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sampler was used 10 collect cross-sectionally integrated suspended-sediment samples using the equal-dischargc- incremenl melhodology (described by Edwards and Glysson [1988]), When used properly, the 0-77 and P-61 suspended-sediment samplers sample different portions of the flow. Therefore, Ihe concentrations and grain-size distri- butions measured wilh Ihese 2 types of samplers are syslemalically different. Because a 0-77 samples 10 no closer than 18 cm of the bed, whereas a P-61 samples 10 wilhin \2 cm of lhe bed [Edwards and Gysson, 1988J, suspended-sand concentrations measured with a 0-77 sampler are somewhat lower than those measured with a P- 6\ sampler, and suspended-sand grain-size distributions measured with a D-77 sampler are somewhat finer than those measured with a P-61 sampler, To monitor the bed sediment at the Grand Canyon gage, we collected samples of the bed material al 3 to 5 locations under the cable way (Slations 140, 190, 240, 290, and 340) on days -1,1-3,5-7, and +1 (Figure I), With the exception of \ sample, these bed samples were collected with a BM- 54 sampler (one sample on day 5 was collected in the center of the channel with a pipe dredge), Because of mechanical problems with the BM-54 sampler, reliable samples of the bed sediment were not obtained on day 4, 4,2. 122-Mile Eddy In order to monitor alteration in the shape and volume of the eddy bar, we measured bathymetry on a daily basis in a large lateral-recirculation eddy on river right, immediately downstream from the mouth of Hundred Twenty-two Mile Creek, These topographic measurements were made using a combination of ground surveys and coupled tracking theod- olile/echo sounder (Hydro) measurements, In addition, suspended-sedimenl concentralion and grain-size infor- mation were collected using a 0-74 depth-integraling sampler deployed from a boat at least twice daily at 4 different sites spanning the width of the eddy, Continuous indirect measurements of sedimenl concentration were obtained at discrete locations in the eddy using oplical- backscatter sensors (OBS), Suspended-sedimenl concentrations and grain-size distri- butions at (he two sites were determined from the samples using the same methodologies. Concentrations of suspended sedimenl were determined using slandard USGS techniques [see Guy. 1969J, Grain-size distribulions of lhe suspended sand in P-61 and D-74 samples were measured al 1/4-<1> inlervals in lhe USGS Colorado Dislricllaboralory using a visual-accumulation lube specifically calibraled for sediment from the Colorado River in Grand Canyon. Typical sediment from this sitc has a mean Powers index of TOPPING ET AL. 75 3,0 and a mean Corey shape factor of 0,7 [Topping. \997J, Grain-size distribulions of the suspended sand in the D-77 samples were measured at 1-41 inlervals in Ihe sedimenl laboratory althe USGS Cascade Volcano ObservalOry usi ng wet sieving. Grain-size distributions of the bed m..Hcrial were measured at 112-41 intervals using dry sieving. 5, RESULTS 5,1, Grand Canyon Gage Reach Our observations during lhe 1996 controlled "ood suggest that many sediment-related processes are common to bolh the pre- and posl-dam sYSlems, During the \996 controlled "ood, bed topography at the Grand Canyon gage measurement cable way, grain-size distributions of both lhe bed and suspended sediment, and suspended-sedimenl concentrations all evolved in manners somewhat similar to lheir evolution during pre-dam "oods, Except for one 24-hr period (day 2 to day 3) during the 1996 controlled "ood, magnitudes of bed aggradation or degradation at the measurement cableway were in approx- imate balance wilh opposing magnitudes of erosion or deposition in 4 cross-sections within 158 m upstream, such thaI lillle change in sedimenl volume occurred in the 158- m.long reach upstream of lhe cableway, The moSl erosion in this reach occurred during the first day of the recessional limb of the "ood, nol during the 7 days of high discharge, Furthermore, less sediment was eroded from the reach during the 7 days of high discharge than during the 3 weeks prior to the "ood, Finally, during the 1996 controlled "ood, as in lhe pre-dam sYSlem [Leopold and Maddock, 1953; Colby, 1964; Howard and Dolan, 198\; Burkham, 1986], lhe bed al the measuremenl cable way aggraded during lhe rising limb of the "ood and then scoured during the latter part of the "ood, Most of Ihis scour occurred during lhe recessional limb of Ihe 1996 "ood, During lhe 1996 controlled "ood, bed grain-size distri- hutions measured at the Grand Canyon gage were similarto lhose measured near lhe peak of the only pre-dam snowmelt "ood for which bed sedimenl data are available (i,e" 1956 snowmelt "ood), and as during the 1956 snowmelt "ood, lhe grain-size distribulion of the bed sediment coarsened (Figure 2), Measured bed.sediment grain-size distribulions (Figure 2) during the 1996 controlled flood were COarser than lhe grain-size distri- hut ion of sedimenl supplied during tributary (e.g., Pari a River) Hoods, similar 10 thal measured near the peak discharge of the pre-dam 1956 snowmell flood, coarser than that measured during the rising limb of the pre-dam 1956 snowmelt "ood. and finer than lhat measured near lhe