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Instream Flows to Assist the Recovery of Endangered Fishes of the Upper Colorado River Basin by Jack A. Stanford Flathead Lake Biological Station The University of Montana 311 Bio Station Lane Polson, Montana 59860 Abstract. The riverine landscape of the Upper Colorado River Basin has been extensively modified by dams, diversions, revetments, and water abstractions. These changes, probably coupled with the introduction of many nonnative fishes, have compromised the existence of four of the native fishes (Colorado River squawfish Ptychocheilus lucius, humpback chub Gila cypha, bonytail chub Gila elegans, and razorback sucker Xyrauchen texanus) of the river system. Efforts to recover these endangered fishes have emphasized reregulation of flows to provide better habitat conditions than existed during the last half century, when ranges and abundances of the fishes declined significantly. Contention emerged, however, with regard to the efficacy of methods used by the U.S. Fish and Wildlife Service to justify flow recommendations to protect the endangered fishes. The purpose of this study was to review the science pertaining to the issue of flow provision, to identify critical uncertainties, and to provide recommendations for determining the instream flow needs of the endangered fishes. Colorado River squawfish, humpback chub, and razorback sucker (in order of relative abundance; all are rare) live in the warm water (downstream) reaches of the Upper Colorado River Basin. Bonytail chub seem to be extirpated. Routine collections of larvae and age structure of populations in the Green and Colorado rivers indicate that adult recruitment of squawfish is occurring almost every year. Recruitment of adult humpback chub and razorback sucker has not been demonstrated, but both are known to produce young, at least in some years. Production of young squawfish seems to be lowest in years of very low or very high flows. However, studies strongly indicate that truncation of peak flows and higher, fluctuating baseflows (loss of seasonality) resulting from river regulation have altered complex biophysical processes that form and maintain low velocity habitats required for survival of the various life history stages of the fishes. An ecological tradeoff apparently exists: Very high flows are needed occasionally to produce habitats that the fish need to survive, but at the expense of reproductive success. The apparent importance of variable, but clearly seasonal, flow regimes and associated biophysical interactions was the key rationale for the flow recommendations made by the U.S. Fish and Wildlife Service. For the Yampa, Green, and Colorado rivers, flows were recommended that would increase amplitude of the spring peak and reduce short-term fluctuations from hydropower operations at baseflows. However, on the Green River, the peak flows recommended for wet years were considerably less than flows of record and allowed substantial flow fluctuations during the late summer, fall, and winter (baseflow) period in all years. Moreover, a complex flow-habitat model was used to support flow recommendations on the Colorado River, but model output was discarded on the Green and Yampa rivers. Review of models currently used to determine an incremental relationship between flow and river conditions favorable to the endangered fishes revealed that none, including the one used on the Colorado River, was sufficiently well developed to be used exclusive of many other ecological measures. Inconsistencies in rationale and perceived need for a predictive model compromised the science that