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Hoping to boost water supplies during future droughts, Silicon Valley’s largest water provider is working on a plan to build a new $800 million dam and reservoir in the remote hills of eastern Santa Clara County, just off Pacheco Pass.

The idea, still in the early stages, could result in the construction of one of the largest reservoirs in the Bay Area — a lake that would be twice the size of Crystal Springs Reservoir along Interstate 280 in San Mateo County — and the first new reservoir built in Santa Clara County since 1957, when Uvas Reservoir near Morgan Hill opened.

“It remains to be seen if it is feasible. But it definitely is worth exploring,” said Garth Hall, deputy operating officer at the Santa Clara Valley Water District. “This is a major opportunity to find new storage.”

On Feb. 28, the board of the water district, a government agency based in San Jose, voted to pay consultants up to $900,000 to study the idea. If those studies show the project has promise, Hall said, the district will apply for funding under Proposition 1, the $7.5 billion water bond that California voters passed in 2014 to help pay for new reservoirs, underground storage, conservation programs, water recycling, desalination and other water projects.

The project faces considerable hurdles, from its price tag to tricky geology.

“The good dam sites were taken long ago,” said Jonas Minton, senior water policy adviser at the Planning and Conservation League, a Sacramento environmental group. “What’s left are projects that are more expensive and provide less water supply.”

The new reservoir would be built on, or slightly upstream from, an existing reservoir, Pacheco Lake, in the rugged ranch lands about half a mile north of Highway 152 near Casa de Fruta. That lake, owned by the tiny Pacheco Pass Water District, sits on Pacheco Creek behind North Fork Dam, a 100-foot earthen dam built in 1939. The existing reservoir is small and holds only 6,000 acre-feet of water. The new reservoir would hold 130,000 acre-feet. An acre-foot is 325,851 gallons, or roughly the amount of water a family of five uses in a year. By comparison, Anderson Reservoir, the largest reservoir in Santa Clara County, holds 90,000 acre-feet.

The plan would be to tear down North Fork Dam and build a new dam either on the same location or up to a mile upstream, Hall said, in the oak woodlands of the Diablo Range. The new earthen dam would be at least 200 feet tall and potentially 300 feet tall. The district would take water it now stores in nearby San Luis Reservoir and pipe it into the new reservoir, filling it in wet years. There’s already a pipe, known as the San Felipe Project, running from San Luis Reservoir through the mountains into Anderson Reservoir, so building a connection to bring the water into the San Jose area would be relatively easy, district officials believe.

The idea, as with many dam projects, is expected to face controversy.

The district studied two locations in the same area as far back as 1993 for a new reservoir of roughly between 150,000 and 400,000 acre-feet. Both would have submerged part of Henry W. Coe State Park. When plans moved ahead in 2003, the outcry and potential lawsuits from park lovers and environmental groups led the district to drop the idea.

The 1993 study noted that shale geology in the area is unstable in places, which would make building a dam difficult. One site, known as Pacheco B, located 1.5 miles upstream from the existing reservoir, had solid geology, however. A 150,000 acre-foot reservoir there would cover about 1,200 acres, a 2002 district study found, and would require a 305-foot-tall dam and the acquisition of 25,000 acres of surrounding ranch land to protect the watershed.

The new proposal would not cross the Henry Coe park boundary, Hall said, and could provide more water and passage to help endangered steelhead trout migrate up Pacheco Creek into the state park.

Another challenge is the cost. Any new dam would result in increased water rates in Santa Clara County. The district, which provides water to 1.9 million people, has a long list of costly expenses coming up, including $400 million to rebuild the 67-year-old, seismically unsound Anderson Dam near Morgan Hill.

The district is also exploring other projects. Among them: a partnership with the Contra Costa Water District to raise the height of the dam at Los Vaqueros Reservoir in eastern Contra Costa County by 51 feet, increasing its storage from 160,000 acre-feet to 275,000 acre-feet at a cost of about $800 million, and sharing that water. And it is studying a proposal to store more water at Lake Del Valle in rural Alameda County with the Alameda County Water District.

“With the drought, we realized our vulnerability,” said Gary Kremen, a board member of the Santa Clara Valley Water District. “New projects are expensive, but what’s the cost of reliability? No one wants to run out of water.”

The Los Vaqueros project may be more feasible than a new Pacheco Pass reservoir, said Minton, a former deputy director of the state Department of Water Resources. It has no environmental opposition and would be built at an existing reservoir that already was successfully expanded five years ago.

“This is a new idea, which even if it is worthwhile will take quite a while to prove out,” Minton said. “Los Vaqueros is much further along.”