Michael McHenry | Geology Reports

USGS2019

Complexities, context, and new information about the Elwha River

Recently, Hand et al. (2018) discussed the socio- ecological complexities surround-ing natural resource policy, science, and management in the Columbia River Basin, using a case study of dam removal on the Elwha River in Washington State (WebFigure 1a). We feel compelled to provide additional historical context and correct some of the information provided by Hand et al. about the Elwha. The his-toric removals of the two Elwha River dams – a long- anticipated project that was decades in the making – have been touted by some as an environmental success story following the project’s completion in 2014. As with any complex endeavor, critical analyses and lessons learned continue to emerge, and will likely benefit future projects.

Washington↗

USGS2008

Channel Evolution on the Lower Elwha River, Washington, 1939-2006

Analyses of historical aerial photographs of the lower Elwha River, Clallam County, Washington, reveal rates and patterns of channel change in this dammed, anabranching river between 1939 and 2006. Absolute positional changes of the active-floodplain margins, which commonly exceeded 50 m over that interval, have exceeded 400 m locally. Annualized rates of channel movement were typically ~2 to 10 m/yr; higher annualized rates over some time intervals are attributable to the formation of new channels by episodic avulsion. Channel movement by more gradual lateral meander migration was also common. Anthropogenic modification of the floodplain between the 1940s and 1980s substantially altered channel form and position. This analysis of rates and patterns of channel change over nearly 70 years on the lower Elwha River is intended to characterize the evolution of the river throughout most of the time interval when two large dams have been in place upstream. Channel morphology and rates of channel movement are expected to change significantly in response to removal of the dams and re-establishment of the upstream sediment supply during a major river-restoration project.

Scientific Investigations Report↗