In 2024, four dams were removed from an American river. Over 400 miles of Salmon habitat reopened. Scientists are now watching an ecosystem rebuilding itself

For generations, salmon swimming up the Klamath River eventually encountered barriers they could not cross. Four hydroelectric dams had cut the fish off from hundreds of miles of historic habitat across California and Oregon. In 2024, the last major obstacle came down, completing what NOAA described as the world's largest dam-removal project and giving the river and its salmon an extraordinary chance at ecological recovery.

How Four Dams Changed the Klamath

The four hydroelectric dams were constructed between 1922 and 1965. According to the U.S. Geological Survey (USGS), their construction was among the most significant disturbances to the Klamath Basin ecosystem. Beyond blocking fish migration, the reservoirs affected water temperature, dissolved oxygen and sediment movement. USGS also documented recurring harmful cyanobacterial blooms in the reservoirs and noted that altered river conditions contributed to disease and salmon mortality. The consequences extended beyond wildlife. Declining salmon runs affected Indigenous communities, including tribes for whom salmon have profound cultural and subsistence importance.

A River Reconnected After More Than a Century

Removal of the four dams, Iron Gate, Copco No. 1, Copco No. 2 and J.C. Boyle transformed the river. When crews removed the final temporary barrier in October 2024, salmon, steelhead and lamprey gained access to more than 400 miles of habitat. Some salmon habitat had been disconnected for more than a century. “These final dam removal steps set the stage for salmon to return to reclaimed habitat and expand their population recovery,” Jim Simondet, NOAA Fisheries' Klamath Branch Supervisor, said at the time. The scale is difficult to overstate. NOAA said the project would reopen more Pacific salmon habitat than all previous dam removals in the western United States combined.

Scientists Are Watching What Happens Next

Removing concrete, however, does not automatically repair an ecosystem. Researchers from UC Davis are studying salmon before and after the project to determine whether diverse, healthy populations can establish themselves in the reopened habitat. Scientists can even examine chemical signatures in tiny fish ear bones, called otoliths, to reconstruct parts of a salmon's life history. There are also reasons for caution. Research involving Oregon State University scientists concluded that dam removal should benefit salmon but will not eliminate every threat they face. Disease, changing environmental conditions and habitat quality remain important challenges.

Removing the Dams Was Just the Beginning

The Klamath project is therefore more than a story about demolishing four structures. It is a massive real-world experiment in whether an altered river can regain ecological functions lost over generations. The salmon now have something they lacked for decades: a route home. Whether they can rebuild thriving populations will take years of monitoring to answer, but reconnecting more than 400 miles of habitat has given the Klamath River a chance to begin that recovery.