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The Yampa River is the only large river in the Colorado River basin in which flow patterns have not been substantially altered by water development projects (Fig. 2). Examples of downstream- alterations include modification of flow and temperature patterns, and channel morphology. Upstream loss of fish habitat can occur with stream blockage and impoundment. Construction of the Flaming Gorge and Fontenelle dams on the Green River in the 196O's eliminated spring peak flows and increased baseline discharge in that system. However, the spring and early summer peak in the existing Green River hydrograph below the confluence of the Yampa River is maintained by spring runoff from the Yampa River (Fig. 3). Fishes indigenous to the Yampa River include cyprinids (Colorado squawfish [Ptychocheilus hicius], humpback chub [Gila cypl:a], bonytail chub {G. elegans], roundtail chub [Gila robusta], speckled dace [Rhinic)tt)tys osculus]); catostomids (razorback sucker [Xyrauchen texanus], flannelmouth sucker [Catostonucs latipinnis], bluehead sucker [C. discobolus], mountain sucker [C. platyrhynchus]); salmonids (Colorado River cutthroat trout [Salnio clarki pleuritict~s]; Rocky Mountain whitefish [Prosopium williantsoni]); and sculpins (Cottus bairdi sp.; Tyus et a1.1982a; Behnke and Benson 1983; Woodling 1985). All mainstream fishes persist today despite the introduction of at least 18 nonnative fishes (Tyus et a1.1982a; Wick et a1.1985; U.S. Fish and Wildlife Service, unpublished dataj. Native fishes also dominate the Yampa River fish community as indicated by Miller et al. (1982) and Wick et al. (1985). Using electrofishing and trammel netting techniques, these investigators found that native fishes composed more than 70% of the catch, and Miller et al. (1982] reported that 54% of all fishes captured (including collections of small fishes) were native. Persistence of native fishes is most often observed in unaltered (natural) river systems (e.g., Yampa River and Little Colorado River) and is presumably associated with maintenance of usable fish habitat due to a regimen of fluctuating seasonal and annual flows. Historically, the native cyprinids and catostomids were the dominant fishes in mainstream habitats of the Colorado River basin. The Colorado squawfish, bonytail chub, humpback chub, and razorback sucker were widely distributed and common-to-abundant in major rivers of the Colorado River basin. However, all four species are now threatened with extinction due to the combined effects of habitat ions; regulation of natural flow, temperature, and sediment regimes; proliferation of introduced competitors and predators; and other man-induced disturbances {Miller 1961; Minckley 1973; U.S. Fish and Wildlife Service i9$7). The Colorado squawfish, humpback chub, and bonytail chub are federally protected as endangered species under provisions of the Endangered Species Act of 1973 (U.S. Fish and Wildlife Service 1986). The razorback sucker, a candidate species for Federal listing, is protected by State statutes in Arizona, California, Colorado, Nevada, and Utah (U.S. Fish and Wildlife Service 1985,1987). In the lower Colorado River basin (below Lee Ferry, Arizonaj, the Colorado squawfish has been extirpated; relict populations of bonytail chub and razorback sucker remain in some impoundments; and the humpback chub persists only in the Little Colorado River (Minckley 1973, 1983). In the upper Colorado River basin, the 660 ~~ ...__._.........._ GREENDAIE YAMPA 420 A E ~ 260 H a 140 OCT NOV DEC JAN Fg MAR APR MAY JUN JUL ~~ ~ 560 .~~ _....._......__. GREENDAlE VAMPA 420 2ao 0 140 OCi NOV DEC JAN FHB MAR AMt MAY JUN JM. AVC' acr MONTH Fig. 3. Average annual distribution hydrograph for the Green and Yampa rivers. Upper figure for 1951-62; lower figure for 1464-84. USGS flow records: Jensen =Green River at Jensen, Utah; Greendale =Green River below Flaming Gorge Dam; Yampa = Yampa River at mouth (Yampa River near Maybell, Colorado, and Little Snake River near Lily, Colorado). 3