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002331 downstreani, and he found that the proportion of eroded bars was slightly more at low eleva- tion than at elevations that are now only inundated by post-dam floods. Measurements during the 1996 Controlled Flood demonstrated that the concentration of suspended sediment decreased by approxi- mately one-half during this 7 -day event (Top- ping et aI., 1999). This evidence of supply limitation led to recalculation of historical sediment budgets (Topping et aI. (2000a, 2000b) and realization that many of the as- sumptions on which the sediment-related findings of the Environmental Impact State- ment for Glen Canyon Dam Operations (U.S. Department of the Interior, 1995) are not valid (Schmidt, 1999; Rubin et aI., 2002). 5.0 CHANGES IN THE TOPOGRAPHY OF THE MAIN-CHANNEL BED ..........) Gravel has been eroded from riffles that act as hydraulic controls within 20 km from Glen Canyon Dam, but there is no indication of degradation of gravel and boulders from the rapids that act as channel controls in the next 145 kIn. There is abundant evidence of exten- sive erosion of sand from riffles and pools in Glen Canyon, and there is scattered evidence of degradation of pools in Marble and Grand Canyons. The only evidence for sustained accumulation of fine sediment in the study area during the post-dam era was in 1963 and 1964 when dam releases were very low. 5.1 Pre-dam Changes in the Main-Channel Bed, as Measured at the Lees Ferry and Grand Canyon Gages The characteristics of the Colorado River's bed have been measured at the Lees Ferry and Grand Canyon gages since the early 1920s (Fig. 15 A,B), because discharge mea- surements have been made at these locations. The measurement reaches are a few channel widths in length and include 2 and 3 cableway crossings of the river, respectively. In order to evaluate pre-dam characteristics of the bed, it was necessary to integrate these temporally precise but spatially restricted data with impre- cise but spatially extensive estimates of bed change that were deduced from the mass balance calculations of Topping et aI. (2000b) and estimates of the area of the bed and of EDZs. Topping et al. (2000a, b) described hy- draulics and sediment transport at the Lees Ferry and Grand Canyon gages. Each gaging reach is a ponded backwater, and the down- stream controls are the riffle opposite the Paria River confluence and Bright Angel Rapid, respectively. The magnitude of annual scour and fill was between 1 and 7 m at the Lees Ferry gage and between 1 and 3 m at the Grand Canyon gage. The bed at the Lees Ferry gage scoured during rise of the annual flood, and filling began about 2 weeks after the peak occurred. Filling continued until the pre-flood bed elevation was approximately restored. In contrast, the bed at the Grand Canyon gage initially filled. Scour began about 4 weeks before the annual peak was reached, and scour continued until the channel returned to its approximate pre-flood elevation. The bed remained at this elevation until the next year's snowmelt flood repeated the cycle. Comparison of the magnitude of scour and fill measured at the gages with predictions of average bed elevation changes estimated under different scenarios of seasonal sediment accumulation, proportion of the accumulation stored in eddies and the main channel, and the proportion of the channel where fine sediment storage occurred indicate that the gages de- scribe bed behavior of a small proportion of the channel. In a year when 7.0 x 106 t of fine sediment accumulated between July and the next March, the average elevation change in eddies would have been about 0.6 m and the average change in bed elevation would have been between 0.2 and 0.5 m, if half of the fine sediment accumulated in eddies and half in the 4.0 Previous Studies of Fine-Sediment Flux and Storage 23