With all precincts reporting, unofficial election results showed that 77 percent of San Francisco voters voted no on Proposition F, a measure put on the ballot by a group seeking to restore the valley to its natural state.
The measure would have forced the city to formulate a plan to dismantle the dam, constructed in 1923, and improve local water conservation efforts. The plan would have gone before San Francisco voters as a charter amendment in November 2016.
Nearly every local elected official was against the proposal, including Mayor Ed Lee, who said tonight that Proposition F was "something that was really stupid."
The Bay Area Council, a coalition of local business leaders, had also opposed the measure, and its CEO Jim Wunderman issued a statement lauding its failure.
"This was a vote for common sense," Wunderman said. "It makes no sense to spend $10 billion we don't have to tear down one of the Bay Area's most important sources of clean drinking water and clean power."
Mike Marshall, director of the campaign in support of Proposition F, acknowledged that supporters faced "an uphill climb" against all of the opposition but said he was happy the proposal created a debate about the wisdom of a man-made reservoir in a national park.
"Even though we didn't win, I view today as a big success," Marshall said.