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340 SUMMARY AND SYN1llESIS OF GEOMORPHIC STUDIES TABLE 3. Sand budget for the Colorado River in Grand Canyon during Ihe 1996 controlled flood LITTLE COLORADO RIVER TO GRAND CANYON LEES FERRY TO LITTLE COLORADO RIVER GAGING STATION Average (Mg) Range (Mg) Average (Mg) Range (Mg) Inflow small na 0.8'J06 na High. 0,76'106 0.63100.90'106 o,n 106 0,31 to 0.34 106 elevation sand in eddies High. 0./2'/06 0.0610 0,18'106 0.09'J06 0.044100.13'106 elevation sand on banks Net chanse in .0.65 106 .1.7100.42'106 0.36 106 -055101.3'106 sand in e dies Sand allow- .1.4-106 -2.8 10 _0.20'106 0 o to 0.89'106 elevation in eddies Sandin .0. n 106 .0,9810.0.86'106 - 1.25'106 -0.8910.2.1'106 channel Outflow 0.8'106 na 1.6106 na The flood was also conducted to test the validity of an evolving understanding of the river; that is, the conLrOlled flood was experimental. If results were as predicted, then certainty of understanding was increased; if results differed from predictions, Lhen revision of concepLs was required and new understanding thereby emerged. Success. in these terms, was never in doubt. That many of the predicted outcomes of the flood, such as widespread deposiLion of new sand bars, were realized is gratifying and is testimony to the scientific research Lhat preceded Lhe flood. That there were surprises was exciting, because new understanding was gained and the concept of how controlled flooding could be used in riyer management was thereby refined. If managers determine thaL the changes in alluvial deposiLs that occurred in 1996 are desirable, then future conLrolled floods can be of shorter duration, because the early high rates of fine-sediment deposition rapidly constructed eddy sand bars and channel-margin deposits. The likelihood Lhat large erosion events would remove newly-deposited sediment increased with time. However, other objectives may require longer duration floods, such as the removal of non-native riparian vegetation along the river banks. Flood-formed deposits will erode rapidly after they form. Future floods will probably be timed to coincide with years when the Paria and Little Colorado rivers deliver large loads 10 the Colorado River because suspended sediment concentrations are likely to be highest soon afler tributary floods. However. appropriate timing or floods may be constrained by the volume of water stored in Lake Powell and wiLhin Lhe enlire basin. The longitudinal trends in suspended-sediment transport rate and longitudinal differences in sand storage in eddies and in the channel musL be resolved by future research programs, because it is essential to know if Glen and Marble Canyons are adversely scoured by controlled floods. Thus, expanded monitoring and research will be necessary to provide more precise estimates of the mean change in sand sLorage in eddies upstream from the Little Colorado River, the relative proportion of stored sand Lhat resides on Lhe bed and in eddies, and the suspended-sand Lransport rate. The geomeLry of the Colorado Riyer and its valley changes greatly near river mile 40 [Schmid I and Graf, 1990; MeIis, 1997], and it may be appropriate to com pULe sand budgets ror the upstream and downstream parts of Marble Canyon, respectively. If managers desire to assess the status of river resources from a reach average perspectiye, then statistical character- iz.ations are (he appropriate tool. However. where site specific impacts are critical, deLaiIed surveys and numerical modeling are appropriate strategies to predict channel change. The continued examination of the basic physical processes lhat control sediment transport and geomorphic adjustment on a reach-averaged and sire-specific scale. which was forced by analysis of the research results from the 1996 controlled nood, indicates thai continued research about the physical resources or the Colorado River is essential to the adaptive management of the resources of the Colorado River.