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I j 1 I ! , .~ . . SUMMARY The 1995 snowmelt runoff produced the highest flows in the upper Colorado River since 1984. The peak discharge of 29,600 cfs at the Cameo gauge ranks as the 7th highest in 61 yr of record; the peak discharge of 49,300 cfs at the State line gauge ranks as the 5th highest in 45 yr of record. The total volume of runoff was likewise much higher than average. In order to assess the effects of high flows on the geomorphology of the river, we re-surveyed a series of cross sections used to monitor specific backwater study sites and changes in the main channel. These survey measurements indicate that two of the three backwaters were scoured and enlarged by the recent high flows, but the other backwater was essentially unchanged. Survey measurements of 12 main channel cross indicated localized scour and fill that usually amounted to much less than 0.5 IT!, and minor amounts of bank erosion at several cross sections. We noted only one instance where changes in the main channel were appreciable; this occurred at cross section RM 130 in the alluvial reach immediately downstream of Ruby-Horsethief Canyon. Although our data and observations suggest that high flows in 1995 did not produce large- scale changes in the geomorphology of the Colorado Rive; we do not want to give the impression that these flows did nothing whatsoever. Based on field observations and results from previous studies (Van Steeter et al. 1995), we know that discharges such as those experienced in 1995 are easily capable of moving the gravel and cobble bed material in the Colorado River. However, this alone may not produce widespread changes in the geomorphology of the river unless there is an imbalance in the amount of sediment entering a reach versus the amount leaving. This did not occur on any widespread basis in 1995. In earlier reports (Van Steeter et al. 1995) we suggested that a more complex channel might result from bank erosion and widening during exceptionally high flows, e.g. such as in 1983 and 1984, but flows such as those experienced in 1995 appear to produce this effect only locally. Tnus, while channel change was nO\ a ubiquitous aspect of the 1995 runoff, the Colorado River did reach a stage where sediment transport rates were relatively high and much of the bed was mobilized, and this was clearly important for maintaining gravel substrate quality and for limiting the growth of native and non-native plants on gravel bars. 18