Water is released Thursday from two needle valves from the Friant Dam to meet the requirements of federal regulations governing the rewatering of the San Joaquin River to start the process of restoring the salmon run, in Friant, Calif. The San Joaquin River, whose waters course from the Sierra Nevada to the Pacific Ocean, carried the continent’s southernmost salmon run until the 1940s, when the government dammed it to nurture croplands below.

Fresno, Calif. ? When Darrell Imperatrice was a boy, California’s San Joaquin River teemed with so many king salmon his father could catch 40-pound fish using only a pitchfork.

Then the salmon vanished from the icy river for nearly 60 years, after a colossal federal dam built to nurture the croplands below dried up their habitat.

Now, as federal officials try to bring the fish back through a sweeping restoration program of the state’s second-largest river — opening the valves for the first full day on Friday — those who know it best are debating its value and its virtue.

“There were so many salmon back then, you could fish any way you wanted, even dynamite. But when they built that dam, thousands of fish lay dead on the banks,” said Imperatrice, who at age 82 still treasures his father’s fishing gear. “There’s no real restoration that will bring back the river I knew.”

Friends of Imperatrice’s family helped build the dam after the Great Depression, hauling sand up the channel and running cranes to build the 314-foot concrete wall that now holds back the Sierra Nevada snowmelt.

Thanks to the Friant Dam, that river water sustains more than 1,500 square miles of productive farmland on the east side of the Central Valley.

But for part of its course, in areas where the salmon once flourished, the river also serves as a drain, taking leftover irrigation water from fields, mixed with fertilizers and city runoff, out to a freshwater delta and the San Francisco Bay. Dry stretches totalling more than 60 miles hold gravel and tumbleweeds.

Friday morning, hours after federal officials released the first streams of water down its channels since the 1940s, environmentalists celebrated the beginning of a major restoration effort, the result of their decades-long legal tussle with farmers and the federal government.

The Natural Resources Defense Council filed a lawsuit in 1988, claiming the government’s massive dam and irrigation channels favored commerce at native salmon’s expense.

After years of hearings and negotiations, all parties agreed to a legal settlement in 2006 to return water to two dry stretches of the river and bring back native Chinook salmon by 2012.

President Obama signed a bill implementing the agreement in March, and on Friday, the first test flows snaked toward the dry riverbed, where Bureau of Reclamation officials hope they will revive the river’s ancestral channels.

But as freshwater supplies throughout the state dry up due to a three-year drought, some farmers worry the deal could wither their crops.