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(2) that flows above peak power-plant release (such as the 45,000 cfs flow in (996) can effectively move that accumulated sand from the channel bed to bars, thereby rebuilding sand bars that are eroded by typical dam releases_ Recent Findings Work conducted since the 45,000 cfs release in 1996 has shown that the first hypothesis on which the 1996 ROD was based is false and that the second hypothesis is only partially true, The 45,000 cfs release inl996 increased the amount of sand at high elevations (Figure I), but the sand that was deposited at high elevations came largely from the lower portions of the sand bars (Schmidt, 1999) and not from the channel bed as originally hypothesized_ Under the dam operations imposed by the 1996 ROD, most newly input sand is not stored on the channel bed for long periods of time (Topping et aI., 2000a; Topping et aI., 2000b). Flows above peak power-plant release cannot take advantage of multiple years of sand accumulation, because substantial multi-year accumulation of sand does not occur. Instead, this sand is transported downstream relatively rapidly. The time required to export (transport downstream past the Grand Canyon gage) one-half of a 500,000 metric ton input of tributary sand (the contribution of a typical, moderate, Paria flood) varies from less than one week (for dam discharges of 25,000-30.000 cfs) to roughly one year (for discharges of 10,000 cfs), as illustrated in Figure 2. The time required to export the second half of a tributary input is greater than for the first half (for a constant water discharge), because the second half is coars_er, as a result of winnowing of the bed (Topping et al., 2OOOb; Rubin and Topping, in press), The remaining half, however, is not necessarily sufficient to enable both bar-building and a positive sand balance, For example, the 45,000 cfs release in 1996 exported 700,000 metric tons of sand from Marble Canyon in one week. Thus, a release above peak power-plant discharge is a double-edged sword: high discharges are indispensable for rebuilding high-elevation parts of bars, but high discharges deplete sand resources rapidly (Figure 2), Conducting a release above peak power-plant discharge when recent tributary sand inputs are greatest will tend to minimize the negative impact on the sand resources, Since the 45,000 cfs release in 19%, six kinds of sediment and topographic data have been examined: sediment input and output, changes in grain size of sand on the river bed, changes in sand-bar size, geomorphic mapping, and changes in channel cross-sections, Some of these studies document rapid export of tributary sand (transport past the Grand Canyon gage), whereas others demonstrate a lack of substantial multi-year accumulation of sand, especially in upper Marble Canyon: . Both measurements and calculations of sediment input and output have shown that most fine sediment (sand, sill, and clay) introduced by tributaries is exported within a few months (Topping et aI., 2000a; Topping et aI., 2000b), For example, field measurements show that most sediment introduced by floods on the Paria River in September, 1999, was exported within 6 weeks, On a longer time scale (August 11, 1999 to May 14, 2(00), the Paria supplied approximately 0,8 million metric tons of sand to the Colorado River, while roughly twice this amount of sand (1.5-2 million metric tons) was exported past the Lower Marble Canyon gage. . Changes in grain size of sand on the river bed also demonstrate rapid export of tributary sand. The bed was measurably enriched in finer sand as a result of Pari a floods in September, 1998 (median grain size of Paria River sand is 0.11 - 0.13 mrn). When sampled next (May, 1999), most of the new fine-grained sand on the bed had been winnowed (Topping et a!., 2000b)_ The 2