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<br /> <br />OD2311 <br /> <br />The location of the penstock intakes primarily defines the quality of Glen Canyon Dam <br /> <br /> <br />releases. Variation in temperature, salinity, and other water quality characteristics are greatly <br /> <br /> <br />reduced from pre-dam conditions (Figs. 11 and 13), The penstock withdrawal zone is deep <br /> <br /> <br />enough that it is isolated from thewide.seasonal fluctuations of the epilimnion, except at times of <br /> <br /> <br />epilimnetic withdrawal as described in the previous section. Suspended sediment has essentially <br /> <br /> <br />been removed due to settling in upstream portions of the reservoir. The structure of Glen Canyon <br /> <br /> <br />Dam also creates 6.12 maf (7.55 km3) of inactive and dead storage in Lake Powell below the <br /> <br /> <br />penstock elevation, unavailable for hydropower generation. Cold saline water of high density can <br /> <br /> <br />build up in this area and remain relatively isolated from other mixing processes in the reservoir. <br /> <br /> <br />Organic material entering the hypolimnion will accumulate and decompose, gradually lowering <br />dissolved oxygen levels. This water will remain relatively stagnant until it is displaced by inflows <br />of higher density or flushed by high reservoir throughput. The process of intrusion of saline water <br />into the hypolimnion, followed by decreasing oxygen levels until the hypolimnetic water is <br />replaced appears during three periods in Lake Powell's history, from 1967 to 1973, from 1978 to <br />1982, and from 1991 to 1997. <br /> <br /> <br />b. Annual Release Volumes <br /> <br /> <br />Annual release volumes from Glen Canyon Dam are dictated by the Criteria for Coordinated <br /> <br /> <br />Long-Range Operation of Colorado River Reservoirs (pL 90-537) and the development of an <br /> <br /> <br />annual plan of operation based on streamflow histories, water supply probabilities, anticipated <br /> <br /> <br />depletions, and other legal and institutional requirements. Glen Canyon Dam is legally required to <br /> <br />release a minimum of 10.2 km3 (8.23 mat) of water per year for downstream compact and treaty <br /> <br />requirements. Since construction of the dam, this amount has been exceeded during five separate <br /> <br /> <br />periods. In 1965, releases were increased following 2 years ofIow releases during the initial <br /> <br /> <br />filling stages of Glen Canyon Dam. In the spring of 1973 a court order directed the drawdown of <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 35 of62 <br />