High on the edge of Yosemite, the O’Shaughnessy Dam has collected San Francisco’s drinking water since 1923 as part of the Hetch Hetchy water system.
The system includes 176 miles of pipes and rivers supply water to 2.6 million people in San Francisco and the Bay Area.
On Tuesday, Ellen Levin of San Francisco’s Public Utilities Commission stared across the dam’s placid waters, taking in a rare piece of history that hadn’t been visible for decades.
Across the water, a stone bridge and carved benches sat against the water’s edge beneath the shadow of the dam - forgotten artifacts from before the height of the dam was raised decades ago.
"Normally you wouldn’t see that," remarked Levin. "It would be underwater at this time."
The sight of the old bridge struck Levin as bittersweet.
While it revealed handy-work of the dam’s original engineers, it also reflected how low the waterline had dropped as a result of California’s drought.
"I personally have never seen them," Levin said. "I’ve been with the city 14 years and I haven’t had a year like this."
The O’Shaughnessy Dam has the capacity to hold 117 billion gallons of water. But as of April, the dam sat at about 53 percent of normal, producing a revealing dirt ring around the granite edge.
"I think what’s even worse is our snow pack is only at 35 percent," Levin said. "Usually in a normal year we would be able to fill Hetch Hetchy three times over with the amount of snow melt."
San Francisco utilities officials have asked customers to reduce water usage by 10 percent. So far the utility is meeting its water demand, but another year of dry conditions would raise concern.
But Hetch Hetchy isn’t just a water delivery system; it also generates electricity.
Three hydro-electric power stations outside of Yosemite utilize rushing waters to generate electricity used for everything from powering San Francisco streetcars to San Francisco General Hospital.
Because the utility’s contracts require it to prioritize the delivery of drinking water, the power stations are having to do with less water.
"When we’re in drought, less water is transported from up-country to San Francisco," said Barbara Hale, the PUC’s assistant General Manager in Charge of Power. "We generate less electricity."
San Francisco sells its surplus electricity onto the open energy market, helping to defray the costs of some of PUC’s work. A prolonged drought could sap even more revenue from the agency’s budget.
"Worst case scenarios have us looking at raising rates. Worst case scenarios would have us looking at staff,” Hale said. “Are we doing those things now? No."
But Hale said the same dry condition’s impacting San Francisco hydro-electric plants are being felt at similar across the state. She said a cumulative effect of the drought on hydro-electric production could drive up rates statewide.
Levin said now that the window for a Miracle March of rain has passed, there was still hope for an Amazing April or even a Miracle May. Beyond that, there's a good chance more of the O’Shaugnessy Dam’s history might come to light.