Within two fall migrations after four hydroelectric dams came down on the Klamath River, Chinook salmon moved into 345 kilometers of river and tributaries that had been blocked for more than a century. A peer-reviewed study published Aug. 27 estimates that 7,742 adult Chinook passed the former Iron Gate Dam site in 2024 and 13,310 in 2025. Researchers describe this as rapid recolonization: an encouraging first measure of habitat use, not proof that the salmon population has recovered.
The Klamath flows from southern Oregon through Northern California to the Pacific. The four dams removed were J.C. Boyle, Copco 1, Copco 2 and Iron Gate; their removal was completed Oct. 2, 2024. It reconnected more than 640 kilometers of main-stem river and tributary habitat. Dams had kept migratory fish from reaching the upper river for over a century, the study says.
To track what happened next, researchers placed a stationary, high-resolution imaging sonar near the former Iron Gate site. The device used sound waves to record fish moving through the river, where post-removal sediment made ordinary underwater video difficult. The team reviewed fish size and direction of travel, then used sampling and other monitoring to estimate how many were Chinook. The study reports estimated upstream passage, not a count of every salmon throughout the watershed.
The estimate was 7,742 Chinook in fall 2024 and 13,310 in 2025. The paper calculates that these fish represented a little over 18% of the basin’s reported Chinook return in 2024 and 19% in 2025. That comparison helps show the scale of the movement into newly opened waters, while the estimates and their confidence ranges remind readers that this was a measured scientific estimate rather than a head count.
Researchers also mapped where salmon were observed. Across the Klamath, Link River and 11 tributaries, Chinook occupied 345 kilometers of reconnected habitat, equivalent to 88% of their documented historical river distribution. The farthest-upstream observation was at Beatty Gap on the Sprague River, about 1,250 meters above sea level and more than 580 kilometers from the Pacific. Those figures describe the length and reach of documented habitat use, not the share of the historic salmon population that has returned.
The fish traveled through more than the former dam sites. To reach the upper basin, they passed complex river corridors, fishways at the Keno and Link River dams, and Upper Klamath Lake. The study describes Chinook moving through those obstacles during the first two migration seasons after reconnection. Its authors say the early spread shows that fish could find and use habitat that had been inaccessible for generations.
The findings are especially meaningful in a watershed where salmon are part of local ecology and cultural life. The research team included Tribal, state and federal fish biologists alongside university and nonprofit scientists. Salmon support food, ceremony and traditional practices for Tribal nations in the basin, the paper notes. The new measurements give those partners a clearer baseline for tracking what happens as fish continue to move through restored connections.
There is an important qualification: the fish were not all known to be wild-born. The Iron Gate Hatchery, which stood at the former dam site, contributed to the source population. Researchers say the fish moving into upper reaches included both natural spawners and returning hatchery fish. Regardless of origin, they swam upstream on their own, but future monitoring will need to follow how the mix changes as the hatchery contribution declines.
The paper calls the results an early indicator of success and says longer-term evaluation depends on evidence of sustained population performance, particularly successful reproduction. Chinook in the river are generally two to four years old, so researchers expect several more years of observation before they can assess how the first fish to reach reopened habitat reproduce and whether their offspring return. Habitat conditions, water quality, passage and changing ocean and river conditions also matter.
In an Oct. 12 report, Mongabay spoke by email with lead author Damon Goodman. He said researchers expected Chinook to return but were surprised by the speed and scale, and that recovery still depends on reproduction over generations and improvements to habitat, water quality and fish passage. That caution matches the study’s framing: the first two seasons show rapid re-entry into newly opened habitat; they cannot yet show whether a self-sustaining population will follow.
For now, the clearest milestone is geographic. Thousands of Chinook passed the old Iron Gate site, and surveys found them using most of the historically documented river length within two seasons. Continued counts and reproduction surveys will show whether that first expansion can become a lasting part of the Klamath’s future.
Reader responses