FRESNO (AP) — Tens of thousands of spring-run Chinook salmon are being released into the San Joaquin River, marking a major milestone in the federal plan to restore native fish populations to the state’s second-longest river.

The Bureau of Reclamation and the U.S. Fish and Wildlife Service released 54,000 hatchery-produced salmon into the river from a site near Fresno on Thursday. Though environmentalists are celebrating the release, federal water managers say the state’s drought means not all of the fish will return to spawn.

The San Joaquin River carried the continent’s southernmost Chinook salmon run until the Friant Dam was built in 1942 above Fresno to capture its water for crops.

The restoration effort is the result of a 2006 legal settlement that ended a decades-long legal tussle between farmers, environmentalists and the federal government.