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534 TOPPING ET AL: COLORADO RIVER SEDIMENf TRANSPORT, I TIlbIe L Maximum Demonstrable Annual OIange in F'Ine-Sediment Mass in Marble Canyon and Upper Grand Canyons as a Function of Unoertainty in the Sediment Loads of the Colorado River, Poria and Little Colorado Rivers, and Ungaged Tnbutaries Uncertainties Assigned to Sediment Loads, . % Sediment Year 2t,5t 2t,Illt 2t,2O 2t,30 5,5t 5,Wt 5,2Oi 5,30 1O.5t 10, lot 10,20 10,30 1949 -9.9 -9.8 -95 -9.3 -4.4 -4.3 -4.1 -3.8 0 0 0 0 1950 0 0 0 0 0 0 0 0 0 0 0 0 1951 2.1 1.9 1.7 15 0 0 0 0 0 0 0 0 1952 -3.3 -2.4 -0.6 0 0 0 0 0 0 0 0 0 1953 0.9 0.6 0 0 0 0 0 0 0 0 0 0 1954 4.1 . 3.7 2.8 1.9 2.0 1.5 0.7 0 0 0 0 0 1955 0 0 0 0 0 0 0 0 0 0 0 0 1958 -1.4 -0.6 0 0 0 0 0 0 0 0 0 0 1960 0 0 0 0 0 0 0 0 0 0 0 0 1961 0 0 0 0 0 0 0 0 0 0 0 0 1962 0 0 0 0 0 0 0 0 0 0 0 0 1963 5.8 5.7 5.3 5.0 4.8 4.6 4.3 3.9 3.0 2.8 2.5 2.2 1964 1.4 0.7 0 0 0.8 0.1 0 0 0 0 0 0 1965 -10.8 -9.8 -8.0 -6.1 -9.4 -85 -6.6 -4.7 -7.1 -6.2 -4.3 -2.5 1966 1.6 0.9 0 0 1.3 0.6 0 0 0.7 0 0 0 1967 0.6 0.3 0 0 05 0.1 0 0 0.1 0 0 0 1968 0 0 0 0 0 0 0 0 0 0 0 0 1969 0.7 0 0 0 0.3 0 0 0 0 0 0 0 1970 0 0 0 0 0 0 0 0 0 0 0 0 Values are in million metric tons; positive values indicate nel annual storage of sediment; negative values indicate net annual erosion of sediment; and zero values indicate that the magnitude of net annual storage or erosion of sediment is less than the magnitude of the uncertainty in the sediment budget. iIIlbe fint number indicates the uncertainty assigned to the measured sediment loads of the Colorado River at the Lees Ferry and Grand Canyon gages. The second number indicates the l,ID.ccrtainty assigned to the measured sediment lo.ads of the Paria and Little Colorado Rivers. In each case an uoeertainty of a factor of 3..... assigned to the estimated sediment loads of the uogaged tnbutaries (see Appendix A). tThese uncertainties are thought to be unre8SODllbly low based on f,be discussion in Appendix B. meso are uoeertainties used in this paper. through August, and, given the assigned uncertainties, at least I million t of fine sediment were retained in storage from September through March. 'Then, from April through June this stored sediment was probably eroded and may have been ex- hausted by the end of April. As sbown in section 5.1, relatively tittle difference in predam silt and clay concentration existed between the Lees Ferry and Grand Canyon gage6 (Figure 4a), whereas large flow-dependent differences in sand concentra- tion existed between the two gages (Figure 4b). 'These differ- ences were greatest for the 0.0625-0:25 mm sand (Figures 4<: and 4d); thus the fine sediment seasonally stored in Marble Canyon and upper Grand Canyon was probably largely com- posed of 0.0625-0.25 mm sand. To place these minimal mag- nitudes of seasonal sand storage in perspective, the 1949-1976 mean-annual sand load of the Paria River was approximately 1.76 million t, of which half was finer than 0.125 mm and 90% was liner than 0.25 mm (Topping, 1997]. In contrast to the predam case, given the same set of uncenain~es, sand cannot be demonstrated to accumulate for more than 1 month or be retained in storage for more than 2 months in Marble Canyon and upper Grand Canyon during the average postdam year (Figure IOd). 'The hydrology of the predam Colorado River during the seasons of sand accumulation and storage (July-March) and sand erosion (April-June) is that expected, given the flow- dependent differences in the transport rates of finer (0.0625- O.25-mm) sand at the Lees Ferry and Grand Canyon gages (Figure 4<:). A1; shown in Figure 40, (1) finer sand should have accumulated in Marble Canyon and upper Grand Canyon when the flows were lower than about 200-300 m'/s, (2) finer sand should have been conveyed through Marble Canyon and upper Grand Canyon when flows exceeded 200-300 m'/s, and (3) finer sand should have been either eroded from or con- veyed through Marble Canyon and npper Grand Canyon when flows eJceeded 400-500 m3/s. Indeed, during the season of sand accumulation and storage (July-March), flows exceeded F~ 10. (Opposite) (a) Mean supplies and export of fine sediment each month (with uncertainties) during the average predam year. These values were calculated from the data col- lected during sediment years 1949-1955,1958, and 1960-1962 (i.e., the predam years with complete sediment data). (b) Mean supplies and export of fine sediment each month (with uncer- tainties) during the average postdam year with fluctuating flows. These values were calculated from the data collected during sediment years 1966-1970. Note that the cumulative monthly loads of the Paria and Little Colorado River are highly correlated with the monthly export of line sediment past the Grand Canyon gage, suggesting minimal accumulation of fine sediment in the postdam river. (c) Amount of fine sedi- ment in storage after each month (with uncert~ties) during the average predam year. These values were det~rmined by differencing and integrating the data in Figure lOa. In the average predam year, fine sediment accumulated in Marble Canyon and the upper Grand Canyon during 9 months of the year and then was eroded. This seasonally stored and eroded fine sediment was probably largely 0.0625-0.25 mm sand (see text), (d) Amount of fine sediment in storage after each month (with uncertainties) during the average postdam year with fluc- tuating ~ows. These values were determined by differencing and integrating the data in Figure lOb. In the average postdam year, fine-sediment storage in Marble Canyon Cl:nd the upper Grand Canyon cannot be demonstrated for morc than 2 months. A1; in Figure IOc,this stored and eroded fine sediment was probably largely sand.