Spurned at the ballot box three years ago and facing an even more uphill battle now because of California's historic drought, an environmental group has filed a lawsuit attempting to drain Hetch Hetchy Reservoir, a linchpin of the water supply for 2.6 million Bay Area residents from San Francisco to San Jose to southern Alameda County.

The reservoir in Yosemite National Park, built in 1923, violates California's constitution, according to the suit from the nonprofit Restore Hetch Hetchy, based in Oakland.

Water that flows from the Sierra Nevada, through the Tuolumne River and into the reservoir can be stored in other reservoirs in the Sierra and downstream, the group argues. As a result, it contends, San Francisco, which owns the system, is not following a key provision of the state constitution requiring that water in California "be put to beneficial use to the fullest extent possible" and be used in "reasonable" ways.

"Hetch Hetchy Valley in Yosemite National Park was once one of our nation's most treasured landscapes. Its destruction, allowed a hundred years ago, is widely regarded as a mistake," said Spreck Rosekrans, executive director of Restore Hetch Hetchy. "The time has come to have a conversation about the merits of restoration in a court of law."