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002330 channel bed for multi-year periods prevailed for the next 15 years. Sediment budgets based on sediment-transport measurements made in the mid-l 980s were calculated by Andrews (1990). He found that, "A three- fold decrease in mean annual peak water discharge, plus the large contribution of sediment by tributaries, results in a surplus rather than a deficit of sediment." The potential beneficial role of floods in transferring sand to high elevation was evident from the increase in the number and size of campsites immediately after the 1983 flood. Brian and Thomas (1984) found that the 1983 flood completely eliminated 24 campsites, but that approximately 30% of the remaining campsites aggraded. Erosion was typical in Marble and upper Grand Canyon. The campsites where there was significant aggradation were primarily located in west- ern Grand Canyon. These [mdings were echoed by Beus et aI. (1985) who resurveyed sand bar profiles immediately after the 1983 flood. They found that the 1983 flood cre- ated thick new sand deposits at many sites, but they noted that some sites were com- pletely eroded. Beus et aI. (1985) assumed that the sites of thick sand deposition were more numerous than those that had eroded and concluded that flood-induced deposition reversed the system-wide progressive erosion trends of the previous 15 years: "A substan- tial net gain of sand on the 20 beaches [that were surveyed] more than compensated for the previous eight-year loss. Possibly occa- sional high water 'spills' through the Grand Canyon are desirable to maintain existing campsite beaches." A significant management implication of the assumed accumulation of sand on the channel bed and beneficial role of floods was that multi-y.ear bed accumulation could be redistributed to the channel edge by floods. Smillie et aI. (1993) suggested that accumu- lation rates could be controlled: "flow fluc- tuations and corresponding sand transport in the Colorado River can be managed to achieve a balance with long-term average annual sand inputs from the Paria River." This view of sediment management was described in the Final Environmental Impact Statement for Glen Canyon Dam Operations (U. S. Department of the Interior, 1995) and the associated Record of Decision that proposed a sequence of controlled floods that would redistribute fine sediment from the channel bed to the banks (Schmidt et aI., 1999a). Kears1ey et aI. (1994) adopted an ap- proach similar to that employed by Brian and Thomas (1984) to measure system-wide changes in sand bars. K~arsley et aI. (1994) made semi-quantitative measurements of the size of every campsite in Grand Canyon from aerial photographs taken in 1965, 1973, 1984, and 1990. Thus, they did not assume that the 20 sites measured in detail in the field by Beus et aI. (1985) were representative ofthe popula- tion of all eddy bars. Their measurements were less precise but more spatially robust than those reported by Howard and Dolan (1981) and Beus et aI. (1985). Kearsley et aI. (1994) found that there had been more post- dam erosion than identified by other studies: "at least 30% of all campsites decreased in size during the first 10 years of Glen Canyon Dam operations, and 32% of all campsites decreased in size during the next 18 years." They found that the "benefit" of sand aggradation caused by the 1983 flood was short lived, and by 1991 only a few campsites were larger than they had been in 1973. Webb (1996) compared the size of sand bars in two elevation zones in replications of ground-level photographs originally taken in 1889 and 1890 during the R. B. Stanton expe- dition, and he found a significant difference in the size of bars in the upstream and down- stream parts of the Colorado River corridor. He found that 72% of low water sand bars, defined as those inundated by flows less than the capacity of the Glen Canyon Dam powerplant, had decreased in size during the intervening century. He found that the propor- tion of sand bars that had eroded decreased 22 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ...