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,400 1200 1000 .. "i w ~800 <( I: " Ul o 600 400 200 . P-61 SAND 0 MCS . P-81 SILT & CLAY 0 MCS o o-n SAND 0 MCS o o-nSILT&CLAYOMCS .. 0-74 SAND 0 ES .. D-74SILT&CLAYOES I I . o :Ie . .Ib ... "'1Ib ..t DAY 1 DAY 2 DAY 3 DAY4 DAY 5 DAY 6 DAY7 Figure 3. Hydrogl'llph of 1996 ftood expertmen1 and apatiaUy averaged, depth-Integl'llted aand and slR1>lua-<:1ay concentratlona _sured at maln-channel and eddy altes. Error bars are ona atandard deviation. Traveltime olllood batween maln-channel and eddy sites has been re- moved, so that beginning 01 day one at each alte corresponda to time olllrs! arrival 0111000 wave. ! sand concentration (from 0.11 to 0.06% by vol- ume). Sand was always the dominant portion of the suspended load. varying from 73% of the IOt.aI suspended sediment on March 27, 1996, to 88% on thelast<lay of high flow (April 2, 1996). Notably, tot.al sediment concentration decreased the most during the first day or lWO of !he flood. and continued to decrease at a lower rate until the last day of !he flood. when it appears 10 have in- creased slightly, al!hough !he increase was within !he range of measured variability. Remarkably similar results are found when spatially averag. ing the measurements taken in !he eddy (Fig. 3). Despite !he fact !hat flows in eddies are typically much lower in velocity than main-channel flows, tOlal concenlrations as well as the relative pro- 0.016 0.014 ~ .0.012 -0 ~ Z 0.010 0 ;:: ~ 0.008 I- ffi 0.006 " z 8 0.004 0.002 0.000 A 0.1 --<>-DAY' (n=8) -+-DAY 2 (n = 4) -a-DAY 3 (n = 8) ___DAY 4 (n '" 4) ___DAY 5 (n=8) -.-OAY 61" = 8) -+-DAY7 n=4) GRAIN SIZE (mm) Jl 10 l I 0.15 0.14 0.13 0.12 0.11 0.10 8 z 0.09 ~ z 0.08 ~ .. -4 0.07 6 z 0.06 <- ~ 0.05 1:. 0.04 0.03 0.02 0.D1 0.00 JXJrtion of sand are similar to those in the main channel 150 Ian upstream, wi!h greatesl similar- ity near !he channelward margin of Ihe eddy. Observations of sediment concentration and grain size at National Canyon (about 71 kIn downstream of !he eddy site) are also similar to bo!h the main-channe! measurements al Phantom Ranch and the eddy measurements, suggesting that there was little longitudinal variation over this 220 Ian section of river. At a river discharge of 1290 mJ/s (45400 cfs), particles finer !han aboul 0.3 mm (by far Ihe majority of measured suspended sediment during !he flood) are carried predominantly in suspen- sion. Similarity of measured suspended-sediment concentrations at the main channel and eddy sites illuslI1lteS that eddy suspended-sediment amcen- lration is largely coolrolled by the main-channel suspended-sediment concenlration; the decrease in concentration during the seven-day experi- ment.al flood documents !he supply-limited nature of sediment transpon in the Colorado River in Grand Canyon. The key to understanding lhe coarsening- upward sequences formed during !he 1996 flood is found in the grain-size distribution of sus- pended sedimenl and bed material measured in the main channel and the grain-size distribution of suspended sediment measured within the eddy. During the flood. suspended sedimen1 and bed material in the main channel evolved simul- taneously; the median grain size of the sus- pended sediment and bed material increased fromO.14100.21 mm and OJ to 0.4 nun, respec- lively, as shown in Figure 4 (A and 8). In the main channel. bed material coarsened solely by depletion of the fines, whereas suspended sedi- ment coarsened by depletion of fines as well as an increase in concentration of coarser sand. This same panem was reflected by measure- ments in the eddy. where the suspended sedi- menl """""ned during !he flood, l1Ipidly on !he first few days of !he flood and more slowly !hereafter, beginning al a value of 0.11 mm and increasing 10 0.19 mm on the final day of high flow (Fig. 4C). This increase in grain size occurred despite decreasing velocities in the eddy as deposition occurred. Furthermore. as in the main channel, the observed coarsening can- not be explained purely by a decrease in the quantity of fine material in suspension; the con- centration of relatively coarse material increased. in an absolute sense even while total concentra- tion decreased (Fig. 4, A and C). A1!hough the measurements within the eddy showed. a slight tendency for grain size to decrease away from the main channel (shoreward), similarities in size distributions at different locations are more striking than differences. demonstrating the link:- 0016 --<>-DAY t (n=6) --a- DAY 31" = 81 ____DAY4 n=8 -o-OAY5ln=91 ___OAY6 0=8 -+- DAY7 (n = 8) GRAIN SIZE (mm) Figure 4. Dally mean concentrations by size fraction of (A) suspended sand depicted In Figure 3 measured with P-61 sampler at the main~hannel sRe; (B) bed measured with 8M-54 sampler at maln-channel site; and (e) suspended sand depicted In FIgure 3 measured with 0-74 sampler at eddy site. Number 01 samples In each spatially averaged measurement Is Indicated by n In legend. Heavy vertical lines Indicate median grain sizes tor each day. 60 --&- DAY 1 (n = 5) ---6-DAYS (n=3) __DAY2 (n=4) _DAY6 (n=4) -e-DAY3 (n=5) --+- DAY 7 (n - 4) ~ 0.012 lSO -0 0 ~ :::40 z 0.010 u. 0 0 ~ 0.008 1530 a: I- ;:: ffi 0.006 " ~ 20 " z u. 8 0.004 10 0.002 0 0.1 0.000 0.1 B GRAIN SIZE (mm) C ,0'