Channel evolution on the dammed Elwha River, Washington, USA

Geomorphology
By: , and 

Links

Abstract

Like many rivers in the western U.S., the Elwha River, Washington, has changed substantially over the past century in response to natural and human forcing. The lower river is affected by two upstream dams that are slated for removal as part of a major river restoration effort. In preparation for studying the effects of dam removal, we present a comprehensive field and aerial photographic analysis of dam influence on an anabranching, gravel-bed river. Over the past century with the dams in place, loss of the upstream sediment supply has caused spatial variations in the sedimentary and geomorphic character of the lower Elwha River channel. Bed sediment is armored and better sorted than on the naturally evolving bed upstream of the dams. On time scales of flood seasons, the channel immediately below the lower dam is fairly stable, but progresses toward greater mobility downstream such that the lowermost portion of the river responded to a recent 40-year flood with bank erosion and bed-elevation changes on a scale approaching that of the natural channel above the dams. In general, channel mobility in the lowest 4 km of the Elwha River has not decreased substantially with time. Enough fine sediment remains in the floodplain that – given sufficient flood forcing – the channel position, sinuosity, and braiding index change substantially. The processes by which this river accesses new fine sediment below the dams (rapid migration into noncohesive banks and avulsion of new channels) allow it to compensate for loss of upstream sediment supply more readily than would a dammed river with cohesive banks or a more limited supply of alluvium. The planned dam removal will provide a valuable opportunity to evaluate channel response to the future restoration of natural upstream sediment supply.

Study Area

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Channel evolution on the dammed Elwha River, Washington, USA
Series title Geomorphology
DOI 10.1016/j.geomorph.2010.12.008
Volume 127
Issue 1-2
Year Published 2011
Language English
Publisher Elsevier
Publisher location Amsterdam, Netherlands
Contributing office(s) Pacific Coastal and Marine Science Center
Description 17 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Geomorphology
First page 71
Last page 87
Country United States
State Washington
Other Geospatial Elwha River
Google Analytic Metrics Metrics page