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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 sediment (i.e., the fines will be winnowed from the bed), re- sulting in ooarsening of the bed. This was observed at the Grand Canyon gage following the 1983 Little Colorado River flood, at all measurement locations during the 1996 flood ex- periment. at all measurement locations during the 1997 test flow, and on the bed of Marble Canyon between September 1998 and May 1999. A stratigraphic record of this winnowing process is preserved in the inversely graded flood deposits that were produced during predam snowmelt floods [Rubin el aI., 1998; Topping et al., this issuel, the 1996 flood experiment, and the 1997 lest flow. 7. Conclusions Systematic changes in bed elevation at a cross section during a flood can be used to make inferences about depletion of the upstream sediment supply in rivers. At a cross section that aggrades with increasing water-sunace stage (e.g., the Grand Canyon cahleway cross section), the ohservation that the time of maximum bed elevation occurs prior to either the peak or the receding limb of a flood indicates the presence of sediment supply limitation. However, at a cross section that scours with increasing water-sunace stage, the effects of depletion of the upstream sediment supply cannot be separated from Ihe ef- fects of a delayed response of the bed topography to changing flow oonditions. The grain-size distribution of sediment in rivers evolves sys- tematically as a function of changes in the upstream sedimeTIt supply. In the Colorado River, grain size evolves in response 10 both tributary acIivily and dam operation, resulting in signifi- cant changes in sediment-transport rates over time. Sand- transport rates in the Colorado River have been observed to change by as much as a factor of 55 in response to these changes in grain size. After sediment is added to the Colorado River during tributary floods, it travels downstream as a sedi- ment wave that elongates as finest sizes are preferentially transported downstream (as observed on the bed of Marble Canyon in September 1998). AI; the fine front of a sediment wave reaches a given site, both the bed and suspended sedi- ment will first fine, and sediment-transport rates will increase. On the "receding limb" of a sediment wave both the bed and suspended sediment will coarsen as the upstream supply of the finer grain sizes decreases. In response to the decreased up- stream supply of the finer sizes, fines will be winnowed from the bed, and sediment-transport rates will decrease. During mainstem floods this process results in coarsening of lbe sediment supplied to eddies and produces inversely graded deposits. Because sand-transport rates change substantially in re- sponse to grain-size changes following tributary sediment in- puts, sediment budgets cannot be constructed [or reaches of a bedrock, supply-limited river like the Colorado River by as- suming stable relationships between the discharge of water and sand-transport rates. Such an approach [U.S. Deparrmenl of the Inlerior, 1995] calibrated to a relatively depleted state prefer. entially underestimates sand-transport rates following tribu- tary floods and results in the prediction of substantial sand accumulation in the Colorado River over time. The key to understanding sand transport and therefore sand budgets in the Colorado River is an understanding of the processes that control the short-term fining of sand in the river following large tributary floods and the subsequent coarsening of sand in the river as the fines are winnowed from the bed and either de- posited in eddies or transported downstream. 569 AckaowIedpl...... This work was funded by the Grand Canyon Momtonng and Research Center and was oonducted in collaboration with the USGS Arizona District Arizooa District personnel oollected the 1983 wU.a al the Grand Canyon gage, the 1996 data at the Lower Marble Canyon and National Canyon gages, and the 1997 data at all of the gages. The Bureau of Reclamation provided the P-61 suspended- sediment sampler used to collect samples at the Grand Canyon gage in 1996. Conversations with Ted Metis. Joe Hazel, Steve Wiele, Jack Schmidt, Peter Wilcock, Roberto Anima, Nancy Hornewer, and Mar- garet Franseen helped improve the science. Steve Wiele, Joseph Lyons., Jim Bennett, L.es Vierra, Tom Hopson, Sam Jansen, Curt Crouch, Jack Schmidt, Joe Hazel, Matt Kaplinski, Mark Manone, Greg Williams, Kelly Smith, Steve Bledsoe, and Matthew Fahey helped collect the data presented In this manuscript. 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