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41 Fluctuating releases from reservoirs cause major changes in the physicochemical environment of receiving streams. Water velocity, depth, width, amount of wetted perimeter, temperature and other water quality variables are affected by varying flows, and these changes in turn impact the aquatic biota below dams. Variable flow patterns below dams impact the entire food chain from primary producers (Neel 1963) to invertebrates (Fisher and LaVoy 1972, Trotzky and Gregory 1974) to the fish fauna (Bauersfeld 1978, Becker et al. 1981). In a 1956 U.S. Fish and Wildlife Service (FWS) report, recommendations were made to maintain a minimal instantaneous release of 8,000 cfs from Glen Canyon Dam. The proposal included a recommendation to maintain a variation in the rate of release of 50% or less within any one-hour period (Nelson et al. 1976). These recommendations were made without supportive biological data from the down river region. The Bureau of Reclamation (BOR) rejected both recommendations, though the Secretary of the interior decreed a 1,000 cfs minimum. The consequences of these actions or the effects of variable release patterns from Glen Canyon Dam have never been examined ,for the Glen and Grand canyon reach of the Colorado River. The present study provides the first comprehensive effort to understand the relationships between operation of Glen Canyon Dam and the aquatic resources below. OBJECTIVES The field aquatic portion of the Glen Canyon Environmental Studies began in April 1984 and ended in June 1986. This G included 14 major river trips as well as shorter term data collection at important river locations. An agreement between Arizona Game and Fish Department (AGFD) .1,and BOR established broad objectives in an attempt to better -understand the aquatic ecosystem and its relationship to -fluctuating flows. The broad objectives of the present study kwere to: 1) determine the importance of the mainstream Colorado ?bRiver and the impact of fluctuating flow levels within the Grand Panyon to the native and exotic aquatic species; 2) determine the importance of the tributaries to native and exotic fish life histories and determine tributary importance to the overall ative and sport fisheries of the Canyon; 3) determine the -11-