base, growth and survival of larval fish, and conditioning of adult fish. Summer water temperature is largely a function of specific weather conditions during a particular summer, but is also strongly influenced by the volume and temperature of releases from Flaming Gorge Dam during this period. Water temperature is discussed for each of the three reaches in the following sections. As a general rule, in water years with more water, the water temperatures remain colder into summer. Water years in which snowmelt and runoff occur early (such as in water year 1962, when the peak flow occurred from mid-April to mid-May) are exceptions to this rule. During water year 1995, which had a high volume of water with a long peak-flow duration, water temperatures stayed low well into July. During water years with less water, water temperatures get warmer earlier in the season because base flows are low and reached earlier in the year (Section 3.4.3). 3.5.1.1 Water Temperature in Reach 1 The dominant factor influencing water temperature in Reach 1 is the temperature of water released from Flaming Gorge Dam. Release temperature is adjusted through the use of a selective withdrawal structure. This structure, which withdraws water from different positions in the Flaming Gorge Reservoir water column, was installed in 1978 to allow control of water temperatures for trout. During typical winter operations, water is drawn from deep within the reservoir through a fixed gate at an elevation of 1,789 m and a second mobile gate at 1,802 m. Water at these levels is 40C and is the warmest available at this time of year. During spring (usually early April), the mobile gate is moved within about 12 m of the surface to draw the warmest water available. Reservoir operators adjust the withdrawal system to find a layer of water with a temperature of 130C throughout the summer, so that a constant temperature of release water is maintained until mid-October when the temperature of the release is lowered. The gate maintains a minimum 12-m distance between the surface and the intake to reduce the possibility of a vortex and entrainment of air and debris. Usually by mid-July, gates are at an elevation of 15 m or more below the reservoir surface. With this operation, suitable temperatures for trout extend to upper Browns Park. As the river flows through Browns Park, it widens, and its water temperature increases. From Browns Park, the river enters Lodore Canyon, which has a north-south orientation that limits exposure to direct solar radiation. Water temperature in Lodore Canyon to the confluence with the Yampa River typically increases about 20C as the rock mass of the canyon radiates heat to the air and water. Effects of release patterns from Flaming Gorge Dam on the thermal regime of the Green River in Reach 1 were described by Bestgen and Crist (2000). They developed empirical regression models that predicted water temperature in the reach over a range of discharges and in different seasons. Air and water temperature data were used to develop models for upper Browns Park