SAN JOSE, Calif. — Silicon Valley’s largest water agency will vote Tuesday on whether to support Gov. Gavin Newsom’s plan to spend $20 billion to build a massive, 45-mile long tunnel under the Sacramento-San Joaquin Delta to make it easier to move water from Northern California to Southern California.

The board of the Santa Clara Valley Water District, a government agency based in San Jose, will consider contributing $9.7 million toward planning and geotechnical studies for the project, which is one of California’s most long-running and controversial water proposals.

Newsom’s idea is to build a 36-foot high concrete tunnel to take water from the Sacramento River about 15 miles south of Sacramento, near the town of Courtland, and move it roughly 150 feet deep, for 45 miles under the marshes and sloughs of the Delta to the massive State Water Project pumps near Tracy, reducing reliance on them.

In recent years, courts have ruled that the pumps must be turned down, or shut off temporarily, at certain times of the year, when salmon, Delta smelt and other endangered fish swim near them, limiting when farms and cities like San Jose, Los Angeles and San Diego can receive water. A tunnel bored below Delta mud would allow the state to more easily move water south during very wet winters in “big gulps,” supporters argue, which is particularly important as climate change makes wet storms wetter and drought years more severe.

“With our changing hydrology, the stakes are higher for us to need to move water when we get these big events,” said Karla Nemeth, director of the California Department of Water Resources, which would oversee construction.

Nemeth noted that in the spring of 2023 when California was being drenched with atmospheric river storms, 156,000 cubic feet of water per second — an enormous amount similar to the flow of the Columbia River — was flowing through the Delta and under the Golden Gate Bridge out to the sea. When the state and federal pumps near Tracy are turned up to maximum capacity, they can move about 15,000 cfs. The tunnel would help California move more water south more reliably, into San Luis Reservoir, groundwater storage and other locations, she said.

Had the tunnel been in place last spring during atmospheric river storms, it would have moved 941,000 acre feet of water — enough for 9.8 million people, she added, which otherwise flowed out to sea.

Opponents of the project, who include environmental groups and Delta counties such as Contra Costa, call the project a Southern California water grab that will take too much freshwater, degrading water quality in the Delta and San Francisco Bay and harming populations of salmon and other fish and wildlife.

“The Delta Tunnel would burden our infrastructure and communities with over a decade of unbearable construction, and ultimately increase water salinity and harmful algal blooms, in addition to causing the Sacramento River to flow backwards at times,” wrote Contra Costa County Supervisor Ken Carlson, in a letter to the Santa Clara Valley Water District last week that was also signed by supervisors from Solano, Yolo, San Joaquin and Sacramento counties.

The project has been around for generations. In 1982, former Gov. Jerry Brown called it “the peripheral canal.” That year voters rejected it in a statewide election that became a Northern California vs Southern California water battle. Gray Davis and Arnold Schwarzenegger both tried to bring it back, without luck, and Brown, renaming it the “California Water Fix” pushed a two-tunnel version a decade ago, only to leave office in 2019 with it stalled after the massive Westlands Water District in Fresno pulled out of the partnership.

Newsom took office in 2019 and downsized the plan to one tunnel. He emphasized how it was needed amid climate change, and against earthquakes that could wreck Delta levees.

In December the state certified its final environmental impact statement. Also that month, the huge Metropolitan Water District of Southern California, which provides water to 20 million people, voted to contribute $141 million for its remaining studies.

There are 18 agencies that have agreed to be partners in the project and help pay its costs. The goal is to break ground in 2027, Nemeth said, with construction finished in 2042.

The big question for those local agencies — whether to fund the $20 billion construction cost and who will be on the hook for overruns — will come in 2026 or 2027. The Santa Clara Valley Water District’s share would be about $650 million.

Two other Bay Area water agencies, the Alameda County Water District in Fremont, and Zone 7 Water Agency in Livermore, have also recently endorsed the project and agreed to pay for some of its studies. East Bay MUD and Contra Costa Water District are not partners on the project.

Newsom has called the project one of his top priorities during the final two years of his term.

In 2019, the Santa Clara Valley Water District Board voted 4-3 to endorse the plan and contribute $11 million toward its planning and environmental studies.

“The Delta has many risks,” said Cindy Kao, the district’s imported water manager. “Sea level rise, climate change, seismic issues and environmental issues. This will make our water supplies more resilient and more reliable.”

Critics say water agencies across the state should do more to develop local supplies, like stormwater capture, recycled water and conservation instead of relying on water shipped from hundreds of miles away.

“It’s a really risky and unsure project,” said Katja Irving, conservation chair for the Guadalupe Regional Group of the Sierra Club. “We’re going to be spending a lot of money on it before we know whether it’s going to be feasible. And we could be spending that money instead repairing local dams or lowering water rates.”

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