Klamath River Salmon Comeback Surprises Scientists After Record Dam Removal.
Researchers report that Chinook salmon moved into 88% of their documented historical range on the Klamath River within two migration seasons after four hydroelectric dams were removed in 2024.
Chinook salmon quickly moved into stretches of the Klamath River that had been blocked by dams, according to a new study. The researchers tracked the fish during the first two migratory seasons after four hydroelectric dams were taken out in 2024.
The removals reconnected more than 640 kilometers of habitat for anadromous fish. The authors describe the project as the largest dam removal completed to date. They also note that habitat fragmentation is a global driver of biodiversity decline, while evaluations of how ecosystems respond after rivers are reconnected remain rare.
To measure passage, the team used a stationary, high-resolution multi-beam imaging SONAR. The device quantified adult Chinook salmon (Oncorhynchus tshawytscha) moving upstream into the newly reopened reaches. The researchers estimated that 7,742 fish migrated upstream in 2024 (95% confidence interval: 7,702 to 7,778) and 13,310 in 2025 (95% confidence interval: 12,876 to 13,733). They report that these totals represented 18 to 19 percent of the fish returning to the Klamath River Basin.
Within that period, the salmon recolonized 88 percent of their documented historical distribution. They expanded into 345 kilometers of reconnected habitat and reached elevations as high as 1,250 meters, according to the authors.
The fish did not have an easy route. Recolonization required them to swim through the dam removal sites, complex hydraulic corridors and engineered fishways. The authors say this shows a propensity for rapid recolonization at the scale of a whole river basin, even with those obstacles.
The researchers call the work the first quantitative assessment of salmon response to the world's largest dam removal. The abstract covers only the first two migratory seasons, so it does not address longer-term outcomes.
Note: The page does not say scientists were surprised. It reports rapid recolonization and describes the project as the largest dam removal completed to date.
At a Glance
- Four hydroelectric dams on the Klamath River were removed in 2024, reconnecting more than 640 km of habitat.
- Estimated upstream Chinook migrants into reopened reaches: 7,742 in 2024 and 13,310 in 2025.
- Chinook recolonized 88% of their documented historical distribution, including 345 km of reconnected habitat.
- Fish reached elevations of up to 1,250 meters, passing dam removal sites and engineered fishways.
Sources
This brief was written by Brightside Report from the reporting linked above. Spot an error? Request a correction.