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40000 N 30000 ao 20000 10000 0 Time (days) Figure 9. - Colorado River hydrograph. 1000 800 M? 600 400 200 0 Assuming a sediment density of 2.65 g/cm3, we calculated deposition rates of 0.79 to 0.40 kg/s for periods of 6 and 12 hours respectively. These rates are higher than the range of rates (0.22 to 0.05 kg(s) determined from flume experiments of recirculation zone sedimentation (Schmidt et al., 1993). Because of its close proximity to the sediment source (LCR), this site may not be representative of other sandbars along the river corridor during similar flood events. Deposition rates calculated for the RM62.4 site can be considered an upper limit for rates throughout the Grand Canyon. Comparison of the April 1993 and April 1994 surveys further illustrates the pattern of post-flood erosion at the site (fig. 10). In general, the greatest amounts of erosion within the eddy (-4 to -11.5 m) correspond with areas of maximum current velocity, particularly in the return current channel where currents sweeping across the eddy coalesce and are directed upstream. Large volumes of sediment were also transported along the main river channel between the surveys. Up to 10 meters of change was recorded along the left bank of the main channel. We also used the survey comparison, combined with qualitative visual observations of the site between measurements, to estimate approximate erosion rates at the site. About 5 to 10% of the deposit we measured in April had been eroded by June 1993. We estimate that the erosion rates ranged between 50 to 100 m3/day from April 1993 to June 1993 (Kaplinski et al., 1994a). Dam operations changed from low volume (226 to 368 m3/s [8,000 to 13,000 ft /s]) to high- volume (340 to 540 m3/s [12,000 to 19,000 WAD interim flows on July 1, 1993. Following this change in dam operations, the portion of the bar above the 227-m3/s (8,000-ft3/s) stage elevation had completely eroded within a 2-to 3-week period. We estimated that erosion rates during July 1993 ranged from 2,000 to 2,500 m3/day. The change from low-volume to high-volume interim flow dam operations may have triggered an order of magnitude increase in erosion rates. 16 I0-l? 12?? I??3 ?'?3 31.V