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Anticipated sediment delivery to the lower Elwha River during and following dam removal: Chapter 2 in Coastal habitats of the Elwha River, Washington--biological and physical patterns and processes prior to dam removal

Scientific Investigations Report 2011-5120-2

This report is Chapter 2 in Coastal habitats of the Elwha River, Washington--biological and physical patterns and processes prior to dam removal. For more information, see: Scientific Investigations Report 2011-5120
By:
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Edited by:
Jeffrey J. Duda, Jonathan A. Warrick, Christopher S. Magirl

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Abstract

During and after the planned incremental removal of two large, century-old concrete dams between 2011 and 2014, the sediment-transport regime in the lower Elwha River of western Washington will initially spike above background levels and then return to pre-dam conditions some years after complete dam removal. Measurements indicate the upper reaches of the steep-gradient Elwha River, draining the northeast section of the Olympic Mountains, carries between an estimated 120,000 and 290,000 cubic meters of sediment annually. This large load has deposited an estimated 19 million cubic meters of sediment within the two reservoirs formed by the Elwha and Glines Canyon Dams. It is anticipated that from 7 to 8 million cubic meters of this trapped sediment will mobilize and transport downstream during and after dam decommissioning, restoring the downstream sections of the sediment-starved river and nearshore marine environments. Downstream transport of sediment from the dam sites will have significant effects on channel morphology, water quality, and aquatic habitat during and after dam removal. Sediment concentrations are expected to be between 200 and 1,000 milligrams per liter during and just after dam removal and could rise to as much as 50,000 milligrams per liter during high flows. Downstream sedimentation in the river channel and flood plain will be potentially large, particularly in the lower Elwha River, an alluvial reach with a wide flood plain. Overall aggradation could be as much as one to several meters. Not all reservoir sediment, however, will be released to the river. Some material will remain on hill slopes and flood plains within the drained reservoirs in quantities that will depend on the hydrology, precipitation, and mechanics of the incising channel. Eventually, vegetation will stabilize this remaining reservoir sediment, and the overall sediment load in the restored river will return to pre-dam levels.

Geospatial Extents

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Anticipated sediment delivery to the lower Elwha River during and following dam removal: Chapter 2 in Coastal habitats of the Elwha River, Washington--biological and physical patterns and processes prior to dam removal
Series title:
Scientific Investigations Report
Series number:
2011-5120-2
Year Published:
2011
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Western Fisheries Research Center
Description:
20 p.
Larger Work Type:
Report
Larger Work Subtype:
USGS Numbered Series
Larger Work Title:
Coastal habitats of the Elwha River, Washington--biological and physical patterns and processes prior to dam removal (SIR 2011-5120)
First page:
27
Last page:
46
Country:
United States
State:
Washington
Other Geospatial:
Elwha River