When the dams fall, the fish come home — High Country News – Know the West

0

As dam removal accelerates across the country, experts are learning how quickly rivers and fish are reacting.

In October 2021, conservationists broke the top of a dam on Mill Creek near Davenport, California with a hydraulic hammer. Within hours, the entire structure was down. For at least 110 years, the long-obsolete dam had prevented endangered rainbow trout from reaching important spawning habitat just upstream.

Video courtesy of Sempervirens Fund

For the next three days, workers and scientists from the Sempervirens Fund, the land trust that owns the 8,500-acre reservation around Mill Creek, moved granite and gravel, in an effort to restore the creek to its natural state. . Members of the Amah Mutsun Tribal Band, stewardship partners working on their traditional lands, held ceremonies praying for the return of the salmon, an invaluable cultural resource.

“We thought it would take them five, six years to get home,” said Valentin Lopez, president of the tribal band. Ian Rowbotham, stewardship manager at the Sempervirens Fund, thought the same thing: to spawn, rainbow trout and salmon need gravel beds, which form where material washed away by rain, melt or spring water accumulates in the stream bed. Years of drought suggested that conservationists and fishes had a long wait ahead of them.

Then, in September 2022, the Rowbotham team spotted a juvenile rainbow trout above the old dam site. To their surprise, they also found 15 juvenile coho salmon downstream. This was the first time coho, an endangered species, had been recorded at Mill Creek.

(Left to right) Sean Cochran (Fisheries Biologist, California Department of Fish and Wildlife), Jim Robins (Senior Ecologist), Melisa Cambron Perez (Sempervirens Fund Field Operations Manager), and Mike Podlech (aquatic ecologist) on a September visit to Mill Creek.

Ian Rowbotham/Sempervirens Fund

For Rowbotham and Lopez, the presence of the two fish is testament to the years of work they have done to restore the watershed and reestablish traditional practices. To some extent, luck also played a role: record rains fell just weeks after the dam fell. The experience adds to a growing body of research documenting the rapid recovery of fish and other species after the removal of dams – work that will only grow in importance as After and bigger dam removals are planned across the country.

“We thought it would take them five, six years to come home.”

ALTHOUGH IT OFTEN MAKES headlines, Lisa Hollingsworth-Segedy isn’t surprised when fish return soon after a dam breaks. Pennsylvania-based river restoration director for the nonprofit American Rivers, she has overseen the dismantling of more than 100 dams. “I’ve seen fish trying to jump through the part of the dam we’re removing,” she said.

People build dams to generate electricity and provide drinking water, irrigation and flood protection, often without regard to the damage they cause to ecosystems that depend on the migration of fish to the upstream and sediment flow. Dams large and small have contributed to the extinction of 29% of salmon populations in California and the Pacific Northwest, and to the threat or endangerment of many remaining populations, according to American Rivers . Of more than 90,000 dams in the United States, most are aging, which in recent years has caused a number of catastrophic breakdowns – and some scary ones narrowly avoided.

Since 1912, nearly 2,000 dams have been removed in the United States, and the number is accelerating; 76% of these have been reduced since 1999, when a federal agency first ordered a dam removed because maintenance costs outweighed its benefits. It was a tipping point that paved the way for many more moves and spurred a new era of research. As evidence rapidly accumulated showing how quickly rivers and wildlife are responding, habitat restoration has become a second key driver prompting communities, agencies and landowners to redesign their dams.

Ian Rowbotham/Sempervirens Fund

Many environmentalists consider the 2011–2014 removal of two dams on the Elwha River in Washington — the biggest drawdown yet in the United States — to be a watershed moment. The rapid physical and ecological changes documented there added magnitude to what researchers had learned from small dams. Among other things, the removal of these dams reversed decades of coastal erosion near the mouth of the river as sediment movement was restored. “It was a huge ‘aha’ moment for us to realize that rivers do more work than we humans think they do,” Hollingsworth-Segedy said.

“It gave a lot of hope that we could take on larger, more complex projects and that the river would respond positively,” said Jeff Duda, a US Geological Survey ecologist and leading dam removal expert. .

Duda’s research and that of his colleagues has reached a number of conclusions over the past two decades: Physical changes, caused by sediment redistribution and water movement, occur very rapidly, stabilizing in a few years rather than a few decades. Ecological changes occur on different time scales, but upstream fish migration is one of the first to occur, often within weeks or months. These fish are contributing to other changes: in the year since the first removal of the Elwha dam, conservationists have discovered nutrients of adult salmon in ladles upstream of the dam site – the nutrients they have recently shown to be boosted bird survival.

“It was a huge ‘aha’ moment for us to realize that rivers do more work than we humans think they do.”

Longer-term recovery, from reforestation of old reservoir beds to multigenerational returns of salmon and other fish, is only beginning to be studied. “A lot of dam removals have happened so recently that we haven’t had a chance for those results to fully develop,” Duda said. In the Elwha, most migratory fish have only been documented for one generation. “We’re just waiting and measuring, but a lot of the early feedback is there, and it’s positive.”

AT THE MILL STREAM At the dam site, Sempervirens Fund staff, Amah Mutsun researchers and others had been preparing for years to restore the stream’s flow. They had cleared acres of overgrown vines and laid logs on the creek bed to simulate the natural fall of trees. “But there are only so many things you can predict,” Rowbotham said.

A few weeks after the dam was removed, an atmospheric storm flooded the area with record rainfall, washing sediment and even large boulders downstream. Material collected around logs, creating areas of ideal habitat for fish. In March, Rowbotham’s team discovered that water was flowing throughout the original floodplain, reconnecting the ecosystem.

In September, they leave with hand nets to monitor the fish. The first one they caught was a coho, just a few inches long, likely exploring for better food or habitat from San Vicente Creek downstream, into which Mill Creek empties. “Coho are good indicators of ecosystem health, so this is one of those encouraging ‘OK, this could be a change’ moments,” Rowbotham said.

Juvenile coho salmon (bottom) and juvenile rainbow trout (top).

Melisa Cambron Perez/ Sempervirens Fund

There is still a lot of work to do. Rowbotham plans to install more logs in the creek bed and advocates for further restoration of a second dam upstream. The tribal band was recently awarded $700,000 by the California Ocean Protection Council to expand its efforts, part of $3.6 million grant to five tribes using traditional knowledge to protect California’s coastline. Part of the project will restore habitat for small spawning fish near the coast – fish that they hope will one day feed returning salmon.

Sarah Trent is a writing intern for High Country News based in southwest Washington. Email him at [email protected] or submit a letter to the editor. See our letters to editor policy.

Share.

Comments are closed.