OLYMPIC NATIONAL PARK — Destroying two dams to free the Elwha River could restore salmon habitat but make life more precarious for river trout, a research ecologist said.

The 210-foot Glines Canyon Dam and the 105-foot Elwha Dam will be removed beginning next year in a $308 million National Park Service project.

Jeffrey Duda of the U.S. Geological Survey — who has been working with dozens of other scientists to collect samples and data — told more than 75 people who crowded into the Olympic National Park Visitor Center on Tuesday about researchers’ expectations once the dams are removed.

Over the long term, salmon will benefit, he said. With the dams gone, the fish will be able to swim up to old spawning grounds.

“We likely won’t see the full impact for several generations of salmon,” he added. “Salmon spawn when they are about 4 years old, so when you talk in those terms it will be probably 20 years before we see the full effects.

“And realistically, it could be 100 years before we can tell exactly what is happening.”

Researchers are worried about the river trout.

The dams create artificial reservoirs, which are good trout habitat, Duda said.

Once those are gone, scientists don’t know what will happen, he added.

“Bull trout are listed as a species of concern, so there are great concerns about the effects on the species,” he said.

Trout was not naturally a prominent species of the river, but grew in numbers as the right environment was created by the dams, he said.

Path of sediment

Researchers can’t predict exactly where the tons of sediment now piled behind the dams will go once the barriers are gone, even though they have run several computer models.

“We believe the river will find its original path and some, but not all, of the sediment will be very quickly washed downstream,” Duda said.

That will be a tumultuous time for both fish and plant life in the river, but it should clear up quickly, he added.

Sediment will wash out into the ocean. Models indicate that it will travel east and line the beaches and the west side of Ediz Hook, Duda said.

“Obviously sediment from the Elwha is why the Hook is there in the first place,” he said.

Some nutrients are expected to be washed out with the sediment, but one nutrient in particular will be restored to the upper part of the river, if predictions are correct, Duda said.

Nitrogen is notoriously low in the upper headwaters of the river, he said.

He believes that is because it comes from salmon returning from the ocean. When they die and decay in the river after spawning, their bodies release nitrogen — fertilizing algae and other plants.

Salmon are now blocked by the dams from returning to the upper river, so nitrogen is not prominent, he said.

“The Elwha River is a very young river, and as such is fairly oligotrophic [nutrient poor], so when you talk to people outside of the Northwest, they are flabbergasted that it was home to such huge salmon runs,” Duda said.

Duda and his crews have been collecting samples of sediment, insects and water.

Divers have counted fish and evaluated the proportion of various species at different parts of the river.

The experiments will be repeated during the dam removal, and also several times after the project is finished.

“We believe that this information about the immediate impacts of the removal will be very helpful for a whole host of other projects that include possible dam removals,” Duda said.

Work to tear down the dams is expected to be finished in 2014. Requests for proposals for the work are expected to be issued by the end of the month and awarded by September.

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Reporter Paige Dickerson can be reached at 360-417-3535 or at paige.dickerson@peninsuladailynews.com.