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INSTREAM FLOWS TO ASSIST THE RECOVERY OF ENDANGERED FISHES 3 Introduction Endangered Fishes of the Upper Colorado River Basin Four endemic fishes, Colorado squawfish (Pty- chocheilus lucius), bonytail chub (Gila elegans), humpback chub (Gila cypha), and razorback sucker (Xyrauchen texanus), of the Colorado River are protected under the Federal Endangered Spe- cies Act, and a recovery program for these fishes has been established by the U.S. Fish and Wildlife Service (Wydoski and Hamill 1991). These en- demic, big-river fishes were abundant throughout the potamonl reaches of the Upper Colorado River Basin (Fig. 1) during settlement and initial devel- opment of the basin (circa 1870's-1950's; Minckley 1973; Quartarone 1993). However, current popula- tion size and recruitment of these fishes are re- duced substantially, underscoring the rationale for their listing under the Endangered Species Act. Bonytail chub and razorback sucker are virtually extirpated in the Upper Colorado River Basin. Reproducing populations of humpback chub are known only in five canyon segments (Colorado River: Black Rocks and Westwater canyons; Green River: Gray and Desolation canyons; Yampa River: Yampa Canyon). Squawfish remain comparatively abundant, but their distribution is restricted by dams and diversions (Fig. 1). The decline of these fishes is attributed primarily to habitat loss and other environmental changes associated with con- struction of reservoirs and reduced and regulated flows in the remaining potamon reaches of the fragmented river system (Stanford and Ward 1986a). Predation by numerous introduced species (Minckley et al. 1991; Tyus 1991a, 1991b) and toxic effects of selenium from irrigation return flows (Stephens et al. 1992) also have produced docu- mented pressures on the survival of these fishes. The recovery program emphasizes reregulation of flows and obtaining water rights to ensure long- term stability of flows so that documented environ- mental needs of the fish can be met over the long term (U.S. Fish and Wildlife Service 1987a, 1993). Flow regimes have been formally recommended for the Green River (U.S. Fish and Wildlife Service 1992), Yampa River (U.S. Fish and Wildlife Service 1 The downstream zone of a river continuum characterized by warm, often turbid waters, sandy, unstable bottoms, and complex channels that may be constrained in canyon segments but more often meander through broad valley or coastal floodplains (after Illies and Botosaneanu 1963 and Stanford and Ward 1993). 1990), and the "15-mile reach" of the mainstem Colorado River in the Grand Valley near Grand Junction, Colorado (Kaeding and Osmundson 1989; Osmundson and Kaeding 1991). However, provision of instream flows is contentious, owing to the high value of water development entitle- ments apportioned to Colorado, Utah, and Wyo- ming per the Colorado River Compact. Indeed, the recovery program is predicated on development of these entitlements. Contention also has arisen with regard to the efficacy of technical or scientific methods used to justify flow recommendations. Purpose and Objectives of the Study Owing to contention over flow recommendations developed by the U.S. Fish and Wildlife Service, I was commissioned by the Instream Flow Subcom- mittee of the Recovery Implementation Program for Endangered Fish Species ofthe Upper Colorado River Basin to review and synthesize the science pertinent to the issue. The objectives of the study were as follows: (1) to complete a comprehensive review of past and ongoing technical activities, methods, and knowledge related to the quantification of in- stream flows needed for recovery of the four endangered fish species in the Colorado and Green River subbasins, including the flow rec- ommendations of the U.S. Fish and Wildlife Service; (2) to identify critical uncertainties or key issues, technical and nontechnical, related to provision of instream flows; and (3) to provide recommendations to resolve the tech- nical issues related to quantifying the instream flow needs of the endangered fishes. These ob- jectives could not be met without a thorough reading of the literature describing the biogeo- chemistry of the river system; thus, I also offer perspective on the quality and completeness of the ecological information base in the context of flow provisions to protect and enhance the fish populations of concern. Results of this study will assist the Recovery Program members in their decision-making proc- ess for meeting the needs of fish and directing future instream flow studies for the benefit of the endangered fishes. Moreover, the report also is intended to be a scientific synthesis of what is known about the ecosystem encompassed by the Upper Colorado River Basin, which has been ex- tensively altered by dams and diversions (Fig. 1).