Two years after losing in court and six years after being rejected by voters, a Berkeley environmental group is continuing its long-running battle to drain Hetch Hetchy Reservoir, a linchpin of the water supply for 2.6 million Bay Area residents from San Francisco to San Jose to southern Alameda County.

The reservoir in Yosemite National Park, built in 1923, violates California’s constitution, according to a lawsuit from the nonprofit group, Restore Hetch Hetchy, because the constitution requires water to be diverted in a “reasonable” way, and there are other places to store Hetch Hetchy’s water that aren’t in a national park. Related Articles Readers respond to Hetch Hetchy lawsuit story

The group will pursue its appeal Wednesday morning in the Fifth District Court of Appeal in Fresno.

“Hetch Hetchy Valley was once one of the most spectacular, iconic landscapes in California, if not the world, and once the reservoir is emptied, it will come back,” said Spreck Rosekrans, executive director of Restore Hetch Hetchy. “Visitors from throughout California, across America and around the world will come back to see the valley regain its natural splendor.”

But San Francisco officials, who own the Hetch Hetchy system, have fought the idea ferociously.

“San Francisco voters overwhelmingly rejected the idea of draining Hetch Hetchy Reservoir in 2012, because it was a terrible idea. It’s still a terrible idea today,” said San Francisco City Attorney Dennis Herrera. “As California struggles to deal with recurring droughts and climate change, the last thing we should be doing is draining the main water supply for more than 2.6 million San Francisco Bay Area residents, which also generates clean, greenhouse-gas-free hydroelectric power.”

Hetch Hetchy water serves residents in four Bay Area counties, San Francisco, San Mateo, Santa Clara and Southern Alameda. Residents drink it in 26 cities and water districts — from San Francisco to Palo Alto to north San Jose to Hayward, Fremont and Union City.

Captured in old photographs and paintings, Hetch Hetchy Valley was a scenic landscape that rivaled nearby Yosemite Valley before Congress approved construction of O’Shaughnessy Dam in 1913, submerging the valley under 300 feet of water.

At the time, San Francisco leaders argued they needed a more reliable water source following the fires that burned large sections of the city after the 1906 earthquake. Conservation groups battled the plan fiercely, noting that no dam had ever been built inside a national park. The fight to save the valley was the final battle of Sierra Club founder John Muir’s life. And the valley’s submersion has haunted many environmentalists in the century ever since.

Restore Hetch Hetchy argues that if the lake, which holds 360,000 acre feet of water — twice the capacity of Los Vaqueros Reservoir in Contra Costa County and four times the capacity of Anderson Reservoir in Santa Clara County — were drained, water could still be taken from the Tuolumne River and stored in other reservoirs instead. Among them: Don Pedro nearby, and several of the Hetch Hetchy system’s reservoirs in the Bay Area.

“Hetch Hetchy is not a source of water. It is a storage tank,” said Rosekrans. “And that storage tank can be moved downstream.”

The movement has been endorsed by actor Harrison Ford, Patagonia founder Yvon Chouinard and four former Yosemite National Park superintendents: Bob Binnewies, B.J. Griffin, Dave Mihalic and Mike Tollefson.

But it faces a steep uphill climb. California’s senior U.S. senator, Dianne Feinstein, has adamantly opposed the idea. In 1987, when Reagan administration Interior Secretary Donald Hodel raised it, she called it “the worst idea since selling arms to the Ayatollah.”

In 2012, Restore Hetch Hetchy placed a measure on the San Francisco ballot that would have required the city to conduct an $8 million study on the impacts of draining the reservoir. It lost in a landslide, 77-23 percent.

In 2015, the group filed a lawsuit arguing that the reservoir violated California’s constitution. It was thrown out in 2016 by a Tuolumne County judge, who ruled that the provision requiring “reasonable” diversion of water was written in 1928 — five years after the reservoir was built — and that federal law pre-empts the state law.

Rosekrans noted that part of the 1913 law, known as the Raker Act, says that nothing in the law shall interfere with California’s ability to pass its own water laws. On that issue, his group will base its appeal Wednesday.

Regardless of the outcome, both sides are expected to appeal to the state Supreme Court. So the battle rages on.

A 2006 study by UC Davis found that the reservoir could be drained and its water stored in other reservoirs without causing water shortages in most years. But the project would cost billions in lost hydropower, charges for San Francisco to buy some water on the open market to make up shortfalls in dry years, and the need for San Francisco to begin filtering water stored in other reservoirs. The city is required under federal law to treat the water with chlorine, but not to filter it, because of its high quality, coming from Sierra snow melt.

“The Hetch Hetchy controversy is a nice parable for how California’s water system and problems keep changing,” said Jay Lund, director for the UC Davis Center for Watershed Sciences, “as new technologies and social objectives contend with infrastructure, which was once brilliant when it was built, but would be built differently, or not at all, today.

Start your day with the news you need from the Bay Area and beyond.

Sign up for our new Morning Report weekday newsletter.

“When, sometime in the perhaps distant future, San Francisco is forced to build a water filtration plant, the economics of Hetch Hethcy removal will look much more attractive than they do today.”