SAN JOSE — In a landmark vote closely watched across California, Silicon Valley’s largest water agency on Tuesday rejected Gov. Jerry Brown’s $17 billion plan to build two giant tunnels under the Sacramento-San Joaquin River Delta.

By a vote of 7-0, the Santa Clara Valley Water District’s board of directors chose instead to adopt a set of principles endorsing a significantly smaller, less costly project — with just one tunnel.

“What magnitude of project makes sense?” asked board member Barbara Keegan. “From our perspective that looks to be something less than originally was envisioned.”

“It’s clearly going to be a smaller project than what was originally proposed,” said board member Gary Kremen.

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The vote, following a rejection of the project last month by the Westlands Water District in Fresno, which was to have contributed $3 billion, further throws the future of one of Brown’s major construction priorities into doubt with just 14 months left in his term. If the governor decides to pursue a smaller Delta project, which could be roughly half the twin tunnels’ cost, it could take years for new environmental and engineering studies. And what direction California’s next governor will pursue remains unclear.

Brown personally telephoned water district board members Monday evening urging a yes vote on the two-tunnels plan. After the vote Tuesday, he and his staff worked to put the best face on it, saying that the district had endorsed a Delta project, albeit not the one Brown has been proposing, which the governor has dubbed ”WaterFix.”

“The board’s vote today is a major step forward for California WaterFix and ensures that Santa Clara will have the water it desperately needs,” Brown said in a statement.

John Varela, chairman of the water district board, said that Brown told him Monday he is open to discussing changing “the scope” of the project. Varela said he also spoke with Natural Resources Secretary John Laird, who Varela said told him he too is willing to have further discussions.

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“He said, ‘John, we’re open to the idea of a single tunnel vs. the twin tunnels,”’ Varela said.

The water district, based in San Jose, considered contributing at least $620 million to Brown’s original version of project — more than $1 billion when financing costs are included. But in the end the board decided the cost was too high and the benefits to Silicon Valley too undefined.

Under a plan first proposed by Gov. Arnold Schwarzenegger, Brown proposed building two concrete tunnels, each 40 feet high and 35 miles long. They would be constructed 150 feet under the Delta, the vast network of sloughs and wetlands between San Francisco Bay and Sacramento that is a linchpin of water supplies for two-thirds of California’s residents and millions of acres of farmland.

Supporters, including members of several construction unions who spoke at Tuesday’s meeting, say the project will improve drinking water reliability for cities from San Jose to San Diego by taking freshwater from the Sacramento River south of Sacramento near the community of Courtland and delivering it to giant state and federal pumps near Tracy. That, they argue, would better armor the state’s water system against earthquakes and would also reduce reliance on those pumps, which judges have ordered to be slowed or stopped at times when endangered fish such as salmon and smelt are near them.

“We must go forward. This is not a time to delay,” said Grant Davis, director of the State Department of Water Resources, who attended the meeting. “We can no longer say that the status quo is acceptable. It’s not.”

But critics, who made up the majority of speakers at Tuesday’s three-hour meeting, call the project an expensive water grab by Southern California cities and San Joaquin Valley farmers that could saddle Santa Clara County residents with higher water bills and property tax hikes.

“Rather than build more large infrastructure, there are better ways to address California’s water needs,” said Roberta Hollimon with the League of Women Voters of Santa Clara County, which opposes the twin tunnels project.

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Hollimon said communities around the state should instead boost conservation, recycled water projects, storm-water capture and other sources.

John Sanders, a San Martin resident, told the board Tuesday that construction of the new Bay Bridge was originally supposed to cost $1 billion but ended up costing $6 billion. He also noted that when voters approved high-speed rail in 2008, the cost was projected at $33 billion and is now estimated at $64 billion.

“What happens to us, your customers, if the cost doubles or triples?” he said.

Supporters of the two-tunnel plan said the state needs the jobs and more resiliency in its water system. Even though state officials have said the twin tunnels would provide no more water than farms and cities receive now, without them they could receive less, project backers have argued.

“Great projects have always come with great controversy,” said Bob Jennings, Northern region director for California State Building Construction Trades Council, citing the Hoover Dam in the 1930s.

Brown administration officials have said two tunnels are needed for redundancy, in case something happens to one. The one-tunnel idea, in concept, has won support as far back as 2013 from San Diego, Contra Costa Water District, East Bay Municipal Utility District and some environmental groups, including the Natural Resources Defense Council.

Last week the two-tunnel project was endorsed by the Metropolitan Water District of Southern California, which provides water to 19 million people. It offered $4 billion toward the costs. Kern County Water Agency announced late last week that it would partially participate, committing about $1 billion.

Earlier this week, U.S. Sen. Dianne Feinstein questioned the need for two tunnels.

“Two big, 40-foot-wide tunnels? Running 150 feet underground for 35 miles?” the California Democrat said in an interview in the Los Angeles Times. “When I look at that and see what it would take to get down to them if something happens — there has to be all these shafts — it’s awfully hard for me to see this is the way to go.”

Below is the Santa Clara Valley Water District Tunnels resolution: