Chinook salmon are back in the San Joaquin River after dying off 62 years ago.
When the Friant Dam was built in the 1940s, 60 miles of the San Joaquin ran dry, and the state's second largest salmon population died off.
But now biologists are releasing Chinook salmon into their original spawning grounds below the dam.
Scientists say having salmon back in the San Joaquin brings it back to life.
"In California in general, salmon nutrients were critical to all of the life living around our big river complex, including people. So this is the beginning of putting those nutrients back into the San Joaquin system," said Rene Henery, California Science Director for Trout Unlimited.
These scientists will study the salmon to help make their population grow in the San Joaquin.