FRESNO (AP) — California’s second-longest river once teemed with salmon that swam from San Francisco Bay to spawn near Fresno. But that was before the Friant Dam diverted the San Joaquin River to serve agriculture needs 62 years ago.

On Wednesday, biologists released two Chinook salmon 30 miles below the dam, a milestone in one of the most complex river restoration projects in North America.

The hope is the two fish continue their journey and spawn in the same gravel beds where swarms of their ancestors were so thick that locals said it seemed they could walk across the shallow river on their backs.

If all goes according to plan, in spring the newly hatched salmon will travel 320 miles back to sea along the Central California river.

“We hope that they imprint on the habitat and that area of the river, and as adults they’ll be attracted to the same area,” said Gerald Hatler of the California Department of Fish and Game. “That’s our goal, but what we’re doing now is largely experimental.”

The $800 million San Joaquin River Restoration Project stems from a 2006 settlement of a lawsuit filed by environmental groups to restore the river flows and salmon runs. The suit contended the government violated state law when water diversions into the Friant-Kern and Madera irrigation canals dried up the river and ended spring and fall salmon runs.

“When you have an iconic species coming back, it’s a signal the river is being restored,” said Monty Schmitt, a scientist with the Natural Resources Defense Council, which filed the lawsuit.

The settlement involving farmers, fishing groups, environmentalists and the federal government guarantees that at least 350 cubic feet per second of water will be released daily from the dam, which they calculate is enough to refill the 60 or so miles of river that had run dry.

“You can’t put things back to the way they were originally,” said Margaret Gidding of the U.S. Bureau of Reclamation, which built the dam. “Trying to create a connective river again is giving us quite a challenge.”

One problem is irrigation pumping in the nation’s most productive farming region has caused portions of the riverbed to sink, which poses a challenge to maintaining flow that federal engineers have not yet solved.

In other areas, farmers have encroached on land they never dreamed would be in the river’s path again.

Engineers are also trying to figure out how to bypass smaller dams and diversions constructed decades ago to capture what little water made it downstream in wet years.

“There have been other salmon restorations in the U.S., but the complexity of this one seems to be at the forefront,” Gidding said.

Even if the salmon have a clear path to travel back to their original spawning grounds, there are questions about whether they will find the habitat suitable and whether they will return to complete the life cycle in three years.

“The historic spawning gravel is still present,” said Hatler, environmental program manager of the project. “Is there enough? Do we need to add more gravel? Will they spawn there?”

During the past two weeks, biologists have trapped and released 92 salmon drawn by instinct to the river that hardly exists. They set up nets at the confluence of the Merced River then transport the fish 132 miles past the danger zones.

The salmon weigh up to 16 pounds and will be tagged with transmitters so biologists can follow their progress to spawning areas, but two caught Wednesday went to a hatchery at the base of the dam.

Some juveniles also will be tagged before they head back to sea to track how well they navigate man’s changes to the river.

“We have a lot to learn,” Hatler said.