Downstream effects of the Pelton-Round Butte hydroelectric project on bedload, transport, channel morphology, and channel-bed texture, lower Deschutes River, Oregon.

Abstract

Field, laboratory, and historical data provide the basis for interpreting the effects of the Pelton-Round Butte dam complex on the surface water hydrology and geomorphology of the lower Deschutes River, Oregon, USA. The river's response to upstream impoundment and flow regulation is evaluated in terms of changes in predicted bedload transport rates, channel morphology, and channel-bed texture. Using a hydraulic model, we predicted discharges between 270 and 460 m³/s would be required to initiate bedload transport. Analysis of morphologic change at a long-term cross-section showed general scour and fill beginning at approximately 250 m³/s. Such bed-mobilizing flows have occurred less than 1% of the time during the 70+-year period of record, substantially less frequently than on other alluvial rivers. Historical streamflow records and bedload transport modeling suggest dam operations have had only minimal effects on the frequency and magnitude of streamflow and bedload transport. Analysis of gage data collected just below the dam complex revealed slow, minor degradation of the channel over the entire period of record, indicating the dams have not noticeably accelerated long-term incision rates. The dams also have had only minimal effect on channel-bed texture, as demonstrated by detailed analysis of longitudinal trends in surface and subsurface grain-sizes and bed armoring. This study presents a coherent story of the Deschutes River as a stable alluvial system. This stability appears to be due to a hydrologically uniform flow regime and low rates of sediment supply, neither of which has been substantially altered by the dams or their operation.

Citation

Fassnacht, Heidi; McClure, Ellen M.; Grant, Gordon E.; Klingeman, Peter C. 2003. Downstream effects of the Pelton-Round Butte hydroelectric project on bedload, transport, channel morphology, and channel-bed texture, lower Deschutes River, Oregon. Oregon Water Science and Application. 7: 175-207