Breaching Dams Might Push Silt
by Jonathan Brinckman
Breaching four dams on the lower Snake River would send a pulse of sand and silt down the Columbia River that would settle behind McNary and John Day dams, the U.S. Army Corps of Engineers says.
But the sediment would be undetectable in the 140-mile stretch of the Columbia River downstream from Bonneville Dam, a corps study concludes. The lower river and the estuary are key habitat for young salmon.
Sand might plug irrigation pumps in the lake behind McNary Dam; dredging might be necessary to clear navigation channels; and habitat used by young salmon could be buried, corps officials concluded in a draft analysis of the potential effects of removing the dams. The analysis is part of a $20 million corps study on the probable costs and benefits of breaching.
It comes as the region debates whether the corps should remove earthen portions of Ice Harbor, Lower Monumental, Little Goose and Lower Granite dams to allow the lower Snake to flow freely again. Conservationists say that would aid threatened and endangered populations of Snake River salmon.
The corps and the National Marine Fisheries Service are holding public hearings throughout the Northwest on proposals for restoring the dwindling fish runs. The corps will make a recommendation on whether to remove the dams, but Congress would have the final say.
Opponents of breaching say the dams should be left in place because too little is known about the effects of releasing sediments trapped behind the dams. They say it could make the water too murky for fish and destroy habitat.
Conservationists who support breaching acknowledge that salmon and other fish might be hurt by the initial release of sediment. But they say the river would recover quickly and the long-term benefits would be worth it.
The corps' analysis found that between 100 million and 150 million cubic yards of sediment have accumulated behind the four dams in eastern Washington. If the dams were breached, the free-flowing river would create a gully through the layers of accumulated sediment, picking up about 50 million to 75 million cubic yards and carrying it downstream, the corps said.
Most of the sediment would settle when the water slowed behind McNary, the next dam downstream, said Dave Reese, chief of the hydrology branch of the corps' Walla Walla District. Reese does not expect initial sediment loads much greater than those now carried into the reservoir behind Lower Granite Dam, the easternmost of the four lower Snake dams.
The river slows dramatically when it reaches the pool behind Lower Granite, and most of the silt and sand are deposited there. "Whatever problems we currently see in Lower Granite, we'd basically transfer those problems into McNary pool," Reese said.
The corps' study does raise a concern that resuspension of sediments could expose three chemical contaminants: DDT, dioxin and manganese.
"We have no exact way to predict what will happen," Reese said. "We would have to monitor."
That lack of knowledge is reason enough not to breach dams, said Ted Gahr, who operates a research farm near McMinnville. "I don't think they have the foggiest notion of what will happen. That proposal has so many negatives to it. A big disaster like a sediment release wouldn't help salmon at all."
Conservationists said concern about the effects of sediments should not preclude breaching.
"If the dams stay in place, the fate of these fish is sealed," said Scott Bosse of Idaho Rivers United. "We are willing to risk some short-term acute impacts from sedimentation if it means saving these fish in the long run."
Historically, 10 million to 16 million salmon and steelhead returned each year to spawn in the rivers and streams of the Columbia River Basin, including the Snake. Today only about 1 million fish return, and most are hatchery-bred. Experts blame a combination of causes for the decline, including poor ocean conditions, overfishing, hatchery practices and loss of fish habitat to dams, overgrazing, logging and urban development.
Biologists at the Idaho Department of Fish and Game say that breaching the four dams is crucial to saving Snake River salmon.
"I think the impact of sediment on fish is considerably overblown," said Bert Bowler of Idaho Fish and Game. "There will just be a lot of sand -- beach sand, nice quality."
The corps' sediment report at www.nww.usace.army.mil/.
learn more on topics covered in the film
see the video
read the script
learn the songs