Instream flows to assist the recovery of endangered fishes of the upper Colorado River basin

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.