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57,7 percent of the time during the 1920s, 36.5 percent of the time during the 1930s, 47.5 percent of the time during the 1940s, and 36.6 percent of the time during the 12-year period of the 1950s-early 1960s. During the entire pre- darn period, these discharges were exceeded only 44.3 percent of the time, Likewise, based on discharge alone, the pre-darn mouths of January through March and August through December would be characterized by the accumulation of sand, and April through July would be characterized by the erosion of sand (fig, 23 and Appendix F). Because of the greater tributary supply of sand during July (Topping and others, 2000, fig, lOA), however, sand also accumulated in Marble and upper Grand Canyons during this month (Topping and others, 2000, fig. 1OC). Discharge of the pre-dam river at Lees Fen)' was fairly steady. Therefore, the difference between flow- duration curves computed from either the continuous record of instantaneous discharge from this study or the published record of daily mean discharge is minimal (fig. 24), The pre-dam median daily range in discharge was only 542 ft3/s (fig, 25A), Over decadal time scales, the daily range in discharge of the pre-dam river was somewhat correlated with discharge (fig. 25B). The two wettest decades, the 1920s and 1940s, had the largest median daily ranges in discharge, 808 and 566 ft3/s, respectively. The 1930s were the driest decade with the lowest discharges, but had the second smallest median 1,000,000 - PRE-DAM FLOW-DURATION CURVE COMPUTED FROM INSTANTANEOUS DISCHARGES - - PRE-DAM FLOW-DURATION CURVE COMPUTED FROM DAilY MEAN DISCHARGES o .3 100,000 <.) w "' oc w ~ ~ w w ~ 10,000 " ~ <.) ~ w ~ oc ~ is 1,000 "' " 1000 10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF TIME EQUALED OR EXCEEDED Figure 24. Comparison of the pre-dam flow-duration curve computed from instantaneous discharges with the pre-dam flow-duration curve computed from daily mean discharges. daily range in discharge, 516 ft3/s. The 12-year period from Janu31)' I, ]95], through March 12, ]963, was slightly wetter than the I 930s, but had the smallest median daily range in discharge, 416 ft3/s. The month with the greatest median daily range in discharge was June during the snowmelt flood (fig. 26 and Appendix G), although the daily ranges in discharge were most extreme during the summer thunderstorm season of July through October (fig, 26). The pre-dam daily range in discharge was 131'gest on September 13, 1927, when discharge increased by 68, I 00 ft3/s at Lees Ferry to a peak discharge of 125,000 ft3/s as the result of a flood that mostly originated within the San Juan River drainage basin (fig. 21B). Although such extreme examples exist, large daily ranges in discharge were rare during the pre-dam era. with only I percent of all days having a daily discharge range in excess of 10,000 ft3 Is (fig, 27). Thus. the median daily ranges in discharge during the months of July through October were actually lower than those during the snowmelt flood months of April through June (fig. 26), Effects ofthe Operation of Glen Canyon Dam on Flow Duration, Sub-Daily Discharge Variability, and Sediment Transport To determine the effects of the operation of Glen Canyon Dam on the hydrology of the Colorado River at Lees Ferry and on sediment transport in the Colorado River downstream from Lees Ferry, analyses of flow duration and sub-daily variabiliry in discharge were also conducted on the post-dam p31t of the continuous record of instantaneous discharge, As shown by these analyses, not only has operation of the dam reduced the duration of flood flows, it has greatly reduced the duration of low flows shown by Topping and others (2000) to be important for the accumulation and storage of sand in the Colorado River between the Lees Ferry and Grand Canyon gaging stations (fig. 22A). Prior to the closure of Glen Canyon Dam in ]963, the natural dischmge of the Colorado River at Lees Ferry was lower than 7,980 ft3/s half of the time. By the I 990s, operation of the dam had increased the base flows in the river such that the discharge at Lees Ferry was lower than 8,000 ft3/s only 7 percent of the time. In addition to radically changing the hydrology, operation of the dam for power generation has introduced daily fluctuations in discharge that are much larger and more common than those that generally o~cuITed prior to closure of the dam (figs. 21B and 25A). 02229 Analysis of the Continuous Record of Instantaneous Discharge 45