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LOS BANOS — Last summer it was a jarring symbol of California’s historic five-year drought. San Luis Reservoir — the vast lake along Highway 152 between Gilroy and Los Banos, the state’s fifth-largest reservoir and a key link in the water supply for millions of people and thousands of acres of Central Valley farmland — was just 10 percent full.

A parched expanse of cracked mud, littered with old beer bottles and millions of tiny clam shells, San Luis was at its lowest level in 27 years.

But in a stunning turnaround that highlights the state’s recovery from the drought, the reservoir christened by John F. Kennedy in 1962 is now completely full for the first time in six years. Its water level has risen 192 feet — nearly twice the height of Oakland’s Oracle Arena — in seven months.

“Who would have thought this last summer?” said Howard Berman, who retired in December after 40 years as an interpreter at the lake’s visitor center. “I thought we were in trouble. There were signs all over that said ‘Pray for rain.’ Well, you can take those down now.”

The lake holds 2 million acre feet, enough water for 10 million people for a year. Nine miles long, it contains 12 times as much water as Los Vaqueros Reservoir in Contra Costa County, five times as much as Yosemite’s Hetch Hetchy and 100 times as much as Lexington Reservoir in Los Gatos.

Unlike many dams, which are built on big rivers, San Luis’ 382-foot-high earthen dam holds back a reservoir that acts as a switching yard for California’s water system. The reservoir is filled not by a river, but by people — officials from the state Department of Water Resources and federal Bureau of Reclamation. They pump water from the Delta near Tracy, 65 miles to the north, into San Luis, where it is stored, then sent down canals to 600,000 acres of farms in the San Joaquin Valley and cities as far south as San Diego.

A tunnel through the Diablo Range also sends the water into Silicon Valley, where it is a key part of the water supply.

Now that it is full, farmers and cities that receive water from the State Water Project and Central Valley Project are expecting a plentiful supply this summer.

“It’s a bounty of water,” said Cindy Kao, imported water manager for the Santa Clara Valley Water District. “If we hadn’t received all this rain, it would have been very bleak.”

Unlike Oroville and other reservoirs that filled in chaotic and violent ways over the past three months as atmospheric rivers drenched California, San Luis was filled like a bathtub — very carefully — right to the top.

Come late April or May, as demand from farmers and cities increases, it will be slowly drawn down. On Thursday, state and federal officials stopped filling it when it reached the top for the first time since April 2011.

The reservoir, whose once-parched hills are now emerald green, has not been without controversy. After lawsuits to protect endangered fish in the Delta such as salmon and the Delta smelt, judges have ruled in recent years that the giant pumps at Tracy have to be turned down to protect the fish, making it more difficult to fill San Luis.

Environmental groups and biologists say the smelt — a tiny, nearly translucent fish — is a canary in the coal mine, a critical part of the food chain of the Delta whose health is an indicator of the overall ecological health of the area. Every year, roughly half of the fresh water that flows into the Delta is diverted to other uses, never making it to San Francisco Bay.

Last week, as fishing boats zipped along the lake and puffy clouds floated over emerald green hillsides, Eddy Freitas, a truck driver from Los Banos who was pulling his 18-foot Crestliner boat from the lake, said he isn’t a smelt fan.

“It’s ridiculous. It’s a bait fish,” he said. “If it goes extinct, another bait fish will take its place.”

Freitas said he once caught a 38-pound bass at San Luis. “I’ve been fishing here for 35 years,” he said, beaming. “I wish it could fill like this every year. It’s a pleasure to be out there. This is a blessing.”

The lake’s visitor center highlights the day that construction started, Aug. 18, 1962. That afternoon, President Kennedy, in a well-tailored blue suit, after flying to California on Air Force One, took a helicopter to the construction site. He was met by Gov. Pat Brown, Gov. Jerry Brown’s father, and a crowd of local officials, farmers and others.

It was a time when the elder Brown was pouring concrete across the state, building highways, universities, dams and other structures. That was before Proposition 13 limited tax increases, and before environmental laws like the Endangered Species Act and Clean Water Act slowed the relentless roll of construction equipment as California’s population swelled.

“It is a pleasure for me to come out here and help blow up this valley in the name of progress,” Kennedy joked.

He and Brown both pushed the handle of a dynamite plunger, and after a big explosion rocked a hillside in the distance, everyone clapped.

The reservoir was finished in 1967, two months ahead of schedule.

“I love that speech,” Berman said. “I watch it every year on the anniversary.”

As he looked across the rippling waters that seemed to stretch to the horizon, Berman noted that California’s weather has a habit of going through boom and bust cycles.

“You never know what is going to happen next year. We could go right back into another drought,” he said. “I’d settle for an average winter next year, one without the extremes.”