In August 2024, excavation crews breached the last of four dams on the Klamath River in far northern California, marking the first time in over a century that salmon could swim above the dam sites.

Now, new research published on Thursday shows that in the two years since dam removal, over 20,000 chinook salmon have migrated up the Klamath River, many past the former dam sites and into traditional spawning grounds.

“The biological response has been much faster than I, and most of us, expected,” said Damon Goodman, lead author of the study and Mount Shasta-Klamath regional director for Cal Trout. “We expected salmon to eventually recolonize habitat above the former dams, but within two years we’ve seen thousands of Chinook salmon moving into the upper basin and spawning across an enormous portion of their historic range.”

The salmon runs on the Klamath, which continue to hold ceremonial, spiritual and nutritional significance for indigenous tribes in the region, were once among the most productive in the western United States. The four dams, constructed by PacifiCorp in the early and mid-20th century to generate electricity, blocked passage for migrating salmon and altered the river’s natural hydrology, increasing water temperatures and flooding naturally cold springs.