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tributaries, the Green River proceeds 193 km through Desolation and Gray <br />canyons, a region of whitewater rapids and deep pools. The Price River enters <br />the Green River in lower Gray Canyon. The Green River then enters the Green <br />River valley near the town of Green River, Utah, and receives the San Rafael <br />River about 40.2 km downstream. The Green River continues through Labyrinth <br />and Stillwater canyons in Canyonlands National Park, where it joins the upper <br />mainstem Colorado River. The Green River drops in elevation about 3,048 km <br />from its source to the Colorado River. <br />Flows of most mainstream rivers of the Colorado River basin have been <br />substantially altered by water development projects (Figure 2). Construction <br />of Flaming Gorge and Fontenelle dams in the Green River in the 1960's greatly <br />modified upstream and downstream hydrologic conditions (i.e., stream blockage, <br />flows, temperatures, sediment load and channel morphology) such that the <br />magnitude of spring peak flows was reduced while baseflow at other times of <br />the year was increased. However, the magnitude and timing of spring and early <br />summer peak flows in the existing Green River hydrograph below the confluence <br />of the Yampa River is maintained to some degree by spring runoff from the <br />Yampa River (Tyus and Karp 1989; Figure 3). <br />Historic flows of the Green River typically increased in March with <br />runoff from snowmelt and remained high through July (Figu~e 3). Mean monthly <br />flow during spring runoff (April-July) was about 401.86 m /s (perio~ of record <br />1895-1962, USGS at Green River, Utah). A maximum flow of 1,927.23 m /s and a <br />minimum flow of 7.22 m3/s were recorded at Green River, Utah in 1917 and 1931, <br />respectively (Ugland et al. 1987). Following spring runoff, flows ~f the Green <br />River declined toward an average-monthly baseflow of about 64.81 m /s for <br />September through February (USGS flow records), and large daily fluctuations <br />in river level were uncommon events. Closure of Flaming Gorge Dam in 1962 <br />greatly altered this historic flow regimen. Daily operation of the dam from <br />1967 to 1984 was based on peaking power demand, and the resultant annual <br />hydrograph in the upper Green River (above its confluence with the Yampa <br />River) is relatively flat, with extreme daily fluctuations (Figure 3). Spring <br />peaks below the Dam are nonexistent during most years, and winter baseflows at <br />Green River, Utah have increased substantially (from about 79.24 to 107.54 <br />m3/s, 1967-1984, USGS at Green River, Utah). <br />Fishes native to the Colorado River evolved in a system of fluctuating <br />seasonal and annual flows influenced by wet, average, and dry climatic <br />periods. We consider this recurring cycle of high spring flows followed by a <br />period of lower flows (as indicated by the flow regimen of Yampa River in <br />Figure 3) the "natural, or historic flow regimen" in the Green River basin. <br />This regimen is typified by a hydrologic pattern of high spring and low <br />autumn-winter flows that vary in magnitude and duration depending on annual <br />precipitation patterns and runoff from snowmelt. <br />Green River Basin Fishes <br />The Colorado River system constitutes a unique environment and its <br />endemic fishes are considered a precious resource by many (Carlson and Muth <br />1989; Minckley and Deacon, in press). Fishes indigenous to the Green River <br />basin include cyprinids (Colorado squawfish Ptychocheilus Lucius, humpback <br />5 <br />