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00641 Modelling of sand deposition in Colorado River 389 . without significant recirculation zones, sand was stored in narrow bands along the channel sides under most conditions, primarily in the lee of channel irregularities. Sand volumes deposited above the 708 mJls stage in the Lower Tanner reach and in the critical right bank region (J) in the Upper Unkar reach were, at best, half the deposit volumes in the Above Lava Chuar and Palisades recirculation zones. Recir- culation zones appear more consistent in response to increased discharge and vari- ations in sand supply, and store more sediment for a given reach length. Trends in overall sand storage in reaches dominated by recirculation zones can be generalized, but in reaches where sand storage is dependent on liner-scale morphology of the channel sides, the trends are more variable. High discharge releases. Higher flows, such as the 2830mJ{s flow, tend to deposit sand in sheltered areas even with low sand supply. This is especially evident in the two upstream reaches in this study. Higher water discharges are significantly more elTective in depositing sand in critical areas than lower discharges. This is a result of the greater sand transport rate for given sand conditions and, perhaps more sig- nificantly, the larger accommodation spaces created by higher stages. High-flow duration. Deposition rate depends strongly on the volume of sand already present at a depositional site. Deposition rate falls rapidly as the site fills. As a result, high flows are most effective within the Iirst day or two in filling depositional sites. Longer-duration high releases distribute sand more thoroughly within a depositional site, as pointed out by Anima elal. (1998), but are less efficient at utilizing sand in transport. . Sand supply. High sand concentrations, such as would occur during significant tributary flow, would be most effective in forming significant deposits in critical areas. The possibility and potential advantages of dam releases timed to coincide or shortly follow tributary flows were considered by the authors of the EIS (US Department of Interior, 1995), and were reiterated by Lucchitta and Leopold (1999) with particular emphasis on flows in the Little Colorado River. Careful analyses of sand deposits, sand transport processes and suspended sand measure- ments (Rubin etal., 1998; Topping "0/., 2000a,b) have provided a process-based explanation of the importance of tributary inputs to replenishing sand resources, especially those in Marble Canyon that rely primarily on the sand inputs from the Paria River, and a quantification of the potentia! benefits of high releases associated with tributary inputs. Releasing high flows during or shortly following significant tributary flows would also increase the supply of fine-grained sediment, which would increase the stability of the deposits (as pointed out by Richard Hereford, USGS, oral communication, 1998). High discharges increase the total volume of the deposit and place the sand higher up lhe channel bank and, therefore, would place a larger volume within the gully mouths. It is important to note that these study sites are in a relatively sand-rich reach compared with the channel above the confluence with the Little Colorado River. High discharges may deposit sand at many sites in this region, even with a low sand supply; but upstream of the confluence wilh the Little Colorado River, the lower sand supply in the absence of significant flow in the Paria River would make that reach more vulnerable to erosion (Schmidt, 1999; Topping elal., 2000a,b). . .-.-..' :"'f:-' :l,...-". '1:; r', L;;t~., .... ~"'; '~:1- ". ,