A California appeals court has rejected a long-running attempt by environmentalists to drain Hetch Hetchy Reservoir in Yosemite National Park, a linchpin of the water supply for 2.6 million Bay Area residents.

In a 3-0 ruling filed Monday afternoon, the Fifth District Court of Appeal in Fresno upheld a lower court decision by a Tuolumne County judge two years ago.

That judge, Kevin Seibert, had rejected arguments from Restore Hetch Hetchy, an environmental group based in Berkeley, that the reservoir, built in 1923, violates California’s constitution. The group said that the reservoir and its 312-foot-high dam violate a provision of the state constitution that requires water to be diverted in a “reasonable” way, because there are other places to store Hetch Hetchy’s water that aren’t in a national park.

The appeals court on Monday agreed with Seibert’s ruling, however, that federal law, specifically the 1913 Raker Act, which allowed the construction of the dam, pre-empts that provision of the state constitution, which was written in 1928.

The ruling is the latest in a series of defeats for the environmental group. 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.

The group said Tuesday that it plans to appeal to the state Supreme Court.

“We are disappointed but undaunted,” said Spreck Rosecrans, executive director of Restore Hetch Hetchy. “No other national park has been so damaged by development. Restore Hetch Hetchy remains committed to restoring Yosemite’s Hetch Hetchy Valley while ensuring that not one drop of water supply is lost.”

San Francisco city officials, who own the reservoir, dam and water system, praised the ruling.

“The Hetch Hetchy Reservoir has played a crucial role in supplying water to San Francisco and the surrounding area for more than 100 years,” said San Francisco City Attorney Dennis Herrera. “Draining it is a terrible idea that would jeopardize the water needs of Bay Area families.”

Hetch Hetchy water serves 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.

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.”

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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, and San Francisco would have to buy some water on the open market to make up shortfalls in dry years. Also, water stored in other reservoirs would require filtering. The city is required under federal law to treat the Hetch Hetchy water with chlorine, but not to filter it, because of its high quality, coming from Sierra snow melt.