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DD2310 Regardless of whether the inflow density current overrides or displaces the hypolimnion, there is a consistent annual pattern of apparent upwelling of the more saline hypolimnion each winter into the penstock withdrawal zone. This usually ocCurs shortly after the first of the year and may be the result of the momentum ofthe$inter inflow density current temporarily displacing the horizontal upper boundary of the hypolimnion. This causes, in effect, an internal oscillation or seiche of this boundary. The effect downstream is that during this period ofhypolimnetic upwelling, dam releases suddenly become colder and more saline. This usually coincides with high winter releases from Glen Canyon Dam. 3. --Glen Canyon Dam Operations a. Presence and Structure of Glen Canyon Dam Glen Canyon Dam has three structures from which water can be released (Fig 17). The majority of Glen Canyon Dam releases are through eight penstock intakes which route water to the powerplant turbines with a combined capacity of940 cms (33,200 cfs). The penstocks withdraw water from an elevation of 1058 m (3470 ft) above mean sea level, or a depth of70 m (230 ft) when the reservoir is at full pool with a surface elevation of 1128m (3700 ft). When release requirements dictate, an additiona1425 m3/s (15,000 cfs) can be discharged through the river outlet works which are situated at an elevation of 1028 m (3374 ft), 30 m below the penstock structures. The third means of withdrawal is from the spillways that have a combined capacity of 5890 m3/s (208,000 cfs) and withdraw water from elevations above 1122 m (3680 ft). Releases from the river outlet works and the spillways bypass the powerplant and can not be used for hydropower generation. In 1965, high releases were routed through two bypass tunnels used during the construction of the dam. The upper portions of these tunnels were subsequently plugged; the lower portions became the lower spillway tunnels. 03/06/98 DRAFT Page 34 of62