Scientists Think Dams Cause Beach Erosion Columbia River Sand Not Finding Its Way To Coast

Scientists suspect dams blocking the Columbia River and its tributaries contribute to erosion that threatens beaches and homes on the coast.

For most of this century, beaches along a 100-mile stretch of southwestern Washington and northwestern Oregon grew at a remarkable rate. They were fed sand by a huge, natural reservoir known as Peacock Spit, located just north of the Columbia River mouth.

Now, the spit is gone and beaches that grew most rapidly are eroding. As the beaches disappear, water just off shore deepens, allowing waves to strike with more force. The pounding waves accelerate erosion.

The first severe problem surfaced in 1993, when waves breached a Westport jetty, threatening a sewage treatment plant, a marina and the town’s water supply. In 1995, erosion at Cape Shoalwater began undermining state Highway 105 and threatening cranberry bogs. In 1996, erosion at Ocean Shores threatened condominiums.

Some beaches that support sand-dependent razor clams are turning into gravel, said Doug Simons, a Washington state shellfish biologist. Fort Canby State Park has closed some beach campsites.

Federal, state and local agencies launched the Southeast Washington Coastal Erosion Study in 1996 to find the cause of the problem. What scientists have found surprises them, said Peter Ruggiero, coastal engineer for the state Department of Ecology.

“The major contributor that we’ve seen has to do with the jetties that were installed at the turn of the century,” said Ruggiero.

Those jetties, which protect coastal inlets, changed the course of near-shore currents. The currents constantly fed fresh sand to the beaches. Now that the sand reservoir is gone, the currents simply eat away at the beaches.

But the presence of jetties doesn’t explain the depletion of Peacock Spit, which traditionally was fed by sand flowing down the Columbia.

Americans and Canadians have dammed the Columbia in 12 places, primarily in the past 70 years. Counting the Snake River and other tributaries, the Columbia watershed has more than 100 hydroelectric dams, and 250 water-slowing reservoirs.

Scientists theorize that sand is settling behind dams. For instance, engineers studying the possibility of breaching Washington’s Snake River dams estimate the four dams hold back 100 million to 150 million cubic yards of sand and other sediment - enough to bury Spokane’s Riverfront Park and downtown shopping district to a depth of 100 to 150 feet.

Dams may also contribute to erosion by controlling floods that would have washed sediment into the ocean, said Guy Gelfenbaum, study leader for the U.S. Geological Survey.

Gelfenbaum said there’s no telling how much sand the river used to carry and how much it carries now. So the Columbia’s impact on erosion can’t be documented as easily as the impact of jetties.

Solutions won’t come easily if the Columbia proves to be a major cause of erosion.

With so many dams in the watershed, breaching the four lower Snake River dams to bring salmon back to the river probably wouldn’t bring sand back to the beaches, scientists say.

The Corps of Engineers predicts that the sand trapped behind the four dams probably would settle in the next pool downstream, behind McNary Dam on the Columbia River. Below the McNary pool are three more reservoirs that could catch sand that escapes McNary.

Fine silt, not coarser sand, might make it to the ocean. But silt won’t restore beaches or help razor clams.

Gelfenbaum said that once scientists understand the problem, they may be able to find ways to change the operation of the dams to allow sand to flow more freely. No one knows what that would cost in lost power production, barge traffic and flooding, particularly at cities like Portland.

The Corps of Engineers might help solve the problem, Gelfenbaum said.

The corps, which periodically dredges the lower Columbia, usually dumps the spoils off-shore or inland. Scientists haven’t done the studies necessary to determine where that sand might better be placed so near-shore currents could carry the sand to beaches.

Commercial crab fishermen have opposed dumping dredging spoils in shallow water because it would smother their prey, Gelfenbaum said.