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1989 for the 141-km reach between Lees Ferry and the Colorado River gaging station near Grand Canyon (nwnber 09402500), using hourly discharge data and average sediment transport relations for the gaging stations that constitute sediment inputs to this reach (Lees Ferry, Paria and Little Colorado rivers: stations 09380000,09382000, and 09402000) and the one gage that is the sediment output (Grand Canyon: 09402500). They also computed budgets for the subreaches upstream and downstream from mainstem station 09383100, located just upstream from the Little Colorado River. Their results, consistent with analyses by Howard and Dolan (1981), indicate that the probability of accumulating sediment in themain channel over extended periods (80 years) of alternive sequences of mainstem hydrology and tributary inputs varies from about 30-80%, depending upon how discharges from Glen Canyon Darn are managed. Although net degradation of accumulated sediment occurred between 1983 and 1986 during high release years, under an interim flow regime there is a 70% probability that sediment storage in the main channel will increase over 80-year time periods.. Predictions about long-term change of alluvial bars in Grand Canyon have differed. Although Dolan and others (1974) suggested that widespread erosion of sediment deposits might result from dam operations and Laursen and others (1976) estimated that there was a large deficit in the regulated sediment budget for Grand Canyon, Howard and Dolan (1981) found that sediment deposits had "suffered only a very slight net erosion" after dam closure. Several studies measured patterns of aggradation and degradation associated with the high annual peale discharges that occurred between 1983 and 1986 (Beus et al., 1985; Schmidt and Graf, 1990). On-going sand-bar monitoring (5. S. Beus and C. C. Avery, cds., The inflUence of variable discharge regimes on Colorado River sand bars below Glen Canyon Dam. unpubl. consultants' report to Grand Canyon National Park) indicates that rates of sand-bar erosion decline with time following major floods and that there are many sedimentation ~ses that restore sediment to formerly eroded sites. Schmidt and others (1. C. Schmidt, R H Webb, and P. E. Grams,I994, Using inventory techniques to assess geomorphic change in eddy-dominated rivers, unpubl. manuscript) demonstrated that reattachment bats and channel-margin deposits are more susceptlble to erosion than are separation bars. Changes in campsite availability reflect changes in large, high-elevation sand bats. Kearsley et al. (in press) have shown that at least 30 percent of all campsites decreased in size between 1963 and 1973. During the next 18 yrs, between 1973 and 1991,32 percent of all campsites decreased in size, and campsite capacity decreased by 44 percent Although high discharges in 1983 caused aggradation at a n\DDber of sand bars, by 1991 almost allthese sites had eroded to pre-1983 conditions. Results from sand-bar resurveys, campsite inventories, and computations of sediment mass balance demonstrate that sediment has accumulated on the channel bed while upper surfaces of sand bats have eroded during periods of normal powerplant operation (1965-1979, 1986.1994) when discharges do not exceed power-plant capacity. Entrainment of accumulated sediment on the channel bed and transfer of this sediment to channel margins occurs when flows exceed powerplant capacity and when there is sufficient sand stored on the channel bed. Transfer of sand from the channel bed to the banks is one of the major objectives of flood management. 8