Early 20th century visitors to Hetch Hetchy Valley, a few miles north of Yosemite Valley, saw a rich meadowland and green oak groves, with the clear Tuolumne River winding through them, embraced by towering granite walls. It’s a landscape no one has seen since 1923, when the valley was drowned by Hetch Hetchy Reservoir, the main water supply for San Francisco, 180 miles west.

The reservoir’s existence has long cast a shadow on the city’s otherwise environmentally progressive reputation. “San Francisco, quite frankly, has a lot of guilt,” says Robert Righter, author of the 2005 book, The Battle Over Hetch Hetchy. “You can’t find anyone today that says that dam should have been built. There were alternatives.”

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Despite the obstacles, Restore Hetch Hetchy remains undaunted — and legally speaking, it might have grounds for appeal. The California Supreme Court has established that “reasonable” use of water is an evolving standard, says Barton Thompson, professor of natural resource law at Stanford University, and “a decision made in 1913 certainly could be considered unreasonable now.”

But even if Restore Hetch Hetchy wins the appeal, the group faces “an uphill battle,” Thompson says. “Do (the courts) really want to make San Francisco remove the dam? Would that be reasonable? It would be great to see the restoration of Hetch Hetchy Valley, but it would come at a very high cost.” The group plans on delivering its appeal in early June.