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ABSTRACT <br />TRODU CTION <br />The closing of Glen Canyon Dam in March 1963 marked a change <br />in temporal discharge patterns in the Colorado River in Glen and <br />°?Grand canyons. Prior to closure, there was little daily <br />.variation in discharge of the Colorado River in our study area. <br />Rather, the variation was seasonal, largely due to snowmelt <br />-"junoff with occasional summer floods and periods of drought. <br />n: <br />Since the onset of regulated flows, the seasonal pulse in <br />;'.discharge from snowmelt has continued, although much abate in <br />most years. Much of the variation now occurs on a daily basis <br />with dam releases dictated by needs for hydropower in the growing <br />metropolitan areas of the southwestern United States. <br />This study was conducted as part of the Glen Canyon <br />~ Environmental Studies (GCES), a multidisciplinary effort to gain <br />°"knowledge on the effects of current and proposed operations of <br />Glen Canyon Dam on the resources of the Colorado River and its <br />tributaries in Glen and Grand canyons. Objectives of the present <br />V ,study were as follows: <br />Determine the distribution, relative abundance, habitat <br />utilization, reproductive activity, movement, growth,. <br />and food resource utilization of fishes of the Colorado <br />River and its tributaries; <br />Determine the relative importance of the mainstem <br />Colorado and its tributaries to life history stages of <br />native and introduced fishes in the study area; <br />3. Assess the relative importance of natural reproduction <br />and artificial propagation (stocking) to the <br />maintenance of the trout fishery in the Glen Canyon <br />tailwater; <br />4. Evaluate and predict the effects of daily flow <br />fluctuations, due to operation of Glen Canyon Dam, on <br />native and introduced fishes to include direct effects <br />such as stranding, and those brought about by changes <br />in water quality, habitat availability, and food <br />resources. <br />-1-