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Green River hydrograph below the confluence of the Yampa River is maintained to some degree by spring runoff from the Yampa River (Figure 3). Flows of the Green River increased historically in March due to spring snowmelt and remained high through July (Figu~e 3). Mean monthly flow during spring runoff (April-July) was about 401.86 m /s {period of record 1895-1962, USGS at G~een River, Utah). A maximum flow of 1,927.23 m3/s and a minimum flow of 7.22 m /s were recorded at Green River, Utah in 1917 and 1931, respectively {Ugland et al. 1987). Following spring runoff, flows of the G~een River declined toward an average-monthly base flow of about 64.81 m /s for September through February (USGS flow records), and large daily fluctuations in river level were infrequent events. Closure of Flaming Gorge Dam in 1963 greatly altered this historic flow regimen. Daily operation of the dam from 1967 to 1984 has been 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 non-existent during most years, and winter baseflows at Green River, Utah have increased substantially (from about 79.24 to 107.54 m /s, 1967-1984, USGS at Green River, Utah). Native fishes of the Green River basin evolved in a system of fluctuating seasonal and annual flows characterized by wet, average, and dry climatic periods. We consider the 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 current flow regimen". 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 and runoff from snowmelt. Colorado River Fishes Fishes indigenous to the Green River basin include cyprinids {Colorado squawfish, humpback chub, bonytail chub Gila el~ans, roundtail chub G. robusta, speckled dace Rhinichthvs osculus, and Kendall Warm Springs dace Rhinichthvs o. thermalis), catostomids {razorback sucker, fianne]mouth sucker Catostomus latipinnis, bluehead sucker C. discobolus, mountain sucker C. platyrhynchus), salmonids (Colorado River cutthroat trout Oncorhynchus clarki pleuriticus; Rocky Mountain whitefish Prosopium williamsoni), and sculpins (Cottus species; Tyus et ai. 1982a; Sehnke and Benson 1983; Woodiing 1985). All mainstream fishes persist today despite the introduction of numerous non-. native fishes of which about 20 species are common in the Green River basin (Tyus et al. 1982a; Wick et al. 1985; USFWS, unpublished data). Rare fishes usually occur in greatest numbers in unaltered or natural river systems (e.g., Yampa River, Little Colorado River) which is presumably associated with maintenance of usable fish habitat due to a regimen of fluctuating seasonal and annual flows. The native cyprinids and catostomids were widely distributed in mainstream habitats and dominated the fish fauna of the historic Colorado River basin (Jordan and Evermann 1896). However, four of these fishes, the Colorado squawfish, razorback sucker, humpback chub, and bonytail-chub, are now threatened with extinction presumably due to the combined effects of habitat .loss {including regulation of natural flow, temperature, and sediment regimes); proliferation of introduced fishes; and other man-induced„ disturbances (Miller 1961; Hinckley 1973; U.S. Fish and Wildlife Service 1987a). The Colarado~squawfish, humpback chub, and bonytail chub are federally