Silt, sediment and change: Federal government releases scientific studies on Elwha River dam removal

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PORT ANGELES — Five peer-reviewed studies on the effects of the Elwha River dam removal were released this week.

Authors with the U.S. Geological Survey, Bureau of Reclamation, National Park Service, Washington Sea Grant, National Oceanic and Atmospheric Administration Fisheries, the Lower Elwha Klallam tribe and the University of Washington provide detailed observations about the changes in the river’s landforms, waters and coastal zone during the first two years of dam removal, which began in 2011.

In the largest dam removal project in U.S. history, the Elwha and Glines Canyon dams were demolished, allowing the river to revert to its wild state.

The five new papers can be found in Elsevier’s peer-reviewed journal, Geomorphology, which can be found at www.journals.elsevier.com/geomorphology.

According to a press release from the USGS, one finding that intrigued scientists was how efficiently the river eroded and moved sediment from the former reservoirs.

More than a third of the 27 million cubic yards of reservoir sediment, equivalent to about 3,000 Olympic swimming pools filled with sediment, was eroded into the river during the first two years even though the river’s water discharge and peak flows were moderate compared to historical gaging records.

The release of sediment altered the river’s clarity and reshaped the river channel while adding new habitats in the river and at the coast.

Most of the released sediment was discharged into the coastal waters of the Strait of Juan de Fuca, where the river mouth delta expanded seaward by hundreds of feet.

“The expansion of the river mouth delta is very exciting because we are seeing the rebuilding of an estuary and coast that were rapidly eroding prior to dam removal,” said Dr. Jonathan Warrick, USGS research scientist and lead author of the synthesis paper.

Although the primary goal of the dam removal project is to reintroduce spawning salmon runs to the pristine upper reaches of the Elwha River within Olympic National Park, the new studies suggest that dam removal also can have ecological implications downstream of the former dam sites.

These implications include a renewal of the sand, gravel and wood supplies to the river and to the coast, restoring critical processes for maintaining salmon habitat to river, estuarine and coastal ecosystems.

“These changes to sediment and wood supplies are important to understand because they affect the river channel form, and the channel form provides important habitat to numerous species of the region,” said Dr. Amy East, USGS research scientist and river study lead author.

The final stages of dam removal occurred during the summer of 2014.

Some sediment erosion from the former reservoirs will likely continue.

Research teams are continuing to monitor how quickly the river returns to its long-term restored condition.

“We look forward to seeing when the sediment supplies approach background levels because this will help us understand the length of time that dam removal effects will occur,” said Jennifer Bountry, Reclamation engineer and co-author.

Last modified: February 17. 2015 6:52PM
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