LOS ANGELES — On Tuesday, voters in San Francisco, one of the nation’s most progressive and environmentally aware cities, will be asked to decide just how green they want to be.

For nearly 80 years, the city has been getting pristine Sierra Nevada water piped from behind a dam it erected in a majestic glacial valley in Yosemite National Park.

The 1913 passage of the Raker Act, which allowed the city to turn Hetch Hetchy Valley into a 300-foot-deep reservoir, was one of the biggest defeats in America’s youthful conservation movement. It has remained a wound that never completely healed, periodically prompting calls to demolish O’Shaughnessy Dam and restore a valley that John Muir called “one of nature’s rarest and most precious mountain temples.”

A measure on San Francisco’s ballot asks voters if the city should develop an $8 million blueprint to drain the valley and devise ways to make up for the resulting loss of hydropower and water storage. If city voters say yes, they would decide in 2016 whether to actually carry out the plan and empty Hetch Hetchy Reservoir.

Proponents have an uphill battle. Virtually the entire San Francisco political and business establishment is adamantly against the proposal. Former Mayor Dianne Feinstein, California’s senior U.S. senator, says tearing down O’Shaughnessy “makes no sense.” Mayor Edwin Lee has called the idea “stupid” and “insane.” The Board of Supervisors opposes it and the Bay Area Council, the region’s leading business group, is heading the campaign to defeat the proposal.

“It’s the height of folly,” said Jim Wunderman, the Bay Area Council’s president. “Some things are better off put aside and this is one of those things.”

Environmental pride

San Francisco prides itself on its environmental record. It was the first city in the nation to ban plastic grocery bags and boasts that it diverts a greater portion of waste from landfills than any other city in North America. It has hybrid taxicabs and is expanding its network of public charging stations for electric vehicles.

That its water source clashes with that green image has not been lost on those who would like to see Hetch Hetchy’s glories resurrected from a watery tomb.

“Here is this city by the bay that seems to give thought to every cause imaginable except where their water comes from — much less that it’s part of killing a valley in one of the world’s great national parks,” said David Mihalic, who was Yosemite’s superintendent from 1999 to 2003 and is retired from the National Park Service.

Bounded by sheer granite walls and streaked with cascading waterfalls, Hetch Hetchy has often been compared to its famous sibling to the south, Yosemite Valley. San Francisco’s desire to dam the upper Tuolumne River and flood the winding, seven-mile length of Hetch Hetchy ignited a national fight a century ago that played out in newspaper editorials across the country and ultimately in the halls of Congress.

Muir and conservationists bitterly fought the dam, equating the flooding of Hetch Hetchy with using a cathedral for a water tank. Proponents argued that the valley was little visited and that San Francisco — then the state’s premier city — couldn’t get sufficient supplies elsewhere. What’s more, they insisted, a big lake would be prettier.

With a dam, the valley “will be as accessible as it ever was, and far more attractive, for in one place a beautiful lake will be substituted for an arid plain,” declared the Los Angeles Times, which that same year celebrated the arrival of Owens Valley water in L.A.

O’Shaughnessy Dam was finished in 1923 but the network of pipes and tunnels that transported supplies 160 miles from Hetch Hetchy down the slopes of the Sierra Nevada, across the Central Valley and the coastal range didn’t make its first deliveries to San Francisco until 1934.

The mountain water is so clean that to this day, the city is one of the few urban areas in the country that doesn’t have to filter its water supply. The conveyance system is fed by gravity, generating clean hydropower that runs San Francisco’s cable cars, lights municipal buildings and city streets.

San Francisco uses about one-third of the Hetch Hetchy water and sells the rest wholesale to Bay Area communities, including Palo Alto, Mountain View and Stanford University. Altogether, the Hetch Hetchy system supplies about 2.6 million people.

“What is it about this that needs to be fixed?” Wunderman asked. “These are not things we want to give up … to make this very narrow group of environmental interests happy.”

A 2006 state report estimated that it would cost $3 billion to $10 billion to remove the dam, restore the valley and modify the water system to replace lost storage and hydropower. The authors said more study was needed, but outlined various options for changing the system and concluded there were “no fatal flaws in the restoration concept.”

Conflict and concern

Proponents say much of the restoration money would probably come from state and federal coffers. But San Francisco officials say draining Hetch Hetchy would increase local utility costs in a number of ways, driving up water rates. Replacement power for the lost hydroelectric generation would be expensive; water supplies would be less reliable and filtration would be required because water would no longer be diverted directly from Hetch Hetchy into the conveyance system.

Restore Hetch Hetchy, the group that is promoting the ballot measure, says the state cost estimates are inflated. It also argues that San Francisco owns and operates seven other reservoirs in the system that could be managed to make up for much of the lost storage space.

“We’re not saying there’s zero impact of what we’re looking at and zero makeup required,” said Spreck Rosekrans, the organization’s policy director, who co-authored a 2004 restoration study when he was working for the Environmental Defense Fund. “But it’s just so much less than people seem to think.”

By adjusting operations of some its other reservoirs, Rosekrans says, in most years San Francisco would lose no more than 5 percent of its water delivery capability. In dry years, it would lose a fifth. The city could make that up by stepping up the use of local supplies such as groundwater and recycled water, the restore group says. The city currently doesn’t use any recycled supplies, although it plans to do so.