Turns out, if you want a river to act like it did before it was dammed, you just have to drain the reservoir the dam created.

Letting a river return to its natural course, even if only for a brief period, has other benefits, too. Invasive species disappear and endangered species, like the imperiled salmon of the Willamette River basin, get an extra boost.

That’s according to a group of researchers at Oregon State University, who published their findings Tuesday in the journal Ecohydrology.

For their research, led by recent Oregon State graduate Christina Murphy, the group looked at Fall Creek Reservoir, about 30 miles east of Eugene. Over the last several years, the U.S. Army Corps of Engineers, which operates the dam that holds back the reservoir, has drained the manmade lake in the fall to help young spring chinook salmon migrate downstream.

“Even though the strategy appears extreme, it’s both helping juvenile salmonids pass downstream and promoting a native species-dominated reservoir,” Murphy said in a statement. “Bass and crappie, which are major predators in the reservoir, have been pushed out and into the Willamette.”

It’s not just a matter of pushing the problem species into another part of the river system, either, Murphy said. It’s all about timing.

“The good news is that they were pushed downstream in the fall and winter, when the water is cold and the flow is faster, so they should be at a disadvantage in the Willamette River – compared to when they leak out of the reservoir in the summer,” she said.

Historically, the Willamette River has been home to the spawning grounds of roughly half a million fish, many of them coveted salmonids like spring chinook and winter steelhead. Adult fish swim upstream, past the mists and roar of Willamette Falls, to lay their eggs in Cascade and coast range creeks and streams. Newly hatched juvenile salmon then navigated the same course in reverse, swimming down the unimpeded streams to mature in ocean waters before returning to their inland spawning grounds to hatch the next generation.

That’s how things worked for thousands of years. Then, in the 1950s and ’60s, federally-managed dams sprung up on the McKenzie, Santiam and multiple forks of the Upper Willamette. The dams not only presented physical challenges to migratory fish, they also altered water temperatures in the rivers and reservoirs they created.

Earlier this year, an advocacy group named The Willamette as one of the country’s top-10 most endangered rivers because of the threat the dams create for salmon.

The Oregon State researchers looked at fish sampling data from 2006 through 2017, with the draining of Fall Creek Reservoir began halfway through that time period. Salmon migrating downstream increased after the draining started and invasive species, including large mouth bass and crappie, declined.

“In 2012, we could capture 10 bass an hour. This went down each year. By the summer of 2015 we only caught one during all of our sampling and in 2016, we didn’t capture any. This change was reflected in the data from the trap downstream too,” Murphy said. “The decline occurred with crappie, but faster.”

A small change is water management can make a big difference to native fish species, said Ivan Arismendi, an assistant professor at oregon State and co-author of the study.

“This extreme draining management is only about a week in duration, but has implications for the whole ecosystem,” he said. “It makes the reservoir begin to act as a natural river again affecting the entire fish community.”

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale

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