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<br />OJ238Q <br /> <br />(Gloss et al. 1981, Ward and Stanford 1983, Stanford and Ward 1986, 1991, Angradi et al. 1992, <br /> <br />Stevens et al. 1997). <br /> <br />Concurrently, changes were also made to the Colorado River as it began to form Lake Powell <br /> <br />upstream of Glen Canyon Dam. The river was slowed and began depositing sediment in the <br /> <br />reservoir basin. Vertical temperature and chemical gradients appeared in the reservoir body due to <br /> <br />seasonal density variations ofthe inflows, climatic factors, and the mid-depth location of the <br /> <br />powerplant penstocks. Certain aspects of dam operations over the past 33 years are hypothesized to <br /> <br />have impacts to many of the reservoir?s resources, especially water quality. Comprehensive <br /> <br />scientific assessments have not been conducted to determine the extent of these impacts. <br /> <br />Current and past monitoring programs on Lake Powell have been designed with fairly broad <br /> <br />perspectives in order to understand more about processes that affect various resource areas of <br /> <br />concern. Understanding and monitoring salinity trends and patterns in the Colorado River and its <br /> <br />reservoirs has long been important to maintaining adequate quality for downstream uses under <br /> <br />future water development conditions and has been a primary focus of Reclamation? s long-term <br /> <br />monitoring program. Chemical changes occurring in a filling reservoir have implications to <br /> <br />maintaining fisheries and supplying nutrients to downstream environments. More recently it has <br /> <br />become important to understand the processes in Lake Powell that determine the physical, <br /> <br />chemical, and biological characteristics of downstream releases to the Grand Canyon ecosystem. <br /> <br />This was a primary objective for the revision of the Reclamation's Lake Powell monitoring by the <br /> <br />Glen Canyon Environmental Studies office in 1990. Evaluation of a potential selective withdrawal <br /> <br />structure on Glen Canyon Dam will require an understanding of hydrodynamics, warming <br /> <br />processes, thermal budgets, and biological effects upstream and downstream of Glen Canyon Dam. <br /> <br />The effects of planned or unforeseen operational changes at Glen Canyon Dam may have far <br /> <br />reaching effects on varied physical, biological, and cultural resources in the Grand Canyon and on <br />06/02/98 DRAFT P,r ~ of6l <br />