WASHINGTON — Congress and the Trump administration are pushing ahead with a plan to raise a towering symbol of dam-building’s 20th century heyday to meet the water demands of 21st century California — a project backed by San Joaquin Valley growers but opposed by state officials, defenders of a protected river and an American Indian tribe whose sacred sites would be swamped.

The fight is over Shasta Dam, at 602 feet the fourth-tallest dam in California and the cornerstone of the federal Central Valley Project, which provides water to cities and farms throughout the state. One of its biggest customers is the Westlands Water District in the arid western San Joaquin Valley, which distributes water to numerous large farms.

With enthusiastic support from Westlands, the Trump administration and Republicans in Congress want to raise the dam 18½ feet to store more water and guard against losing farmland to future droughts. Some farmers in the valley received no water at all from the Central Valley Project for two straight years during the five-year drought that ended with the winter of 2016-17.

Proponents also argue that raising Shasta would aid salmon runs decimated by its original construction in the 1940s, by storing more cold water to help the remaining downstream fish survive.

Last month, Congress gave the $1.3 billion project a $20 million cash infusion for design and other preliminary work, and the Interior Department declared that construction would start next year.

The project has been on the boards for years, but President Barack Obama’s administration shelved it because it would flood part of the McCloud River. California law protects the river as wild and scenic because it sustains “one of the finest wild trout fisheries in the state.” Congress would have to declare in separate legislation that federal interest in raising the dam supersedes the state’s authority.

The Trump administration is “pretty clearly setting up an attempt to override state law to build this project,” said Doug Obegi, a water lawyer with the Natural Resources Defense Council, an environmental group. “It hits the holy trinity of destroying Native American sacred sites, violating state law and harming fish and wildlife.”

The resurrection of the Shasta project was made possible by a 2016 law sponsored by House Majority Leader Kevin McCarthy, R-Bakersfield, and Sen. Dianne Feinstein, D-Calif. It instructed the interior secretary to take the lead on recommending water storage projects and moving ahead on dam building throughout the West.

Feinstein and McCarthy’s bill was added as a rider to broad water legislation over the opposition of former Sen. Barbara Boxer, a California Democrat who spent her last moments in office trying to block it.

Acting under this new authority, Interior Secretary Ryan Zinke put Shasta at the top of his list. McCarthy then inserted the $20 million that Zinke requested for Shasta in a catch-all spending bill that Congress passed last month.

John Laird, California’s secretary of natural resources, asked that lawmakers not pursue the project, “which disregards California law, and instead work with the state” on other water measures the state views as more worthy.

Dam proponents argue that the McCloud River would suffer no major harm. They say only two-thirds of a mile of the river would be inundated, and then only in wet years.

The added storage would provide water not just to farms in the San Joaquin Valley, but also to Bay Area cities that rely on Shasta water, they argue.

“Enlarging Shasta Dam will provide water supply, water quality and fishery benefits,” said Tom Birmingham, general manager of the Westlands Water District.

Westlands supports raising the dam “for the simple reason that it is the most cost-effective surface water storage project currently being evaluated in the state,” Birmingham said.

Raising Shasta Dam is indeed among the cheapest of the four big dam projects that the state and federal governments have examined for California. All are so expensive that officials think two at most could be built.

The California Water Commission blocked Shasta from receiving any of the $2.7 billion in funding under Proposition 1, a ballot measure voters approved in 2014 to increase the state’s water storage. The commission is considering three other big dam projects among 11 water storage proposals. All would be in competition with Shasta for federal dollars.

Shasta provides 40 percent of the Central Valley Project’s reservoir capacity. Raising it would enlarge its maximum level by 634,000 acre-feet, or about 13 percent.

But that figure exaggerates how much water raising the dam would deliver. Reservoirs don’t always fill, and most of the available water is already captured by the existing dam. The federal Bureau of Reclamation, which operates the dam, estimates that raising Shasta Dam would increase water deliveries by 51,300 acre-feet a year on average, and less during droughts.

“So it’s not a very good deal, which is why these projects have not gone anywhere,” said Ron Stork, senior policy advocate for the environmental group Friends of the River, which opposes the project.

Raising the dam also would inundate most of what remains of the sacred sites of the Winnemem Wintu tribe, whose lands were flooded when the original dam was built, said tribal Chief Caleen Sisk.

The tribe, which once numbered an estimated 14,000 people, is down to 126 members. Sisk said many of them live in Redding or Sacramento because their ancestral land was flooded and its fish runs blocked by the dam.

The sacred sites include dance grounds, healing rocks and pools in the river. “These all have significant spiritual reverence to the Winnemem people,” Sisk said.

Raising the dam “is going to flood out what we have left,” said Gary Mulcahey, a tribe member. “People are waiting with a finger on the trigger to file a lawsuit as soon as any decision is made.”

California Democrats said the $20 million being spent on construction planing is a waste of taxpayer money because the project will never get state permits to begin pouring concrete. But Congress has the right to preempt state law, and ultimately it could be up to the courts to decide whose authority prevails on the Shasta project.

“There are people who are opposed to any project that will help sustain irrigated agriculture, particularly on the west side of the San Joaquin Valley,” Westlands’ Birmingham said. “Environmental groups can and will file lawsuits for many often spurious reasons. Whether they win those is another question.”

Westlands bought the 3,000-acre Bollibokka Fishing Club along the McCloud River in 2007 in anticipation of raising the dam, paying $35 million for the property. It contains many Winnemem Wintu sites and would be inundated by the dam raising.

In a congressional hearing last month, Zinke assured Rep. Jared Huffman, D-San Rafael, who opposes the project, that none of the $20 million Congress approved will be spent buying the property from Westlands.

Westlands isn’t the only district that would welcome the Shasta expansion. The San Luis and Delta Mendota Water Authority, which supplies water to Santa Clara County, told federal officials that it wanted to share the cost of raising Shasta dam. Doing so would provide a critical state partner for the project.

Seven environmental groups shot off a warning letter to the agency, saying water districts are agencies of the state and are banned from participating in a project that “violates California law.”

“It would have been nicer to see a letter coming that’s more, ‘Let’s have a dialogue and sit down and figure out are there paths forward,’ instead of thinly veiled threats to sue people,” said Cannon Michael, chairman of the San Luis and Delta Mendota agency.

Michael said the dam has to be raised not just to help farms, but also fish.

Although dams are the chief culprit behind the calamitous decline of the state’s native fish species, three-quarters of which are threatened, the Bureau of Reclamation argues that dams can help fish by mimicking nature’s springtime influx of cold water into rivers and streams. Providing cold water to salmon has become one of Shasta reservoir’s key functions, and the bureau lists helping fish as one of the main benefits of raising the dam.

The U.S. Fish and Wildlife Service disagreed, saying in documents obtained by environmental groups that the benefit to fish was “not substantial” and that further restricting the Sacramento, McCloud and Pit rivers that flow into the reservoir would inflict more damage.

Michael said climate change is making it harder for both fish and farms to survive, and that raising Shasta Dam would help both.

“We know climate change is going to make it almost impossible for (freshwater) fish to survive in the Sacramento River as the temperatures continue to warm,” he said. Dam operators are “taking the lion’s share of Shasta for cold water, and it still doesn’t appear it’s going to be enough if we continue with climate change.”

There is one dam-raising project that has drawn enthusiastic backing from environmental groups: expansion of the Los Vaqueros reservoir near Livermore. It promises to be a source of water for San Joaquin Valley wildlife refuges that often go dry in drought years.

“The wildlife refuges in the San Joaquin Valley never receive all the water they need to support Pacific Flyway birds and other wetlands creatures,” said Rachel Zwillinger, water policy adviser for the environmental group Defenders of Wildlife.

Opponents of raising Shasta Dam fear it will divert money from such projects.

“This project was dead,” said Stork of Friends of the River. “Some people were thankful for that because their project then has a chance for more money.

“Then the election happened.”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com Twitter: @carolynlochhead