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<br />Abstract <br /> <br />The riverine landscape of the Upper Colorado River Basin has been extensively modified by <br />dams, diversions, revetments and water abstractions. These changes, probably coupled with the <br />introduction of many nonnative fishes, have compromised the existence of four (Colorado River <br />squawfish, humpback and bony tail chub and razorback sucker) of the native fishes of the river <br />system. Efforts to recover these endangered fishes have emphasized reregulation of flows to <br />provide better habitat conditions than existed during the last half century when ranges and <br />abundances of the fishes declined dramatically. <br />However, contention emerged with regard to the efficacy of methods used by the U.S. Fish <br />and Wildlife Service to justify flow recommendations to protect the endangered fishes. The <br />pwpose of this study was to review the science pertaining to the issue of flow provision, to identify <br />critical uncertainties and to provide recommendations for determining the instream flow needs of <br />the endangered fishes. <br />Colorado River squawfish, humpback chub and razorback sucker (in order of relative <br />abundance; all are rare) live in the wann water (downstream) reaches of the Upper Colorado River <br />Basin. Bony tail chub appear to be extirpated. Routine collections of larvae and age structure of <br />populations in the Green and Colorado Rivers suggest that adult recruitment of squawfish is <br />occurring almost every year. Recruitment of adult humpback chub and razorback sucker has not <br />been demonstrated, but both are known to produce young, at least on some years. Production of <br />young squawfish appears to be lowest on years of very low or very high flows. However, studies to <br />date strongly indicate that truncation of peak flows and higher, fluctuating baseflows (loss of <br />seasonality) resulting from river regulation have altered complex biophysical processes (detailed <br />herein) that form and maintain low velocity habitats required for survival of the various life history <br />stages of the fishes. An ecological tradeoff apparently exists: very high flows are needed <br />occasionally to produce habitats that the fish need to survive, but at the expense of reproductive <br />success. <br /> <br />i <br /> <br />G, 0010 <br />