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Field Notes

Covering the natural wonders of the Pacific Northwest

June 20, 2012 at 8:30 AM

Seeing is believing: Sediment is rebuilding the Elwha

With sediment trapped behind two dams for a century, the shore by the mouth of the Elwha has become rocky and eroded. The river itself has also starved for the sediments locked up behind the dams.

But that’s starting to change. Amy Draut, geologist with the USGS Pacific Coast and Marine Science Center in Santa Cruz, nearly had her boots sucked off by the soft sediment transported by the Elwha, now that the lower dam is out, when she came out to examine the river. “I never envisioned a mud flat on the Elwha would rip a wading boot off.

“It’s this really gray, soupy mud,” Draut said of the muck she encountered in her visit to the Elwha during the first week of April, right after contractors took the last of the Elwha Dam out of the river.

“Lake Aldwell was just completely gone by then, and on the river what we were seeing is what I would call a shoulder zone of of mud deposition on the recent high water line. If you wade into the river you feel like you are walking in mud up past your ankles to your calf, but you can feel the cobble underneath.”

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Usually ankle-turning cobble has been transformed to soft sediment that will pull your boots off — just ask Amy Draut of the USGS, going toe to toe so to speak with the Elwha.

Photo by Mark Mastin, USGS

“I got stuck in the new mudflat in the estuary, it was so cohesive and sticky that it ripped my wading boot clear off. I’m holding the boot that I pried free of the mud,” she wrote of what was going on in this photo.

One surprise was seeing the mud all over — not only in places without strong current. “We were seeing it everywhere, not just in quiet pools but in areas with current. Even upstream of gravel bars,” Draut said. “It was silt and clay.” Not enough of it to raise the elevation of the river bed, or cause flooding. But enough to notice, that is for sure.

She also documented how the sediment is working its way into the tiny spaces between the river cobbles, potentially worrisome, she noted for salmon eggs in redds that need oxygen to survive.

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Fine sediment between cobbles could pose a challenge for life in the benthos, whether its salmon eggs in redds, or the tiny invertebrates that stoke the food chain. Amy Draut, photo

Here’s what she says about what she was seeing: “We see mud thinly blanketing the former riverbed cobbles, with cobbles still sticking up through the mud surface. So the overall amount of new sediment deposited isn’t (in most places) enough to raise the bed elevation very much … but the interstitial mud that now fills pore spaces between cobbles could have ecological effects.” That remains to be seen, especially as a far larger amount of sediment still trapped behind the upper dam upriver, Glines Canyon, starts to break free as dam removal continues, probably beginning this fall.

Animals are already using the new mudflats, as Draut’s photos show. She saw the tracks of racoon and beaver in the mud at the estuary after the high tide dropped.

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Beaver are busy indeed as these tracks in the new mudflats of the Elwha show. Amy Draut, photo

Even the tiniest lives are getting a new start, such as this snail, making its way across new ground.

snails_on_mud.JPG

The estuary below the high tide line shows tiny signs of life. Amy Draut photo

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