Diving in the Silt Plume of the Elwha River
By Chad Schulze, Steve Rubin, and Sean Sheldrake
Some of you may have followed our previous blog posts about EPA’s scientific diving program in It’s Our Environment, but we also wanted to share some recent work led by U.S. Geological Survey (USGS) and supported by EPA divers near the mouth of Washington State’s Elwha River here in It All Starts with Science.
Now that removal of the Elwha River dams is well under way, USGS scientists, assisted by divers with the Lower Elwha Klallam Tribe, EPA, and Washington Sea Grant, will continue studying the impacts of removal-related sediment to the Strait of Juan de Fuca.
As the first EPA crew to visit the site this year, we didn’t know what to expect.
What we did know—the removal of the Elwha River dams will affect marine habitats in the Strait of Juan de Fuca, primarily from the flow and deposition of released sediment that had accumulated behind the dams for nearly 100 years. That sediment can affect marine life in many ways, including: burial, reduced aquatic reproduction, shading and light reduction, damage to animal gills and filter feeding structures, and changing how different species behave individually and together with their different tolerances and responses to the sediment.
Diving in on the first day, we found the conditions to be very different from before the dams were in place—last year visibility might be up to 50 feet! Not so this year, with some freshwater layers discharging from the Elwha with maybe 6 inches of visibility.
As we descended through this floating “halocline” of different salinity layers (less dense freshwater will sometimes float over the ocean saltwater until it mixes), it was like a “cloud” over the saltwater below. Visibility improved when we made it through, but it was DARK. Where last year the sun was sometimes visible on the seafloor, this year, we needed lights to see the bottom.
Things have changed. For starters, where there had been algal forests, we found much less growth compared to last year. We and our partners will continue to survey Elwha nearshore undersea communities during and after dam removal. Measuring responses to short and long term changes in deposited and suspended sediments offers an unprecedented opportunity to gain insight relevant to managing these important marine resources, and will help to inform how future dam removal projects can be conducted to minimize impact to downstream plants, insects, fish and animals.
For more information on the study, please see this story on the USGS web site: http://walrus.wr.usgs.gov/elwha/
We’ll follow up with another post as we continue to work. Stay tuned!
About the authors: Sean Sheldrake and Chad Schulze are part of the Seattle EPA Dive unit. Chad is the lead pesticide enforcement in the Northwest, and Sean is also a project manager working on the Portland Harbor cleanup in Oregon. Steve Rubin is an aquatic biologist specializing in algal species with the USGS and a lead scientist on the survey.
Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action.
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