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Field Notes

Covering the natural wonders of the Pacific Northwest

April 30, 2012 at 11:30 AM

Elwha sediment not just mud, it’s nourishment

The sediment loads in the Elwha River are spiking because the reservoir behind former Elwha Dam is now completely gone. That means the settling of fines that used to occur in the lake is no longer happening so all that material is pouring into the river, and heading on down to the Strait of Juan de Fuca. It’s a dramatic sight. Check out these photos shot by Tom Roorda from his plane on April 22:

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The distinct line is caused by the difference in density between the fresh water of the Elwha and the salt water of the Strait of Juan de Fuca. The heavy sediment loading is coming primarily from the area that used to be Elwha Dam. In this photo the Elwha River, right, meets the Strait of Juan de Fuca. The plume is flowing east with the tide — up, in this photograph, as Roorda flies north.

And while the amount of sediment is large — about 50 times normal levels for the Elwha — don’t call it mud. Sediment is a single word for a whole range of material that the river has been depositing behind the two dams for the past 100 years: rocks, gravel, cobble, sand, silt, and clay. About 40 percent of that material is expected to eventually make its way out to sea.

Restarting the river’s natural transport capacity is one of the most important aspects of Elwha River recovery. Big mountain rivers like the Elwha eat a steady diet of wood and rocks and sand and gravel, moving the material with the energy of their perpetual flow down gradient to the sea. Wood and sediment rebuild the natural structure and complexity of the riverbed: meanders, side channels, gravel bars, pools and riffles. A big mountain river like the Elwha naturally transports a fantastic amount of material — but it’s all been stuck up behind the dams, some 24 million cubic yards worth. Well now with the dams coming out — and Elwha Dam already completely gone — that material is on the move.

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The Elwha makes delivery on a big back order of sediment, stuck behind Elwha Dam for a century.

It is expected to eventually help rebuild beaches at the river’s mouth that today are eroding in part because of they are starved for the sediment moved by the river. This material is meant to be where it’s headed — it’s just a little late.

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Starved for sediment from the Elwha River because of the dams, beaches at the mouth of the river have eroded to bare cobble. Photo by Douglas MacDonald, taken on beach at Lower Elwha Klallam Tribal reservation lands, east side of river mouth in 2009

“Sediment is the inorganic material that is moving through a river system,” says Gordon Grant, research hydrologist at the USDA Forest Service Pacific Northwest Research Station in Corvallis. “Everything from microscopic clay particles carried in suspension in the water, to large boulders that sit on the bed of the reservoir. It’s clay, silt, gravel, cobble.

“In terms of understanding the fate of sediment and the impact of sediment it is really important to keep those size distinctions — they do different things. Most of what is coming out now is fine sediment, almost certainly silt and clay. They are fine particles held up by the internal turbulence of the flow. When you stand by a river, you think of it as only going downstream but if you were a tiny particle of sediment it would be like being an airplane in a tremendous thunderstorm. Updraft. Downdraft. Tremendous acceleration and deceleration. It is being carried out, and it can be carried out really for substantial distances because the particles are so fine and so small. That is that beautiful plume. It goes from the river from the Elwha into this essential near shore marine river.

“The river has been doing this since time immemorial. But one way to think about it is the sediment that was captured by Elwha and Glines Canyon reservoirs was in a sense a deposit in a bank. And a deposit is coming due. And now the river is reclaiming it. As far as the river is concerned, it was just a brief stop over.”

Eventually, all that material is expected to rebuild the cobble beaches at the mouth of the Elwha. Covered in rock today, they used to be primo clam harvesting territory for the tribe. Today the harvesting of the beach at the mouth of the river is best for a rock collection.

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The beach has been eroded to bare rock at the mouth of the river on the Lower Elwha Klallam Tribe’s reservation. Some 22 acres of land at the river mouth have been lost to erosion and the rate of erosion is increasing. Douglas MacDonald photo, east side of river mouth on Lower Elwha Klallam Tribal reservation, 2009

So fierce is the erosion that the Ediz Hook to the east has to be continually armored with rock just to keep it from washing away. The rock armoring is piled so high on the west side of the hook — where it is pummeled by the prevailing current — that the beach and water are not even visible, driving the hook in a car. The once-scenic view to the west is now a rock wall.

On the Dungeness Spit to the east, it’s a very different story. There, without the industrial alteration that has so altered the near shore of the Elwha, the spit is soft and sandy. Scientists studying the effect of taking down the dams on the near shore at the Elwha are looking at the spit as a control model of a more natural system.

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The Dungeness Spit is soft and sandy, and offers a comparison of a more natural near shore system. Photo by Douglas MacDonald in 2009, west side of the spit.

To get a good sense of how all this works, take a walk on the beach at Discovery Park. Up in the area of the park, where feeder bluffs nourish the beach, the walking is sandy and soft, and the slithering of small creatures can be seen, traced in the surface.

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Soft sand on the beach at Discovery Park. Fine material sloughed off from feeder bluffs above nourishes the beach. Douglas MacDonald photo, 2009

Tribal members at the Lower Elwha Klallam Tribe’s reservation in the lower river have for decades watched acres of their land simply wash away. They hope one day to see the erosion stop, and to harvest clams again from the beach, as their elders did. It will never be the way it was: construction of a water line in the 1930s along the beach armors the feeder bluffs that should build the beach.

But turning the flow of sediment back on in the river will help. “Right now we don’t really know how long it is going to take the coarser particles to come out of the project, but I would expect to see some growing of the beach,” said Andy Ritchie, Elwha restoration project hydrologist. “That amount of fine sediment coming out there is a pretty good chance to build some sand beds. I don’t think it will have any problem being recolonized.”

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Maybe someday clams like these on the beach at Discovery Park will again be seen on the beaches at the mouth of the Elwha. Douglas MacDonald photo

For more on sediment and its role in the ecology of the river, read my story in The Seattle Times.

Watch a video about the Elwha Restoration project.

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