For nearly 100 years, environmentalists have dreamed about draining Hetch Hetchy Reservoir and turning the 300-foot-deep man-made lake in Yosemite National Park back into a glorious Sierra landscape -- a second Yosemite Valley with green meadows, rich forests and waterfalls cascading down granite walls.
San Francisco voters head to the polls in five weeks to consider Measure F, which would require the city to conduct an $8 million study to determine whether it makes sense to empty the reservoir and replace the electricity it generates and the water it supplies to 2.5 million Bay Area residents. A vote on whether to drain Hetch Hetchy could come as soon as 2016.
The politics and economics are daunting: The project could cost billions of dollars, and many of the state's top political leaders oppose the idea. But as voters become aware of the measure, the questions are growing: Is it even possible to restore Hetch Hetchy to the way it was? Would any of us live to see it? Can you put the ecological toothpaste back in the tube?
Yes, say scientists who have studied the issue over the past 25 years.
"It would require a lot of dedicated work for many years. You'd have to stick with it, and it would cost a lot of money to maintain it and monitor it. But it's not an immense area. It's feasible," said Steve Botti, a botanist and former acting chief of Yosemite's resource management division.
The reservoir was created after crews finished building O'Shaughnessy Dam on the Tuolumne River in 1923. The project was fought bitterly by Sierra Club founder John Muir, who lost his battle when San Francisco leaders made the case to Congress that the city needed a more reliable water supply after it burned in the 1906 earthquake.
After the 312-foot concrete dam was completed and the 7-mile-long valley submerged, the landscape was essentially frozen in time. But during severe droughts since then, the water level sometimes fell so low that clues of what lies underneath have been exposed.
One such year was 1977. Botti, a Yosemite employee at the time, wandered in.
"The valley looked pretty much like it did in 1923," said Botti, now retired in Idaho. "I saw axes lying there where people had chopped the trees down. The river was still in its old banks. There was no vegetation. It wasn't pretty. But I thought, 'This is possible.' I could envision it the way it was."
Although no exhaustive research projects have been done, botanists, biologists, hydrologists and other scientists who have studied the valley say that if the reservoir is ever drained, an ugly landscape left from the dam's construction -- with thousands of huge tree stumps, two abandoned quarries, a railroad track and miles of gray silt -- would come into view.
But within five years, according to a 1988 National Park Service study written by Botti and other scientists, grasses and shrubs would create new meadows, and rainbow trout would come back. Deer, black bears, coyotes and other wildlife would begin wandering through the valley.
Within 10 to 20 years, thousands of small ponderosa pines, sugar pines, Douglas firs and other trees -- planted by large crews of restoration biologists -- would be 20 feet tall. Within 50 years, oak woodlands would emerge. Conifer trees would grow to 90 feet. The old stumps would be largely decayed or hidden by the new forest cover.
"Most people could go there in 40 or 50 years and not even realize that there was ever a dam there," said Sarah Null, an assistant professor of watershed sciences at Utah State University who has studied the area.
"There would be some clues for astute people, like no 300-year-old trees, or the bathtub ring, but it would be a nice place."
The "bathtub ring" is a huge discolored area along the granite walls, formed when the reservoir's waters killed the moss and lichens growing on the rocks. Nobody knows for sure how long it would take to go away. The 1988 National Park Service study, still considered the most exhaustive look at restoring Hetch Hetchy, estimated it would take up to 120 years for the lichens to fully grow back.
Perhaps the best news for people who hope to one day restore the valley is that the reservoir is not full of mud. Other dam removal projects have been hampered by billions of pounds of sand and silt that pile up behind many dams and must be removed if the structures come down. But because the Tuolumne River drains Yosemite's high country, which is mostly granite with thin soils, the silt levels are no more than a few inches deep.
Even so, enormous challenges would loom.
The group Restore Hetch Hetchy, the Sierra Club and other supporters of draining the reservoir say most of its 360,000 acre-feet of water storage can be replaced through more water conservation, recycling and storing water in other reservoirs, such as the massive Don Pedro Reservoir nearby. But critics, such as U.S. Sen. Dianne Feinstein and San Francisco Mayor Ed Lee, who calls the idea "insane," say water is a precious commodity in California and that no city should take the risk of giving up a reliable water supply. Silicon Valley business leaders also say that because two-thirds of Hetch Hetchy water customers live outside San Francisco, in places like Palo Alto and north San Jose, they should get to vote, too.
A 2006 study by the state Department of Water Resources estimated that restoring Hetch Hetchy would cost $3 billion to $10 billion, although some environmentalists say it could be done for as little as $1 billion.
A key question is what to do with the dam. Removing it would involve constant blasting and thousands of truck trips. A rail line might have to be built to carry away the debris.
One 2004 study by the University of Wisconsin said the reservoir should be drained in steps, over years, so scientists could experiment by restoring small patches of dry ground and then use the most successful techniques for bigger areas.
There would also be major biological hurdles. Without enough human intervention, the valley could become overgrown quickly with invasive plants such as star thistle, knapweed and cheat grass, which wouldn't provide adequate food or shelter for wildlife.
"It would have to be constantly maintained and attacked. The park is not a super big fan of using herbicides," said Tim Ramirez, natural resources division manager for the San Francisco Public Utilities Commission, which operates the reservoir. "You are talking about hand crews and a lot of valley. It gets pretty overwhelming."
Also, there would almost certainly be battles over American Indian archeological sites, public access fights and erosion problems because the original dam base was built 118 feet below the ground. Excavating it would change the river gradient and cause massive erosion.
Huge political and economic debates remain. But so far no biologists have come forward to say Hetch Hetchy Valley would not return to life after about 100 years.
"You get major cleansings of the landscape all the time. Fires and other kinds of things like 100-year droughts happen," said Peter Moyle, a professor of fish biology at UC Davis.
"There's not a lot of uncertainly in the science. You'd get a pretty good ecosystem in 50 years after the reservoir had been drained. In the end, it's a political question."
Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn.