Channel Bed Mobility Downstream from the Elwha Dams, Washington

Abstract

Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross-sections. The relationship of average boundary shear stress (τ_o) to critical shear stress (τ_cr) served as the basis for determining channel bed material mobility under the two-year and ten-year flood events. The channel had the greatest channel bed mobility at the natural cross-section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run-of-the-river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River.

Key Words:

• dams
• channel change
• bed mobility
• dam removal
• river restoration

Notes

*This research was supported by a National Science Foundation Graduate Research Fellowship, a Geological Society of America Graduate Research Grant, and an Arizona State University Graduate Research Support Grant. This study would not have been possible without the invaluable field assistance of the K'lallam Tribal Fisheries Office, and support from a number of other federal agencies including the National Park Service, the Bureau of Reclamation, the Army Corps of Engineers, and the Geological Survey. The author is grateful to Kent and Rosemary Brauninger and Paul Crawford for their instrumental role in the logistical arrangements for the field season.