Abstract:
River regulation imposes changes in the supply of sediment and the frequency of competent flows. As a result, impounded rivers often exhibit downstream changes in channel processes, particle size, and morphology. The magnitude of change in competent flow frequency and sediment load together can be used to predict the pattern of bed material size downstream of a dam. A conceptual model is used to examine possible geomorphic impacts to the Lower Deschutes River, OR below the Pelton Round Butte Hydroelectric Project. Flow duration and flood frequency curves constructed separately
for the pre- and post-hydrocomplex time periods illustrate both a remarkable uniformity in river discharges and relatively minor shifts in hydrology following river impoundment. This implies that the frequency of bed-mobilizing flows has changed little following impoundment. There is a predominant lack of longitudinal trends in the texture of the gravel and cobble bed material. Tributary inputs of bed material do not produce abrupt shifts in mainstem grain-size distributions. In general, morphologic changes since impoundment have been minor. Major flood events in 1964 and 1996 led to planform changes concentrated at the confluence of Shitike Creek and fluctuations in bed elevations at cross-sectional sites. That little textural or morphologic change occurred during the record February 1996 flood suggests that sediment transport rates are relatively low over the longer term. Overall, there is no indication of progressive channel adjustment with distance downstream of the Project or with time. Instead, the macrophyte Ceratophyllum demersum influences the grain-size distribution locally by trapping fine sediment. The predominant lack of longitudinal patterns in bed material texture or channel morphology suggests that adjustments are limited for rivers with low sediment transport rates. Although measurable geomorphic impacts during the past half century are apparently subdued on the Deschutes River owing to its geologic context, large flood events (>I 00 year recurrence interval) deprived of sediment supply over the longer term could induce channel changes with consequences to fish habitat and populations.
