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Thus, operation of the dam has reduced the natural range in the monthly median discharge by a factor of cight. Furthermore, because no post-dam months have median discharges less than 9,000 ft3/s, operation of the dam has biased all months of the year toward either the conveyance of sand through or the erosion of sand from Marble and upper Grand Canyons. Operation of Glen Canyon Dam for hydroelectric- power generation has increased the daily range in discharge by a factor of 15,8, with tbe post-dam median daily range in discharge, 8,580 ft3/s, actually exceeding the pre-dam median discharge, 7,980 ft3/s (figs_ 22A and 25A), In addition, dam operations have increased the median daily range in discharge by a factor of 10,6 over the pre-dam decade with the largest daily range in discharge, the 1920s (fig. 25B), Because of this increased sub-daily variability in discharge, daily mean discharges arc not adequatc to characterize the hydrology of the post- dam river. Thus, the post-dam flow-duration curve computed ii-om daily mean discharges underestimates the duration of higher discharges and overestimates the duration oflower discharges (Jig, 31), Of the four post- dam decades, the I 970s had the greatest median daily range in discharge, 13,700 ft3/s, and the 1990s had the smallest median daily range in discharge, 4,940 ft3/s 1,000,000 o z o frl 100,000 w ~ w ~ ID w ~ () ~ 10,000 () ~ w " ~ <( I ~ 1,000 6 -- POST-DAM FLOW-DURATION CURVE COMPUTED FROM INSTANTANEOUS DISCHARGES - - POST-DAM FLOW-DURATION CURVE COMPUTED FROM DAILY MEAN DISCHARGES -- . ,r 100 o 10 ~ ~ ~ 50 00 m 00 00 100 PERCENTAGE OF TIME EQUALED OR EXCEEDED Figure 31. Comparison of the post-dam flow-duration curve computed from instantaneous discharges with the post-dam flow-duration curve computed .from daily mean discharges. (fig. 32). The mcdian daily rangcs in discharge during the 1960s and 1980s were intermediate to these values at 8,840 ft3/s and 9,830 ft3/s, respectively, For comparison, the pre-dam median daily range in discharge was 542 ft3/s (fig. 32), As in the case of discharge, operation of Glen Canyon Dam has also eliminated the seasonality associated with the daily range in discharge (fig. 33), Operation of the dam for hydroelectric-power generation has increased the daily range in discharge during all but 0, I percent of all days (fig. 34). Not only has operation of the dam greatly reduced the duration of lower discharges, it has eliminated low- discharge months (lig. 35), Prior to closure of the dam, minimum discharges could be lower than 5,000 ft3/s each month of the year (figs_ 30A and 35A), and maximum discharges could be lower than 9,000-10,000 ft3/s in several or more months each year (fig, 358), After 1965, discharges released fi-om the dam were not held lower than 9,000 ft3/s for more than I month until the summer of 2000, when the discharge released irom the dam was held steady at 8,000 ft3/s from June through early September, 100,000 o z o () w w " ~ 10,000 ~ w w ~ () " ~ () ~ ,.7:":::...::::::...:- \ ........h........ \,~ - - - - - - - -- '''-, - -- ........... " ~..... ...., ....\ \~ -'\ \. ~. '\ 'l \ '''-, "-, ill 1,000 " " <( I () w 6 ~ w " z <( " ,. ~ " o "- ',-- '--, ',-- '-- w ~ ~ ~ z <( 6 w ~ 100 100 10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF TIME EQUALED OR EXCEEDED EXPLANA liON -1960s (March 14, 1963-Decomber31, 1970) - -1970s (January 1, 1971-December 31, 1980) .............1980s (January 1, 1981-December31, 1990) 1990s (January 1, 1991-Seplember 30, 2000) -----. PRE-DAM (May 8, 1921-March 12, 1963) Figure 32. Exceedance curves of the daily range in the discharge of the Colorado River at Lees Ferry for the four post-dam decades and the entire pre-dam period of record. ~ 50 Computation find Analysis of the Instantaneous-Discharge Record for the Colorado River at lees Ferrv. Arizona-May 8, 1921, through September 30, 2000