As dam removal nationwide accelerates, experts are learning just how quickly rivers and fish respond.
In October 2021, ecologists shattered 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 kept threatened steelhead from reaching important spawning habitat just upstream.
Video courtesy Sempervirens Fund
For the next three days, workers and scientists from Sempervirens Fund, the land trust that owns the 8,500-acre preserve around Mill Creek, moved granite and gravel, aiming 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 salmon — an invaluable cultural resource — to return.
“We were thinking it was going to take five, six years for them to come home,” said Valentin Lopez, chairman of the tribal band. Ian Rowbotham, stewardship manager at Sempervirens Fund, thought the same: In order to spawn, steelhead and salmon require gravel beds, which form where material flushed by rain, melt or springwater collects throughout the streambed. Years of drought suggested that ecologists and fish alike had a long wait ahead.
Then, in September 2022, Rowbotham’s team spotted juvenile steelhead above the former dam site. To their surprise, they also found 15 juvenile coho salmon downstream. It was the first time coho, an endangered species, had ever been recorded in Mill Creek.
(From left) Sean Cochran (Fisheries biologist, CA Department of Fish and Wildlife), Jim Robins (Senior Ecologist), Melisa Cambron Perez (Sempervirens Fund Field Operations Manager), and Mike Podlech (Aquatic Ecologist) during a September visit to Mill Creek.
Ian Rowbotham/Sempervirens Fund
For Rowbotham and Lopez, the presence of both fish is a testament to the years of work they’ve put in to restore the watershed and re-establish traditional practices. To some degree, luck was also a factor: Record rain fell just weeks after the dam came down. The experience adds to a growing body of research documenting the speedy recovery of fish and other species after dams are removed — work that will only grow in importance as more and bigger dam removals are planned nationwide.
“We were thinking it was going to take five, six years for them to come home.”
THOUGH IT OFTEN MAKES headlines, Lisa Hollingsworth-Segedy isn’t surprised when fish come back soon after a dam is breached. A Pennsylvania-based river restoration director for the nonprofit American Rivers, she’s overseen the teardowns of more than 100 dams. “I have seen fish attempting to jump through the part of the dam that we’re actually removing,” she said.
People build dams to produce power and provide drinking water, irrigation and flood protection, often with little regard for the damage they cause to ecosystems that rely on fish migrating upstream and sediment flowing down. Dams large and small have contributed to the extinction of 29% of the salmon populations in California and the Pacific Northwest, and the threat or endangerment of many that remain, according to American Rivers. Of the more than 90,000 dams in the U.S., most are aging, which in recent years has caused a number of catastrophic failures — and some frightening near misses.
Since 1912, nearly 2,000 dams have been removed in the U.S., and that number is accelerating; 76% of those have come down since 1999, when a federal agency for the first time ordered a dam removed because the maintenance costs outweighed its benefits. It was a tipping point that set the stage for many more removals and spurred a new era of research. As evidence quickly accumulated showing just how rapidly rivers and wildlife respond, habitat restoration became a second key factor prompting communities, agencies and landowners to rethink their dams.
Ian Rowbotham/Sempervirens Fund
Many ecologists consider the 2011-14 removal of two dams on Washington’s Elwha River — the largest removal so far in the U.S. — to be a threshold moment. The rapid physical and ecological changes documented there added scale to what researchers had learned from smaller dams. Among other things, removing those dams reversed decades of coastal erosion near the river’s mouth as sediment movement was restored. “That was a huge ‘aha’ moment for us to understand that rivers do more work than just what we humans think they do,” Hollingsworth-Segedy said.
“It provided a lot of hope that we can undertake larger, more complex projects and have the river respond positively,” said Jeff Duda, a U.S. Geological Survey ecologist and a leading expert on dam removal.
Duda’s research and that of his colleagues has led to a number of conclusions over the last two decades: Physical changes, caused by sediment redistribution and water movement, happen very quickly, stabilizing within years rather than decades. Ecological changes manifest at different time scales, but upstream fish migration is one of the first to occur, often within weeks or months. Those fish contribute to further shifts: Within a year of the first Elwha dam removal, ecologists found nutrients from adult salmon in dippers upstream of the dam site — nutrients that they’ve recently shown have boosted the birds’ survival.
“That was a huge ‘aha’ moment for us to understand that rivers do more work than just what we humans think they do.”
Longer-term recovery, from reforestation of old reservoir beds to the multigenerational returns of salmon and other fish, is only beginning to be studied. “A lot of the dam removals have happened so recently that we haven’t had a chance for those outcomes 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 returns are in, and they’re positive.”
AT THE MILL CREEK dam site, Sempervirens Fund staff, Amah Mutsun researchers and others had been preparing for years to restore the creek’s flow. They’d cleared acres of invasive vines and laid logs across the creek bed to simulate natural treefall. “But there’s only so many things you can predict,” Rowbotham said.
A few weeks after they removed the dam, an atmospheric river storm drenched the area in record rainfall, washing sediment and even large rocks downstream. Material collected around the logs, creating zones of ideal fish habitat. In March, Rowbotham’s team found water flowing across the entire original floodplain, reconnecting the ecosystem.
In September, they set out with hand nets to survey fish. The first one they netted was a coho, just a few inches long, likely exploring for better food or habitat from San Vicente Creek downstream, which Mill Creek flows into. “Coho are good indicators of ecosystem health, so it’s one of those heartening moments of, ‘OK, this might be a shift,’” Rowbotham said.
Juvenile coho salmon (bottom) and juvenile steelhead (top).
Melisa Cambron Perez/ Sempervirens Fund
There is still a lot of work to be done. Rowbotham plans to install more logs in the creek bed, and advocate for further restoration at a second dam upstream. The tribal band was recently awarded $700,000 by the California Ocean Protection Council to expand its efforts, part of a $3.6 million grant to five tribes using traditional knowledge to protect California coastline. One part of the project will restore habitat for small fish spawning near the coast — fish they hope will one day feed the salmon that return.
Sarah Trent is an editorial intern for High Country News based in southwest Washington. Email her at [email protected] or submit a letter to the editor. See our letters to the editor policy.
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