Chinook salmon return to historical habitat following removal of four US dams.

The Klamath River, which runs through Southern Oregon and Northern California, was heavily dammed during the 20th century. The dams blocked spawning Chinook salmon, and the river's spawning population fell by roughly 90%, according to Mongabay's report on a recent study.

Four dams were removed from the river in 2024. Shortly afterward, 7,742 Chinook salmon migrated upstream past the former dam sites. In 2025 the count rose to more than 13,000. The fish spread into 345 kilometers (214 miles) of formerly blocked river, recolonizing 88% of their known historical habitat, the study found.

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First author Damon Goodman told Mongabay by email that the team expected the salmon to return but did not know how long it would take. He said the speed and scale of the response surprised the researchers. According to Goodman, the fish got through difficult rapids, engineered fishways and a large lake to reach stretches of river that had been inaccessible for more than a century.

To count the fish, the researchers used an underwater sonar camera that produces moving images from sound waves instead of light. Goodman described it as "an underwater video camera that can see through muddy water, even in complete darkness." The team placed it near the former Iron Gate Dam site and monitored the fall migration continuously. They measured individual fish and could tell Chinook apart from other species. The river stayed very turbid after the dams came out, which made ordinary underwater video impractical.

Goodman said he is optimistic about a substantial recovery but cautioned that the work is not finished. Returning to historical population levels will require successful reproduction over multiple generations, along with better habitat, water quality and fish passage across the basin. In his words, "Dam removal was an enormous step forward, but it doesn’t eliminate all the challenges facing salmon." Scott Hinch, a salmon ecologist at the University of British Columbia who was not part of the study, told The New York Times that the speed of the recovery is remarkable.