The Klamath River in the U.S. was heavily dammed over the 20th century, blocking spawning Chinook salmon. As a result, the population of salmon that spawned in the river, which runs through Southern Oregon and Northern California, declined by roughly 90%. However, a recent study found that removing the dams resulted in a quick return of the fish to the river. In 2024, four dams were removed from the Klamath, allowing Chinook salmon (Oncorhynchus tshawytscha) to recolonize habitat that had been unavailable to them for roughly a century. Shortly after the dams were removed, 7,742 Chinook salmon migrated up the Klamath River past the former dams. In 2025, that number jumped to more than 13,000. The fish expanded into 345 kilometers (214 miles) of formerly blocked river, recolonizing 88% of their known historical habitat, the study found. First author of the study Damon Goodman told Mongabay in an email that they expected the Chinook to return to the river but didn’t know how long it would take. “What surprised us was the speed and scale of their response – it felt like we were running to keep up with the fish! These fish navigated challenging rapids, engineered fishways, and a large lake to reach habitats that had been inaccessible for more than a century.” Goodman and colleagues employed a specialized underwater sonar camera that uses sound waves instead of light to produce moving images of fish swimming through the river. “Think of it as an underwater video camera that can…This article was originally published on Mongabay

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