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<br />Once epilimnetic withdrawal has been established, releases cool rapidly with the <br />continued cooling of the mixed surface layer of the reservoir. By late February or early <br />March, winter cooling has reached a maximum, the conductivity of the epilimnion <br />increases, and minimum temperatures and maximum conductivity levels appear in dam <br />releases. An upwelling of the hypolimnion due to displacement influences of high- <br />density winter inflows to the reservoir may also affect these patterns. Because of the <br />proximity of the zone of maximum stratification to the penstock level, storms on Lake <br />Powell can cause significant temporary shifts in release water quality during this period <br />due to oscillations of stratification within the reservoir caused by the weather disturbance. <br /> <br />With spring surface warming, the reservoir begins to stratify, isolating the penstock <br />withdrawal zone from surface processes and terminating epilimnetic withdrawal. <br />Through the upcoming summer months release temperature gradually increases and <br />conductivity decreases. <br /> <br />Glen Canyon Dam release temperature has varied in the last decade between 7 and 12 <br />deg C, Specific conductance levels have ranged between 600 and 1000 J.lS (390 to 650 <br />mgIL TDS) and typically fluctuate by around 200 J.lS (130 mgIL TDS) on an annual <br />basic. During the middle 1990s there was a decreasing trend in salinity of dam releases, <br />corresponding to above average inflows from the Upper Colorado River basin. <br /> <br />Recent Patterns <br /> <br />Water Year 2000 exhibited an unusual release pattern from Glen Canyon Dam. Because <br />of decreasing inflow forecasts, a decision was made to carry out the Low SUmmer Steady <br />Flow (LSSF) experiment, recommneded by the US Fish and Wildlife Service for the <br />benefit of native fish, This began on March 25, 2000 when daily fluctuations ceased and <br />steady releases of 8000 cfs were begun. Releases were increased to 17000 cfs on April 8, <br />2000 and held steady until May 3, 2000, when a 3-day high flow of 3??oo cfs was <br />released, near powerplant capacity. This was followed by another 17000 cfs flow period <br />and 2 other brief periods at 19000 cfs and 13500 cfs. On June 1,2000,8000 cfs was <br />released and remained steady at this level until September 5, 2000 when another 3-day <br />powerplant capacity flow of 3??oo cfs was released. Releases were then returned to <br />8000 cfs and, with the exception of a brief power emergency on September 18, 2000, <br />remained steady at that level until October I, 2000, after which normal fluctuations were <br />resumed. <br /> <br />Warming patterns under the last year's steady flow experiment (Figure 3) are compared <br />with those of 1997 (Figure 2), a year in which flows were steady for the most part but at <br />levels above 2??oo cfs for the majority of the year. Figure 4 shows'a warming increase <br />of over 0.5 deg C from June to September of 2000 over that of the same time period in <br />1997. <br /> <br />10/25/00 <br /> <br />DRAFT <br /> <br />4 <br />