YOSEMITE NATIONAL PARK, Calif. >> It is one of the oldest environmental battles in the U.S., and it involved one of the country’s most famous national parks, one of its most liberal cities, leaders of Silicon Valley and a perennial source of conflict in California: water.

In 1913, Congress approved the construction of a dam and an eight-mile-long reservoir, called Hetch Hetchy, in the northwest corner of Yosemite National Park to supply cheap water to San Francisco.

But the Hetch Hetchy reservoir, which submerged a valley that many have likened to Yosemite Valley in its grandeur and is credited with giving birth to the modern environmental movement, has lost none of its power to arouse strong emotions. In November, San Francisco will vote on a measure that could ultimately lead to the draining and restoration of Hetch Hetchy Valley — and force the city to look elsewhere for most of its water.

San Francisco’s mayor, Edwin M. Lee, has dismissed the measure as “stupid” and “insane.” But its supporters say that San Francisco can find other sources of water and that the valley’s restoration could have a positive impact not only in California but also across the nation and the world.

“It will say that decisions that cities made, based on what they knew a hundred years ago, can be revisited for the benefit of the environment,” said Mike Marshall, executive director of Restore Hetch Hetchy, an environmental organization. “It will inspire restoration efforts across the country that are less visible.”

Restore Hetch Hetchy collected about 16,000 signatures from registered voters in San Francisco — far more than the 9,702 needed — to get the measure on the ballot on Nov. 6. If it is approved, the city would be required to spend $8 million to draw up plans for a new water system; that blueprint would be submitted to voters in 2016.

Opponents in San Francisco and the rest of the Bay Area, which also relies on the reservoir for its water, say that draining Hetch Hetchy will jeopardize the water supply for 2.6 million residents. Removing it, they say, will increase water rates and make the Bay Area’s water supply vulnerable to droughts. Unproven alternatives, they say, will require costly and less environmentally friendly filtering and pumping of water.

“It basically sends a signal to businesses that a basic necessity — water — is at risk, and it calls into question whether or not businesses would want to stay and invest here,” said Mike Mielke, the vice president of environmental policy at the Silicon Valley Leadership Group, an umbrella organization of 375 technology companies.

Other critics say that San Francisco voters alone will get to decide the fate of the reservoir, which is owned by San Francisco but also supplies water to three other counties in the San Francisco Regional Water System. Arthur Jensen, the chief executive of the Bay Area Water Supply and Conservation Agency, which represents municipalities outside San Francisco, said his members have no say in the Hetch Hetchy debate even though they pay two-thirds of the cost of operating and maintaining the system.

“We have a lot of skin in the game, but we don’t vote in the San Francisco election in November,” Jensen said.

One of nine reservoirs that store water destined for the Bay Area, Hetch Hetchy catches snowmelt and rain runoff from the High Sierra. The Hetch Hetchy system’s supporters say that it has one of the smallest carbon footprints of any water system in the United States because its water is of such high quality that it requires no filtration and is delivered by gravity through 160 miles of pipelines and tunnels. The system currently provides about 85 percent of San Francisco’s water supply.

In 1913, despite a national outcry, Congress gave San Francisco — whose water system was partially destroyed in the 1906 earthquake — the authority to create a water reservoir by damming the Tuolumne River here. Completed in 1923, the O’Shaughnessy Dam submerged Hetch Hetchy Valley, which was often compared to Yosemite Valley, about 17 miles south of it.

The naturalist John Muir, who fought the project, called Hetch Hetchy Valley, with its sheer granite walls and waterfalls, “one of nature’s rarest and most precious mountain temples.”

Over the decades, environmentalists have pressed for its restoration. The state of California has estimated that restoration, which would entail dismantling the dam and draining the reservoir, would cost from $3 billion to $10 billion. Restore Hetch Hetchy puts the cost at $1 billion.

Officials in San Francisco, a city otherwise known for spearheading environmental causes, have been put on the defensive. High-profile Democrats, including Nancy Pelosi, the House minority leader, and Sen. Dianne Feinstein, both of whom are from San Francisco, oppose the measure.

Republicans have led calls to remove the Hetch Hetchy reservoir. Rep. Dan Lungren, a Republican whose district borders Yosemite, asked the Interior Department to investigate whether San Francisco had been turning to other sources, including recycled water, groundwater and rainwater, before using the water from Hetch Hetchy, as required by a 1913 law. Brian Kaveney, a spokesman for Lungren, said the congressman had not yet received a reply from the department.

Michael Carlin, the deputy general manager of the San Francisco Public Utilities Commission, the agency that owns the Hetch Hetchy reservoir, said that recycling and groundwater projects were currently under way.

Late Friday afternoon, half a dozen cars belonging to tourists were parked near the reservoir.

Jan Larson, who retired recently from her job as an elementary school teacher, and her husband, Craig, had decided to visit Yosemite for the first time. The Larsons, who live in Colorado, were sitting in folding chairs, nibbling on cheese and crackers, as they gazed out at O’Shaughnessy Dam.

Their visit, they said, had filled them with awe, but they also said they opposed the ballot measure, which they had heard about before coming here.

“If the dam weren’t already here, I would probably say, ‘Don’t build it,”’ Larson said. “But it’s here, it works, it’s going to cost millions to replace it, and it’s going to take years to restore it to what it was like before. There are so many other wonderful areas in Yosemite. So to me, it’s like, leave it alone.”

Michelle and Harry Von Schmidt, a couple from New Jersey who were walking atop the dam, were split. Michelle Von Schmidt said she was “grateful for what had been preserved,” while her husband said he was “more inclined toward” those advocating restoration.

“I’ve seen before and after pictures, and it’s so pretty here in a way that’s hard to imagine,” Harry Von Schmidt said. “I feel for John Muir, who tried for years to save it.”

© 2012 The New York Times Company