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Proceedings of the Seventh Federal Interagency Sed,mentation Conferenct!, March 25 to 29. 2001. Reno. Nevada . EXAMPLES Calculation of a An example of grain-size regulation of sediment transport occurred during an experimental flood on the Colorado River in Grand Canyon in 1996 (Rubin et aI., 1998; Topping et aI., 1999), For the seven days of the flood experiment, clear water was released from Glen Canyon Dam at the rate of 1270 rJ/s, In response to this erosive flow, the bed at the Grand Canyon gage coarsened, which caused suspended sediment to both coarsen and decrease in concentration. The resulting negative correlation between suspended-sediment concentration and grain size (Fig. 2a) demonstrates that grain-size regulation was important during this event. Calculated values ofa for data co\Iected at 3 reaches during the flood were .1.5, ,3.3, and .6.3, indicating that changes in sediment transport were regulated primarily by changes in bed-sediment grain size (10.) > I). Calculation of B The same observations of suspended~sediment concentration used to calculate a can be used to calculate changes in the relative coarseness of sediment on the bed during the flood (Fig. 2b). Comparison with sampled bed sediment not only shows good agreement, but the smoother trend of the calculated values suggests that the calculalions may be more representative of the system than measurements at a single cross-section. In this case, where river discharge was constant, changes in ~ reflect actual changes in grain size of sediment on the bed. In other situations, where discharge is free to vary, calculated changes in P can reflect changes in grain size on the bed, as well as changes in the region of the bed that is accessible to the flow, The predicted values of bed ,sedi men I diameter are in close agreement with observed values. The predicted values have less scatter than the values observed at a single cross-section and may be more representa tive of the river. APPLICATIONS . The technique developed by Rubin and Topplng (in press) and summarized in this paper has important applications with respect to: (I) designing sediment-transport measurement programs, (2) constructing sediment-transport models, (3) providing a starting point for the accurate determination ofTMDLs for sediment, and (4) determining whether changes in upstream sediment budgels are positive or negative. First, this technique allows one to best design a sediment-transport measurement program. If, in a given situation, the dominant regulator of sediment transport is the flow, then an approximately stable relationship exists between the discharge of water and sediment transport (i.e., a stable sediment rating curve exists). In this case. measurements can be collected so that they best define a sediment rating curve (i.e., they are uniformly distributed across the entire range of flows). In contrast, if the dominant regulator of sediment transport is the grain size of the bed sediment, then sediment transport is controlled by changes in the upstream sediment supply and no stable sediment rating curve exists; measurements need 10 be closely spaced in time. In a similar manner, this technique provides a guide to knowing the pertinent physical processes to include in a sediment transport model. If the dominant regulator of sediment transport is the flow, then a model can be constructed that predicts a stable sediment rating curve by assuming that the channel geometry and bed sediment are in equilibrium with the flow. However, if the dominant regulator of sediment transport is the grain size of the bed sediment. then a totally different type of mooel needs to be constructed. one that routes sediment downstream from its source. A third application of this technique involves the Calculation of TMDLs for sediment In the case where Ihe now is the dominanl regulator of sediment transport, TMDLs can be easily calculated using stable sediment rating curves (either measured or modeled). However, in the case where bed sediment grain size is Ihe dominanl regulator of sediment Iransport, TMDLs Call only be calculated by detennining the maximum naturally occurring daily supply of sediment. Finally, Ihis lechnique allows upstream sediment budgets to be inlerpreted based on data from only one site. By determining whether P is increasing or decreasing over long time scales, one can detcnnine whether the upstream supply of fine sediment is decreasing or increasing over long time scales. . 1-203