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338 SUMMARY AND SYNTHESIS OF GEOMORPHIC STUDIES ciem sediment ayailabIe to build eddy bar.; everywhere. Hazel et 01. Ithis volume] and Thompso/l et 01. lI997J showed thai there were large increases in the volume of newIy-deposiled sand in Marble Canyon and thaI Ihe proportional increase in bars was greatest rhere. However, area-weighled daily deposilion was much lower in Ihe 2 siles in lower Marble Canyon measured by Andrews el 01. [this yolume] than in Ihe 3 sites further downstream (Figure 2). The sites in lower Marble Canyon were eXlensiveIy eroded on the first day of Ihe flood, and it took many days of slow aggradation to replace the sedimenl that had been initially eroded. Other evidence also suggests thaI the extenl of aggra- dation was less upstream from the LiltIe Colorado River. More than 40% of eddies in the Tapeats Gorge and the Big Bend reaches downstream from the Lillie Colorado River filled 10 more than 50% of their capacity. FUrlher upstream, less lhan 30% of eddies filled to this extent. Similarly, more than 25% of eddies downstream from the Lillie Colorado River had nel nonnalized aggradalion (NNA) values greater than 0.25; upstream, less than 15% of eddies had NNA values greater than 0.25 [Schmidt el aI., this yoIume]. The average net topographic change in eddies, averaging high-elevalion deposilion and low-eleyalion erosion, was negaLiye for measured sites upstream from the LillIe Colorado River and positive for the sites between the LillIe Colorado River and the Grand Canyon gage [Hazel et 01., this yoIume]. These differences between the extent of deposition upstream and downstream from the LillIe Colorado River are probably due to the fact that suspended-sedimenl concentrations were much lower in Marble Canyon and that the magnitude of eddy deposition is dependent on Ihe concentration of suspended sediment Topping el 01. [Ihis volume] showed that Ihe suspended-sediment concentra- tions at the gage upstream from the LilLIe Colorado River were approximately half of the concentralion downstream, and the total transport of sand from Marble Canyon was ahout hair of thaI transporled pas I the Grand Canyon gage. As argued by Rubin elal. [19941, low sediment concentra- tions in the main flow, IimiLed sedimenl repIenishmem from only one large sedimenl-supplying tribulary, and limited sediment storage on the bed have the potemialto cause net scour in rcaches nearesl to Lees Ferry. These observations highlight Ihe need 10 evaluale the sand balance of the 1996 conlrolled flood. 7. SAND BALANCE One elusive goal of Grand Canyon researchers has been fo provide managers an accur;He budget of inflows, outflows, and changes in storage of sand. Such a budget could provide essential insight inlO the mass of sand available for redistribution to high elevation. An accurate budget for Ihe 1996 controlled flood could be used to analyze the efficiency with which high.elevation sand bars were deposiled in relation to the mass of sand delivered to Lake Mead and to determine the source of sand deposiled on the channel banks. Unfortunately, only imprecise sand budgeLs ror lhe reaches bel ween Lees Ferry and the Linle Colorado River and bel ween Ihe Liule Colorado River and Ihe Grand Canyon gage can be developed. However, devel. opment of such budgels proyides a conceptual framework within which the status of the current understanding of physical processes in Grand Canyon can be eyaIuated. Based on the measurements of Konieczki et 01. [1997], Ihe tolal mass of sand transported 10 Lake Mead by the 1966 controlled flood was approximately 1.55'106 Mg, because the transporl rates past Ihe Grand Canyon and Nalional Canyon gages were each approximately LS5'106 Mg (D.J. Topping, U.S. Geological Survey, written commun., 1998). The mass transported past the Geological Suryey gaging sLation upstream from the Little Colorado River was half this amounl. All of Ihe transported sand musl have come from the channel bed, eddies, or banks because no sand was released inlo the Colorado River from Lake Powell reservoir during the flood. A number of assumplions and generalizations musl be made in order to estimale and partition these changes in storage, because the measurement of changes in sediment storage are not comprehensive. Estimates of the neL change in Ihe mass of sand in eddies was based on multiplying the ayerage change in measured eddies [Andrews el 0/., this yolume; Hazel el 01., Ihis volume] by the estimated number of eddies and their ayerage size (Table 2). It is necessary to distinguish Ihe mass of sand deposited at high eleyalion from the nel change in storage in eddies because the cemers of large eddies eroded and thick deposition occurred al the margins of eddies and near their upstream and downstream ends (Andrews el 01., this volume; Hazel et aI., Ihis volume; Schmidt et 01., this volume]. Hazel el 01. [Ihis volumel measured the average Ihickness of high.elevation deposits; the area of these Ihick deposits were eSlimated from Schmidl el al.'s [Ihis volumel measurements of the area of significant deposition. Allhough Ihe sand budget for Ihe /996 controlled flood is imprecise (Table 3). the ralio of the mass of sand exported 10 the magnilude change in sand storage differs greatly for the IwO reaches. The sand exporled from Ihe reach upstream from the LiuIe Colorado River was approximately equal to Ihe mass deposiled along rhe channel margins in eddies and