River connectivity is essential for healthy river ecosystems and the people and communities that depend on them. Connected rivers allow fish to move between habitats, complete their life cycles, and access areas important for spawning and rearing.
Dams can disrupt this connectivity by blocking fish passage and fragmenting river ecosystems. Removing dams offers an opportunity to restore these connections, but the extent to which fish can quickly reclaim historically accessible habitat is less clear.
In 2024, four hydroelectric dams were removed from the Klamath River in Oregon and California, reconnecting hundreds of kilometers of habitat that had been inaccessible to migratory fish for more than a century. Researchers examined how Chinook salmon responded to the removal of the dams, focusing on whether salmon would quickly recolonize their historical range in the upper Klamath River.
Goodman, D.H., Denton, K., Pess, G.R., Whelan, J., Stefankiv, O., Soto, T., Corum, A., Ortega, J., O’Rourke, O., Hereford, M.E. and Giudice, D. 2026. Rewilding the upper Klamath River Basin: rapid recolonization of Chinook salmon following the world’s largest dam removal. Scientific Reports, 16(1): 25322. DOI: 10.1038/s41598-026-65437-0
Researchers used stationary, high-resolution multibeam imaging SONAR near the former Iron Gate Dam to monitor fish movement through the newly connected river. They combined SONAR observations with underwater video and other fish surveys to identify species, estimate the number of salmon passing the former dam site, and track the timing and extent of their upstream movement during the 2024 and 2025 fall migration seasons.
The results showed a rapid response. Within two years of dam removal, Chinook salmon had recolonized approximately 345 km of river, representing 88% of their documented historical distribution in the upper basin. Salmon successfully moved through the former dam site and into upstream habitats, including areas at elevations as high as 1,250 m. Researchers also detected Coho salmon, steelhead, and Pacific lamprey in newly accessible habitat.
The findings contribute to the growing evidence that large-scale dam removal can rapidly restore river connectivity and expand the distribution of migratory fish. Reconnecting the upper Klamath River may also provide opportunities for the return of spring-run Chinook salmon, which have been absent from the upper basin for decades. While continued monitoring will be needed to determine whether salmon establish successful spawning populations, the rapid recolonization demonstrates the potential for dam removal to restore connectivity and recover access to important river habitats.
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