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Field Notes

Covering the natural wonders of the Pacific Northwest

April 19, 2012 at 7:55 AM

Here it comes: sediment starts to really hit the Elwha

Long expected, the heavy loads of sediment created primarily by demolition of Elwha Dam are starting to hit the river. Here’s a photo from the air of just how cloudy the river is with sediment lately:

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The Elwha River is brown with sediment as demolition continues on Elwha and Glines Canyon Dam. Together the dams trapped sediment that normally would have rinsed down the river over the last 100 years. Photo by Tom Roorda.

The sediment behind the dams is one of the factors that makes the $325 million Elwha restoration the largest dam removal project ever anywhere, with some 24 million cubic yards of sediment to manage.

Much of the material will remain behind in the watershed, distributed along the river’s middle and lower run. But a lot of it is also rinsing out to the Strait of Juan de Fuca, where it is expected the material will help rebuild the near shore and beaches eroded to ankle-turning cobble. Here’s a look again from the air at just how big the plume is:

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Sediment trapped behind the Elwha dams makes its way into the Strait of Juan de Fuca. Elwha Dam is already gone, and demolition is continuing on Glines Canyon Dam. Tom Roorda photo.

Restoring the natural flow of sand, cobbles, rocks, and wood from the middle and upper watershed is one of the most important aspects of the Elwha restoration. They are the material a big mountain river like the Elwha eats everyday, to build its gravel bars, meanders, side channels, log jams, and other structural complexity that make a healthy river.

The river is expected to show elevated levels of sediment for about five years as the dams come down, with the biggest slugs of material coming in the early stages. That’s one of the reasons dam removal is paused every two weeks, to give the river time to catch up and sluice material out. Dam removal is also stopped altogether when fish are migrating in the river.

Imagine swimming in this and trying to get your food and air supply too through this water:

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The Elwha will be carrying elevated levels of sediment for about five years as it moves material trapped for decades in the middle and upper watersheds.No mere mud, the gravel, sand, cobble and rock that will be moved by the river is valuable material that will rebuild the structure of the river and near shore. Tom Roorda, photo.

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