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546 8 7 6 5 4 f 3 UJ 2 ~ 1 .... (j) 0 -1 -2 -3 -4 -. -6 a) 8 7 6 5 4 f 3 UJ 2 ~ , .... 0 (j) -, -2 -3 -4 -5 -6 b) TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 I I ",0... WATER-SURFACE STAGE __ -lrrl-. -:-~EANBEDSTAGE .. 11 f 'iiZ! IIF .. '2.<;;-2'2 ? E~~M~8N r f rlf,s;-2'Q'Q-Q t 300 "1<")(- .x- .)t..~..~' ..>c-" x..~..~. )C.- X"'X 200 "')("X-X..)Ii..)(. ~"'\l("-x...". x ,.. .)(._)(.- 100 , Jen Fob Mar Apr May Jun Jul Aug Sep Oct o __._m.. WATER-SURFACE STAGE --+- MEAN BED STAGE j ~ i\ft'v^.r. .~!,.f'.,..:, foAf'l; i .....\,.. .. 'ro",.J'<I.'."~i. ~..._-1 Nov Doc \ \ \ -o/VV\^M-'^_h \ : '.. '-Vl"YlO........ \ j '---' MARCH 5 10 20 APRIL 25 .0 30 5 15 20 15 Figure 3, (a) Water-surface stage. mean bed stage, and minimum bed stage in 2-week bins during the average predam year. Values in Figure 3a were computed from the 3690 discharge measurements made from November 12, 1922, through December 31, 1962, by the USGS at the Grand Canyon cableway; stage shown is tbat at the lower gage in Figure 2. Here n is the number of data in each 2-week bin; error bars are 1 standard deviation. (b) Water-surface and mean bed stage measured at the Grand Canyon cableway during the 1996 flood experiment. Cross-hatched area indicates the range of values depicted in Figure 3a of the mean bed stage at the measurement cableway during the predam era. As during the average predam year, the bed at the cableway aggraded as the water-surface stage increased and began to scour prior to the receding limb of the flood. other locations where suspended.sand concentration and grain size were measured during the flood (Figures 1 and 4b). Each day, the concentrations of suspende-d sand were similar along 127 km of the river, from the Grand Canyon gage to the National Canyon gage, with the concentrations of suspended sand being about a factor of 2 less at the Lower Marble Canyon gage (Figure 4b). Thus, during the 1996 flood. the upstream supply of sand at the Lower Marble Canyon gage was about half of that at the sites downstream in Grand Canyon. This relationship changed substantially by November 1997 as a re- sult of a series of large floods on the Paria River that intro- duced large quantities of sand to Marble Canyon. Over the 7 days of high discharge during the 1996 flood experiment, bed aggradation at the Grand Canyon cablev.'ay was offset by scour upstream, such that relatively little change in sand volume occurred in the 158-m-Iong reach immediately upstream rrom the cableway (Figures 5 and 6). Indeed, less sand was eroded from the reach during the 7 days of high discharge than during the 3 weeks prior to the flood (Figure 6b). The measured longitudinal pattern of scour and fill during the 1996 flood (Figure 5) supports the conceptual model or flood~induced rearrangement of sand in the Grand Canyon gage reach depicted in Figure 7 or Howard and Down {1981]. Because aggradation during floods at the Grand Canyon cable- way is somewhat balanced by scour upstream, the dominant control on this rearrangement of sand in the reach is probably the redistribution of the boundary shear stress field as the stage increases (i.e., reach-geometric effects). During the 1996 flood experiment the topographic response of the bed to the measured depletion of Ihe upstream supply of sand lagged about 3-4 days behind the measured coarsening of both the bed and thc suspended sand (Figure 3a); this impor-