Rewilding the upper Klamath River Basin: rapid recolonization of Chinook salmon following the world's largest dam removal.

Habitat fragmentation is a global driver of decline in biodiversity, yet evaluations of ecological response following re-connection remain rare. In 2024, four hydroelectric dams on the Klamath River were removed, reconnecting over 640 km of anadromous habitat and marking the largest dam removal project completed to date. We used a stationary high-resolution multi-beam imaging SONAR to quantify adult Chinook salmon (Oncorhynchus tshawytscha) passage into newly reopened reaches during the first two migratory seasons following dam removal. An estimated 7,742 (95% CI: 7,702-7,778) Chinook salmon migrated upstream in 2024 and 13,310 (95% CI: 12,876-13,733) in 2025, representing 18-19% of fish returning to the Klamath River Basin. Within this period, Chinook salmon recolonized 88% of their documented historical distribution, expanding into 345 km of reconnected habitats and reaching elevations of up to 1,250 m. Recolonization required fish to swim through dam removal sites, complex hydraulic corridors and engineered fishways, demonstrating the propensity for rapid recolonization at basin scale despite these challenges. These findings provide the first quantitative assessment of salmon response to the world's largest dam removal and demonstrate how reconnection can catalyze large-scale recolonization in historically fragmented river networks.