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S44 TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 ~ 113D UTAH ARIlIlNA . USGSGAGlNGSTATlON o USGS WATCR-OlJA11TY STATION o 1996 FlOOO DEPOSIT 1.OCt\11ON . 1997 TEST -FlOW DEPOSIT lOCA T10N Il 1996FUX1J& 10ll7TEST.flOW DEPOSIT LOCATION 37. 112D P_R LEES FERRY GAGE GLEN CANYON DAM lEES fERRY ~ . 'I GRANOCANI'ON GAGE MrNE DIAMOND CREEK GAGE l, o 3ll. " ()/I'('roo<:;:' kII1 LeR HEM CAMERON GAGE , 50 Figure 1. Map of Grand Canyon region showing measurement locations. Glen Canyon lies upstream of the mouth of the Paria River; Marble Canyon extends from the Paria River to the little Colorado River; Grand Canyon lies downstream of the little Colorado River; the portion of Grand Canyon between the mouth of the little Colorado River and the Grand Canyon gage is herein referred to as upper Grand Canyon. The names and station numbers of the depicted U.S. Geolngical Survey (USGS) gaging and water-<juaIity stations are Colorado River at Lees Ferry, station number 09380000 (herein referred to as the Lees Ferry gage); Paria River at Lees Ferry, station number 09382000 (herein referred to as Ihe Paria River Lees Ferry gage); Colorado River above liItle Colorado River near Desert View, station number 09383100 (herein referred 10 as the Lower Marble Canyon gage); little Colorado River at Cameron, station number 09401200 (herein referred to as the Highway 89 bridge at Cameron); Uttle Colorado River near Cameron, station number 09402000 (herein referred to as the LCR near Cameron gage); Colorado River near Grand Canyon, station number 09402500 (herein referred to as the Grand Canyon gage), Colorado River above National Canyon near Supai, station number 09404120 (herein referred to as the National Canyon gage); and Colorado River above Diamond Creek near Peach Springs, station number 09404200 (herein referred to as the Above Diamond Creek gage). Numbers next to the locations of Rood deposits indicate the river miles of these locations. was not supply-limited during all seasons [Topping el al., this issue]. In Ihe predam river, sand would accumulate in Marble Canyon and upper Grand Canyon when Rows were lower than about 200-300 m'js and would either be conveyed through or be eroded from this reach when flows were higher. Seasonal sediment budgets suggest that sand would accumulate in this reach for 9 months of the year (July-March), a period when flows were less than 250 m'js about 73% of the time. This stored sand would then be depleted from April through June during the annual snowmelt flood, a period when flows were greater than 250 m'js about 90% of the time [see Topping et aI., this issue, Figure 11 J. Seasonal depletion of the upstream sup- ply of sand during the annual snowmelt flood was associated with coarsening of the sand in the river (both on the bed and in suspension) and led to the production of inversely graded flood deposits [Topping et al., this issuel. In the postdam river, Rubin et al. [1998] and Topping el al. [1999J have documented similar coarsening in response to the depletion of the upstream supply of sand. In March 1963, the Colorado River was altered by the clo- sure of Glen Canyon Dam, 24 km above the downstream terminus of Glen Canyon. Dam operation for power genera- tion has flattened the shape of the annual hydrograph by not only removing the annual snowmelt flood but also by removing the low flows that were predominant during the predam season of sand accumulation and storage (July-March), such thaI flows of 250 m'js are now exceeded 74% of the time [Topping er al., this issue). Thus operation of the dam has eliminated most ftows in the discharge range that caused sand to accumu- late in predam Marble Canyon and upper Grand Canyon. Given that higher flows now dominate the entire year and that sand is supplied by the tributaries downstream from the dam primarily during 3 months of the year (July-September) [see Topping el aI., this issue, Figure lOb], the postdam Colorado River in Marble and Grand Canyons is essentially a large sediment-feed flume, with a feed device that functions only intermittently. Therefore the postdam river is the ideal natural laboratory for investigating the response of bed elevation and sediment grain size to changes in the upstream supply of sand. In this paper, we present and anal}'2C data collected in the Colorado River in Marble and Grand Canyons (1) during high main stem flows in 1996 and 1997 and (2) during and following large tributary floods in 1983, 1997, and 1998. 2, Systematic Changes in Bed Elevation, Sand Grain Size, and Suspended-Sand Concentration During the 1996 Flood Experiment By virtue of conservation of mass, changes in bed topogra- phy are caused by divergence in the flux of sediment. During periods of changing discharge. divergence in the flux of sedi- ment can be driven by two phKesses: (1) redistribution of the