Working to expand water supplies for California’s next drought, a coalition of 12 Bay Area water agencies took a significant step Monday toward an $800 million expansion of one of the largest reservoirs in the Bay Area — Los Vaqueros Reservoir in the rolling hills near the Alameda-Contra Costa county line.

The plan would raise the reservoir’s earthen dam by 55 feet, to 273 feet high. That would make it the second tallest dam in the Bay Area, eclipsed only by Warm Springs Dam, at 319 feet tall, on Lake Sonoma near Healdsburg.

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Water service to shut off overnight in areas of El Sobrante next week The construction would expand the size of Los Vaqueros from its current 160,000 acre-feet capacity to 275,000 acre-feet, enough water when full for the annual needs of 1.4 million people.

The Santa Clara Valley Water District, East Bay Municipal Utility District, Alameda County Water District, San Francisco Public Utilities Commission and other agencies are contributing to the studies of the project, and see it as a cooperative solution to water shortages and a way to better tie their systems together as insurance against emergencies like earthquakes.

“This is an opportunity to help us deal with extended droughts,” said Jennifer Allen, a spokeswoman for the Contra Costa Water District, based in Concord. “It’s a way to expand the benefits of the reservoir and make it work for a larger group.”

On Monday, the Contra Costa Water District, which owns the reservoir, released new environmental studies and scheduled a series of six public meetings from July 11 to July 27 in Concord, Oakland, San Jose, Brentwood, Sacramento and Los Banos to discuss the specifics in the new documents, which are officially called the draft supplement to the final environmental impact study.

Supporters of the idea are hoping that up to $400 million of the $800 million cost will come from Proposition 1, a water bond passed by California voters in 2014. The deadline for water agencies to apply for funding under the measure is August 14.

The remainder of the cost would be paid by the customers of each participating agency. Exactly which agencies will decide to formally join the project, and who will pay how much is yet to be determined. Costs will likely be decided based on the amount of water each agency would get.

“Having more partners brings in more customers, and that helps spread the costs out more widely,” said Allen.

Still, more studies, negotiations and permits are needed. Construction would begin in 2022 and be completed in 2027.

Los Vaqueros Reservoir is 3 miles long. It was built in 1998 with $450 million from the 500,000 customers of the Contra Costa Water District. In 2010, the same customers approved an advisory measure to increase the reservoir’s size from 100,000 acre feet to 160,000 acre feet, raising the dam 34 feet.

That project was finished in 2012. The reservoir, which is filled by water from the Sacramento-San Joaquin River Delta, is currently 95 percent full, the highest level it has ever been, as a result of this recent drenching wet winter.

The idea of expanding Los Vaqueros Reservoir has been around for a while. Nearly a decade ago, it was discussed, but at the time, the economy was struggling, and there were few state matching funds, and Contra Costa officials could not find other agencies to help foot the bill.

Now, after the historic 5-year-drought, they are lining up.

“We’re interested in studying this because an expanded reservoir provides an opportunity for us and others to capture surplus water that ordinarily would flow to the ocean,” said Colleen Valles, a spokeswoman for the Santa Clara Valley Water District, which provides water to 1.9 million people in Santa Clara County. “It would allow us to store more for drier years.”

The Santa Clara Valley Water District also is submitting its own plan for an expanded reservoir, a proposal to increase the size of Pacheco Reservoir, a remote lake located along Highway 152 between Gilroy and Los Banos. But that project is considered more of a long shot. It has no major water agencies as partners, and is in an area that has had geologic issues in the past.

Unlike with many other plans to build new dams on rivers, which would submerge huge areas of wilderness and kill endangered salmon and other species, environmentalists have not come out in opposition to increasing the height of Los Vaqueros, which is an off-stream reservoir filled with water brought in from the Delta via pipes and a canal.

The project would have a relatively minor impact, because a reservoir is already there. When Los Vaqueros’ dam was raised five years ago, the Contra Costa Water district purchased roughly 5,000 acres in Alameda, Contra Costa and San Joaquin counties to provide habitat for the endangered San Joaquin Valley kit fox and other species whose habitat was submerged when the lake level rose. A similar environmental project would be likely if the lake rises again.

The project would require the entire reservoir to be drained, since the dam would need to be worked on at the reservoir side, Allen said. Trails in the 20,000-acre watershed would need to be rerouted, and the boat marina would need to be relocated. Also, a 9-mile long pipeline from a transfer station near Los Vaqueros would need to be built to Bethany Reservoir nearby, so the water could be put into the South Bay Aqueduct and delivered further south.

The meetings to discuss the idea are scheduled for: