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ers), were widely distributed in main- stream habitats of the historic Colorado River basin (Jordan and Evermann 1896). Four of the big river fishes, Colorado squawfish (Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila cypha), and bony tail (Gila elegans), are now threatened with extinction due to the combined effects of habitat loss (including regulation of natural flow, temperature, and sediment regimes), proliferation of in- troduced fishes, and other man-induced disturbances (Miller 1961; Minckley 1973, 1982, 1983; Minckley and Deacon 1991). Stocks of these four fishes have declined to the point that all are federally listed as endangered species. That this decline con- tinues is evidenced by the recent listing of razorback sucker, a species first proposed for federal listing as threatened in 1980, which has deteriorated to the point that only relict stocks of nonrecruiting fish pre- vail (FWS 1991). Conversion of the mainstream lower Colorado River into a system of dams and diversions has been accompanied by a sig- nificant change in river fauna. Change in the natural flow regimen, stream blockage, and conversion of many miles of warm- water stream habitat to reservoirs and cold tailwaters have been accompanied by stocking a plethora of nonnative fishes. Native fishes there have been largely ex- tirpated and replaced by a new fauna of about 44 forms (Minckley 1982), many of which were introduced from more mesic environments. Of these, 20 species are abundant either locally or regionally (Minckley 1982). In the lower basin, Col- orado squawfish has been extirpated; relict populations of bony tail and razorback sucker remain in some impoundments, but neither species is presumed self-sustain- ing;and humpback chub reproduction is restricted to the Little Colorado River in the Grand Canyon (Minckley 1973,1983; Kaeding and Zimmerman 1983). Recovery prospects for most of the en- dangered fishes may be greatest in the up- per Colorado River basin because about 2,000 km of occupied habitat remains. The native fish fauna there includes six species that are endemic large-river cyprinids and catostomids, and six headwater forms that also occur elsewhere. Although 42 intro- duced fishes are presently reported, less than 10 are considered abundant (Tyus et al. 1982). Colorado squawfish persists in the Yampa River, the Green River below its confluence with the Yampa River, the upper Colorado River mainstream, and the lower San Juan River (Tyuset al. 1982; Pla- tania et al. 1991). The humpback chub is reproducing successfully in the Yampa and upper Colorado rivers (Kaeding et al. 1990; Karp and Tyus 1990a). The razorback suck- er persists in the lower Yampaand Green rivers, the mainstream Colorado River, and the lower San Juan River (McAda and Wy- doski 1980; Tyus et al. 1982; Platania et al. 1991), but there is no indication of recent recruitment in these remnant populations (FWS 1991). The remaining endangered large-river fish, the bony tail, is extremely rare in the upper Colorado River basin (Valdez and Clemmer 1982; Kaeding et al. 1986) but it occurs elsewhere only in hatch- ery or relict reservoir stocks. CONTRASTING PHILOSOPHIES Past efforts to determine instream flows for the endangered Colorado River fishes have been based on two different philos- ophies: (1) flows required for population survival (Beecher 1990), and (2) flows needed for recovery. The first one attempts to maintain populations of endangered fishes at their current levels and recom- mends flows based on current habitat use information. In this case, emphasis is placed on preventing extinction. The second would increase standing crops of fishes and numbers of reproducing populations to the point that the species could be de listed un- der the Endangered Species Act (16 D.S.C. ~ 1531 et seq.), a goal that would presum- ably require improvement of existing con- ditions; that is, nondegradation of suitable habitats and restoration of others (Beecher 1990). In the upper Colorado river basin, most efforts have been directed at deter- mining survival flows, even though this goal is seldom identified. Survival Flows The goal of most habitat and streamflow protection efforts for endangered fishes has I H. M. Tyus 2911~