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2 CLARKSON ET AL. habitats during the course of the fishes' evoIutionary history. We emphasize that the presence of predatory and competitory non-native fishes has contributed to, and indeed is a descriptor of, this habitat degradation. Our 'objectives ,for enhancement of nativefish.populations in Grand Canyon incIude: (1) promote successfuI reproduction in the CoIorado River and its tributaries; (2) increase sUlVival of early life stages; (3) increase recruitment to aduIt, reproducing segments of populations, and; (4) minimize detrimental effects (competition, predation) of non-native fishes. Complete restoration of the native fauna must include reintroduction of those species eliminated from the system following closure of Glen Canyon Dam. . , Stmcture of the Document-To allow managers and administrators with limited time and training in fisheries science the opportunity to consider our arguments, we begin this document with a relatively brief section that makes recommendations for the operational elements of hydrology, water temperature, and sediment. Hydrology initially is considered independently, because there are restrictions to any near term modification of the other elements that neCessarily are imposed by the need for environmental compliance, further studies, planning, engineering, and construction of a muIti-Ievel intake structure or means of sediment' augmentation. We assume that future annual water delivery will continue to conform to Colorado River ~mpact.aIIocation of at least 8.23 million acre-feet. We stress th;I.~bydroIogy, water temperature, and, sediment were inextricably related environmental varia.ples ll(, the evolutionary ~ry of the native fishes. Failure to adequately recognize theSe interrelationships and integrate them into future operations of Glen Canyon Dam has been a major contributor to the recent impasse in selection of a pref~ alternative. . '.' : ~ .-~ ,"':. :.' : For~ thosereaders with the_time and 'training to critically evaluate .~ui :reasoning. and -. . judg~ent, we provide a subsequent, more lengthy background section. This section provides the basis for our position, which is rooted in scientific literature, unpublished results of ongo,ing studies, arid our combined expC:riences with fishes and aquatic ecosystems in the GraJ!d. Canyoll and elsewhere..- . , . . StoIement of the Problem-The closure in 1963 of GIen Canyon Dam on the CoIorado River above Grand Canyon drastically aItered hydroIogical, physical; and biological functions and cl1lUacteristics of the downstream riverine ecosystem (Carothers et aI. 1980, Maddux et aI. 1987, Andrews 1991, Blinn andCoIe 1991, Carothers and Brown 1991. Minckley 1991, Stanford and Ward 1991). Flow patterns were modified dramatically froin pre-dam conditions; a predictabIe pattern of seasonal flow variation was replaced 'by one of daily . di~harge fluctuations. Large reductions in the amount of sedimenttransjlorted through Glen and Grand canyons occurred with entrapm~nt of tens of millions of tonS annually in upper reaches of Lake Powell. Cold water re1~ from the depths of Lake Powell aItered . .