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Elwha Sediment Detailed in New Document

February 11, 2014
As the largest dam-removal project in history moves into its third year, scientists from the U.S. Geological Survey released a new report that documents the movement of sediment down the Elwha River in Washington State.
Image: New River-Mouth Delta
USGS hydrologic technician Andrew Spanjer walks along a newly deposited sand and gravel beach at the mouth of the Elwha River, Washington. In the first two years of the largest dam-removal project United States history, the Elwha River deposited 3-4 million cubic yards in a growing offshore delta. The USGS is measuring turbidity, surveying sediment accumulation, and monitoring responses of the offshore marine ecosystem as Elwha River sediment delivery to Puget Sound is restored.Elwha River (near Port Angeles, WA).Public domain

TACOMA, Wash. — As the largest dam-removal project in history moves into its third year, scientists from the U.S. Geological Survey released a new report that documents the movement of sediment down the Elwha River in Washington State.

"During last winter's high flows, we set a record for the largest concentration of sediment thus far measured on the Elwha River," said USGS hydrologist, Chris Curran. "We expect more high flows this season and are interested to see how much of the sediment remaining behind the former dam sites is flushed toward Puget Sound."

In the lower Elwha River, a suite of sediment-detection instruments and samplers operate 24 hours a day, 7 days a week to track the downstream flux of sediment. Periodic measurement of sediment concentration in the river allows scientists to calibrate the sediment-detection instruments and calculate the total load of sediment carried by the river.

The release of millions of tons of sediment from the former reservoir areas is restoring the natural function of the downstream river, estuarine, and marine ecosystems. Understanding the timing and magnitude of mobilized sediment provides a foundation for cross-disciplinary basic research in the Elwha River with benefits far beyond dam-removal projects.

"Deposition patterns on the floodplain, estuary, and beaches are all influenced by the timing and caliber of arriving sediment," says USGS’s Jon Warrick. "The pre-removal models gave us a rough approximation of system response, and now we get to document the fascinating details of change in the downstream river channel, floodplain, and marine delta.  This includes the massive growth of a new delta along a shoreline that historically eroded at rapid rates."

Image: Sampling Mud in the Elwha River Estuary
USGS hydrologic technician Andrew Spanjer shovels Elwha River mud into a bucket for latter analysis in the lab. The USGS is monitoring turbidity at several locations in the Elwha River during the largest dam-removal project in United States history. The mud will be mixed with water to assess how different turbidity sensors react to different sediment concentrations using Elwha River sediment.Elwha River (near Port Angeles, WA).Public domain

Calculations of suspended-sediment concentration in the lower Elwha River, recently updated to include the first two years of the restoration project, are available in a USGS document, "Suspended-Sediment Concentrations during Dam Decommissioning in the Elwha River, Washington."

This research and monitoring was funded by the USGS, the U.S. Environmental Protection Agency, and the National Park Service.


Image: Retrieving Sediment Instrumentation, Elwha River, Washington
USGS researchers Chris Curran and Raegan Huffman retrieve instrumentation to measure sediment concentration from the Elwha River, Washington. Two large dams on the Elwha River are being incrementally removed from 2011 to 2013 to restore river function in an important salmon-bearing river. The USGS is sampling sediment from the river during the dam-removal project to quantify the magnitude and timing of sediment released during the restoration project. One third to one half of 24 million cubic yards of sediment is expected to be released and flow downstream during the next 3 to 6 years. Public domain

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