Long thought tamed, the Elwha River is proving to hold a few last surprises for scientists.
The river, despite being dammed for nearly a century, has never quite lost its wild side, said Amy Draut, a U.S. Geological Survey research geologist.
Draut has spent the past five years studying the river, poring over maps and aerial photos, and paying close attention to the ebb and flow of this long, restrained stream.
To her surprise, she found the river downstream of its two dams to not be quite as docile as might be expected.
“It’s a really dynamic system,” Draut said. “It changes really fast.”
The USGS study is the first on a dam’s influence on an “anabranching river,” she said.
Such rivers are noted for having branches that divert from the main stem and later reconnect downstream.
Dams’ impact
The dams have never been used for flood control, but they still have an impact on a river by trapping sediment.
The silt can influence a river’s direction when deposited along its banks, Draut said.
It also increases erosion.
For instance, the stream 2.5 miles downstream from the Elwha Dam — located at river mile 5 — is armored with large, coarse rocks.
That makes it much more stable and is a result of fine sediment being trapped behind the dams.
“The coarser the grain size . . . the harder it is for the channel to move,” Draut said.
The researcher said she expected much more of the river downstream to have those traits.
But instead, the last 2.5 miles of the stream have shown little influence from the dams and have changed dramatically over the past 80 years. (There are no reliable maps of the river before the dam was built in 1913.)
Carved through floodplain
Like a dissatisfied sculptor, the river has ceaselessly carved its way through the floodplain, ever changing its direction and creating and destroying swaths of land in the process.
“It’s nothing like it was in the 1930s,” she said, noting that it has moved hundreds of meters in some areas.
Draut said the large amounts of fine sediment that still exist in the valley make that possible.
“The channel is cannibalizing its own floodplain,” she said.
How the river will change when the Elwha and Glines Canyon dams are removed has not been modeled.
Draut said it’s likely that an uncontrolled stream carrying large amounts of fresh sediment would lead to the creation of new channels and overall make the river even less predictable.
“We could see parts of the floodplain become even more mobile and more dynamic than they are now,” she said.
Tribe ready
This is a reality that the Lower Elwha Klallam tribe has been preparing for and, in some ways, welcoming, said Robert Elofson, the tribe’s river restoration program manager.
The tribe’s reservation is located in the floodplain, where about 400 members live.
But Elofson said the tribe does not think the river is going to be a hazard, once freed.
The levee on the reservation has been extended by up to 1,500 feet and reinforced in preparation for dam removal, making them “better protected” than ever, he said.
Plus, the tribe has given the river plenty of room to grow.
As much as 400 out of the 850 acres the tribe owns in the floodplain have been set aside for the river.
Further restraining the stream is one of the last things the tribe, which has long awaited the demolition of the two dams, would want to do, Elofson said, adding:
“We have always felt with the river, from dependency for subsistence and culture and history for so many centuries; we still feel the river is very important.”
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Reporter Tom Callis can be reached at 360-417-3532 or at tom.callis@peninsuladailynews.com.
