When a Dam Comes Down, the Fish Come Back
American eels were on the decline in Virginia’s Rappahannock River when the United States Army Corps of Engineers removed the Embrey Dam from the waterway in 2004. Within four years, the endangered eels’ numbers more than doubled, and the fish was being found 100 miles upstream of the dam site.
Atlantic salmon began to reappear just days after the century-old Mill Dam was removed from Sedgeunkedunk Stream in Maine in 2009.
“Salmon, if you get of their way, can recolonize their former spawning and rearing grounds fairly quickly,” said research ecologist Jeffrey Duda.
So can many other fish, Duda and two colleagues found when they analyzed data from about 126 dam removals in the United States and other countries, according to a study published in the journal Science.
Since two dams were removed from northwestern Washington state’s Elwha River last year, Duda has witnessed the phenomenon personally. “After the first dam was removed,” he said, endangered chinook salmon “made it into Olympic National Park. When the Glines Canyon Dam was removed, further upstream, they were seen, really, within days of the final blast.”
The U.S. experienced a dam-building boom from the 1940s through the 1960s, said Duda, who works with the U.S. Geological Survey in Seattle. Now many of the smaller dams have become obsolete and not worth the expense of maintaining. Anti-dam sentiment has grown, along with concerns over the harms of damming rivers and streams to the survival of migratory fish, the flow of sediment, and other facets of river ecology.
These factors combined have helped to spark a nationwide dam-removal movement. The rate of removals shot up in the U.S. in the past 20 years, according to the study, with 548 dams removed between 2006 and 2014, compared with 298 between 1996 and 2005 and 147 between 1986 and 1994.
Not every organism in a newly free-flowing waterway bounces back, the coauthors found. “Responses have been mixed for less mobile bottom-dwelling plants and animals in former reservoirs and downstream channels,” they wrote.
Removing a dam usually releases a lot of sediment that has piled up behind it, so downstream communities or industrial users may have to plan for increased water treatment, he said.
There may also be situations in which removing one or more dams might be a bad idea, because it would allow nonnative species to spread farther upstream. “In the Great Lakes, there are a number of cases where dams are limiting the ability of sea lamprey to parasitize” waterways by blocking their movements, Duda said.
That’s why potential dam removals need to be decided on a case-by-case basis, he noted. Still, Duda expects the fast pace of dam removals in the U.S. to be sustained, because many have had positive environmental effects.
But as the nation’s stock of obsolete and aging small dams wanes, he said, the complexity of possible removals will likely rise, along with the size of the dams that remain.
“Also clouding the future is climate change,” he and his coauthors wrote, because the need to cut carbon pollution may heighten demand for emissions-free hydropower, while warming temperatures and more intense droughts will increase the pressure to store drinking water in reservoirs.