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TOPPING ET AL: COLORADO RIVER SEDIMENf TRANSPORT, 2 549 200 -' t <( ZOE 100 Q.... - -' ....w"' -' 0> 0> u: w- cD 0 en.... OJ 0:: 005 '" :J enW z -100 0 00:: 0 0 0::<( <( -0- X-S 4 en ow w -<>- X-S 3 t zO:: 0:: -200 -<( <( -u- X-S 2 we e Clz z __X-51 z<( <( -300 --+- X-S 0 :2(1) (I) 0 -400 N .. ~ in iD F'" .. a) w > > > ~ ~ ~ > > ~~ 8 ~" < ~ < < < ,. '" <z B B B B ~ <z '" o~ ~ 8 III ~ III 0< <110 ., do .. ",'" '" 0 '" '" '" ~ ~ '" '" '" sooo wW ::<::< :J:J 0 -,-' 00 - >>wE ~!i;; -5000 <(<( 0> enen zO ~ -I- .10000 WW z Cl<!: 0 :i~ :I:w -15000 00:: -20000 b) Figure 6, (a) Measured change in cross.sectional sand area over lime relative to the bed topography surveyed on the day before the 1996 flood experiment. (b) Changes in sand volume in the 158-m-long reach upstream from the Grand Canyon cableway during the period encompassing the 1996 flood experiment. Volumes were computed using the data in Figure 6a. In Figures 6a and 6b, the cross-hatched region indicates the 7 days of steady high discharge; the scale of the x axis is compressed during the 3 weeks prior to and after the flood. but in a different manner than at the Lower Marble Canyon gage. At this site the fraction of the fine sediment on the bed composed of 0.0625- to O.l25-mm sand decreased from 2.6 to 0.33%, while the median size of the fine sediment on the bed actually decreased from 0.41 to 0.35 mm. 4. Vertical Trends in the Grain-Size Distribution of Colorado River Flood Deposits 4.1. Deposits of the 1996 Flood Rubin el al. (19981 sampled the deposits of the 1996 flood experiment in trenches on five eddy bars between Lees Ferry and Diamond Creek (Figure 1). Just as the suspended and bed sediment coarsened during the 1996 flood (Figures 4 and 8), the sediment deposited during the flood also coarsened (Fig- ure 8). Prior to Rubin el al. [1998], production of inversely graded deposits during floods had been documented by lseya (1989] in Japanese rivers. Production of the inverse grading in the 1996 flood deposits occurred both by coarsening of the sand and a reduction in the content of silt and clay. As with the sus- pended sand, coarsening of the sand OCUIITcd nOI merely by the removal of fines but also by an increase in the modal size and an increase in size oflhe coarsest fraction [Topping et al., 1999]. 4.2, Deposits of the 1997 Test Flow Deposits of the 1997 test flow were sampled in trenches on eight eddy baTS between Lees Ferry and the Grand Canyon gage (Figure 1). At each site, samples were collected at mul- tiple elevations between the base and top of the deposit. At several sites (i.e., upper Eminence Break and Tanner), vertical sample sets were also coUected at different distances from the edge of the main channel. Where deposits were sampled in different lateral locations, they generally fined toward the bank. Just as the suspended and bed sediment coarsened dur- ing the test flow (Figures 7 and 9), the sediment deposited during the 2-day test flow also coarsened (Figure 9). As with the 1996 flood deposits, this occurred by both coarsening of the sand and a reduction in the content of silt and clay. Also, as in 1996, coarsening of the sand occurred nOl merely by the re- moval of fines but also by an increase in the modal size and an incrc;1<;e in size of the coarsest fraction.