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}.i" i' ~ "1 :~, 12 I -...-- - . ! ! . . . . I . I I 11 -.....---+---t--- I I I [ 10 ----L----+-- -- I . . 9 ---I------~- !i 8 Q f 7 8 I a- S - l. 4 j s B- e ~ 1 i--T--- I I , I .-.... ------ -- ----------- ---r--- I I' I I I I I ,I I I I I I ____1.___ _____ ___ ____' _____ ______ j i 1500 ) ;j 1400 1300 1200 1100 1000 ~ 900 'E' 8 800 g- 700 600 500 400 ~~: \~ ;.~~, ..' ~ "'1'". .. o ----- - - 300 19l111 'W89 1990 1991 1992 1995 1994 1995 1998 1997 1998 1999 2000 2001 Figure 1 Water quality patterns below Glen Canyon Dam i'"., The quality of dam releases is largely a function of density patterns in the reservoir immediately upstream of the dam, The penstock withdrawal zone is fixed at an elevation of 3470 ft ams!.. Density is primarily detennined by water temperature and dissolved mineral content, increasing with lower water temperatures and higher conductivity. Therefore, temperature and conductivity (a function of dissolved mineral content) of dam releases follow an inverse pattern to each other as the density of dam releases changes. Release temperature increases as the reservoir warms through the summer months. Conductivity decreases as the water near the dam is influenced by the previous spring's runoff. With autumn and early winter surface cooling, the epilimnion of the reservoir mixes to greater depths, deepening the thermocline, and warming the water near the thermocline. With the deepening thermocline, the epilimnion begins to influence the penstock withdrawal zone, dominating releases by the end of the year. The cooling epilimnetic water is still warmer than deeper waters of the reservoir and releases reach their maximum temperature at this time. Because the surface of the reservoir is more dilute due to influence of the previous season's snowmelt runoff, release conductivity reaches a minimum value at this time of epilimnetic withdrawal. .,' :.... i~ .-; "',' . ' " 10/25/00 DRAFT 3