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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 1 533 200 t,'j SUPPLY il1l EXPORT _'00 . < 2 u < 1i E < ~ I 50 .... a: 0 Q. X W .. >- -' Q. Q. ::> <II .... 20 z w " 5 w <II W Z "' 10 5 c) ~ ~ ~ ~ ~ m ~ I Z ~ Q Q Q ~ ~ Q ~ Q Q m SEDIMENT YEAR Figure 9. (continued) Ferry and Grand Canyon gages in the postdam era, are the Paria and Little Colorado Rivers. During their respective pe- riods of sediment record the mean-annual suppfies of fine sediment were 3.0 :!: 0.6 million t from tbe Paria River (of which about 50% was sand) and 8.6 :!: 1.7 million t from the Little Colorado River (of which about 30-40% was sand). During the period of sediment years 1949-1970 the mean- annual supply of fine sediment from the ungaged tributaries between Lees Ferry and the Grand Canyon gage was estimated at 0.72 :!: (a factor of 3) million t [after Griffiths el aI., 1998]. Because the periods of record at each of the four USGS sediment stations were different, a reasonable sediment budget for Marble Canyon and upper Grand Canyon could only be constructed for sediment years 1949-1955, 1958, and 1960- 1970 (Figure 9c). This period included 11 years from the pre- dam era and the first 7 yeaTS of the postdam era. During both the predam and postdam periods in this sediment budget the mean-annual fine-sediment supply generally equaled the mean-annual export to within the measurement error (Figure 9c and Table I). As discussed in Appendix B and shown in the analysis in Table 1, the uncertainties in the sediment loads would have to be much lower than is reasonable to result in the prediction of substantial multiyear storage of fine sediment in either the predam or postdam periods. Given the uncertainties in the sediment budget, the only years in which the flne- sediment supply can be shown to have exceeded the export were sediment years 1954 and 1963, and the only years in which the export can be shown to have exceeded the supply were sedimenl years 1949 and 1965. 6.2. Magnitude ODd Duration of Seasonal SedIment Storage Our analysis of the predam differences in sand-transport rates between the Lees Ferry and Grand Canyon gages (Figure 4b) suggests tbat sand accumulated in Marble Canyon and upper Grand Canyon during low 8aws and was eroded from Marble Canyon and upper Grand Canyon during high 8aws. Therefore, to determine the mean seasonal patterns of fine- sediment accumulation, storage, and erosion both under oat- ural conditions and under normal dam operations, we aver- aged, differenced, and integrated the monthly sediment-load data for the 11 predam years with complete data and the 5 postdam years with fluctuating flows. This approach is justified because the annual fine-sediment supply to and export from Marble Canyon and upper Grand Canyon during tbese periods were equal to within the uncertainties in the sediment budget, with the exception of only 2 of the II predam years. The mean supplies (Colorado River at Lees Ferry, Paria River, Little Colorado River, and ungaged tributaries) and export (Colo- rado River at Grand Canyon) of fine sediment each month during the average predam and postdam years are shown in Figures lOa and lOb. The amount of fine sediment in storage after each month during the average predam and postdam year are shown in Figures IOc and IOd. Substantial differences exist between the magnitude and du- ration of seasonal fine-sediment storage in the average predam and postdam years (Figures IOc and IOd). During the average predam year the most rapid accumulation of fine sediment in Marble Canyon and upper Grand Canyon occurred [rom July