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remaining sand in the channel was generally 100 coarse to be transported onto the high-elevation areas of sand bars, . Topographic surveys of II sand bars in the flTst 76 miles downstream from the dam document a continuing depletion of sand-bar area from 1991 to 1999 (Figure IA). High flows in 1996 and 1997 temporarily reversed this ttend but did not halt the continuing decrease in sand-bar area, The sand bars (above 20,000 cfs) were 22% smaller in surface area in 1999 (Figure IA), although they contained 2-3% more sand than in 1991 (Figure IB), . Topographic surveys of 35 sand-bar sites documented scour of sand during the 45,000 cfs release in 1996, followed by net accumulation (1. Hazel, personal communication)_ Comparison with tributary-input data for the same time, however, indicates that most of the observed accumulation occurred when there was no substantial tributary sand input. . Repeated surveys of channel cross-sections from 1991 to 1999 have shown relatively large and rapid fluctuations in the amount of sediment present (M. Flynn and N, Homewer, personal communications), These fluctuations are interpreted to represent temporary storage and subsequent down-river transport of sediment. These studies have not detected multi-year accumulation of sediment. . Analysis of bed -elevation data at the historical Marble Canyon dam sites suggests considerable loss of sediment from the 1950's to the present. Not only does the post-dam river contain less sand than the pre-dam river, but the remaining sand is generally coarser (Rubin and Topping, in press), I' , , t; '" ., -\'1 ~ -~ )>> "~~, ,. :,' . Geomorphic mapping indicates that deposition of the 45,000 cfs release in 1996 was least near Lees Ferry and was greatest downstteam from the Little Colorado River (Schmidt et aI., 1999; " H. Sondossi, personal communication), The magnitude of "improvement" is greatest further downstteam where more tributaries have delivered fine sediment to the channel, Thus, the "improvement" caused by any specific release above peak power-plant discharge differs both tempora11y and spatially, depending on how enriched or depleted a particular reach is at the time, Inlplications for Current Manaeement Actions ?:/ The features listed above characterize a system where increases in sand abundance result not from incremental multi-year accumulation but rather from temporary storage of individual tributary inputs, In such a system, where increases in sand abundance are temporary, the goal for building sand bars should be to exploit tributary inputs as soon as possible, because the volume of sand available for bar-building is greatest immediately after large tributary inflows, To be effective in rebuilding sand bars, releases above peak power-plant discharge should occur soon after these tributary inflows, before the new sand is lost downstream (Figure 2), Large Paria tributary inflows typically occur during late summer and early fall. Under the rules of the 1996 ROD, however, releases above peak power-plant discharge cannot be implemented on a schedule that takes advantage of such inputs, If a release above peak power-plant discharge cannot be scheduled immediately following a tributary input, another option might be to maintain low flows until a release above peak power-plant discharge could be implemented; the low flows would reduce the amount of sand lost downstream, The magnitude of an acceptable low flow that limits the rate of sand export depends on the volume of sand introduced by tributary flooding, the length of time following the tributary input, and what loss of sand downstream is considered acceptable, At dam releases that are typical of recent years, half of the sand introduced by a tributary flood can be exported within days or weeks (Figure 2)_ Retention of sand for more than a few months 3 ~,' . ?~; '-'~ : <:i., ~ .-?~~ i_I' :....,." f'f. - . e%