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tributaries, the Green River proceeds 193 km through Desolation and Gray canyons, a region of whitewater rapids and deep pools. The Price River enters the Green River in lower Gray Canyon. The Green River then enters the Green River valley near the town of Green River, Utah, and receives the San Rafael River about 40.2 km downstream. The Green River continues through labyrinth and Stillwater canyons in Canyonlands National Park, where it Joins the upper mainstem Colorado River. The Green River drops in elevation about 3,048 km from its source to the Colorado River. Flows of most mainstream rivers of the Colorado River basin have been substantially altered by water development projects (Figure 2). Construction of Flaming Gorge and Fontenelle dams in the Green River in the 1960's greatly modified upstream and downstream hydrologic conditions (i.e., stream blockage, flows, temperatures, sediment load and channel morphology) such that the magnitude of spring peak flows was reduced while baseflow at other times of the year was increased. However, the magnitude and timing of spring and early summer peak flows in the existing Green River hydrograph below the confluence of the Yampa River is maintained to some degree by spring runoff from the Yampa River (Tyus and Karp 1989; Figure 3). Historic flows of the Green River typically increased in March with runoff from snowmelt and remained high through July (Figu~e 3). Mean monthly flow during spring runoff (April-July) was about 401.86 m Is (periog of record 1895-1962, USGS at Grejn River, Utah). A maximum flow of 1,927.23 m Is and a minimum flow of 7.22 m Is were recorded at Green River, Utah in 1917 and 1931, respectively (Ugland et al. 1987). Following spring runoff, flows ~f the Green River declined toward an average-monthly baseflow of about 64.81 m Is for September through February (USGS flow records), and large daily fluctuations in river level were uncommon events. Closure of Flaming Gorge Dam in 1962 greatly altered this historic flow regimen. Daily operation of the dam from 1967 to 1984 was based on peaking power demand, and the resultant annual hydrograph in the upper Green River (above its confluence with the Yampa River) is relatively flat, with extreme daily fluctuations (Figure 3). Spring peaks below the Dam are nonexistent during most years, and winter baseflows at G~een River, Utah have increased substantially (from about 79.24 to 107.54 mis, 1967-1984, USGS at Green River, Utah). Fishes native to the Colorado River evolved in a system of fluctuating seasonal and annual flows influenced by wet, average, and dry climatic periods. We consider this recurring cycle of high spring flows followed by a period of lower flows (as indicated by the flow regimen of Yampa River in Figure 3) the "natural, or historic flow regimen" in the Green River basin. This regimen is typified by a hydrologic pattern of high spring and low autumn-winter flows that vary in magnitude and duration depending on annual precipitation patterns and runoff from snowmelt. Green River Basin Fishes The Colorado River system constitutes a unique environment and its endemic fishes are considered a precious resource by many (Carlson and Muth 1989; Minck1ey and Deacon, in press). Fishes indigenous to the Green River basin include cyprinids (Colorado squawfish Ptvchocheilus lucius, humpback 5