A raging wildfire in Yosemite National Park has rained ash on the reservoir that is the chief source of San Francisco's famously pure drinking water.
Utility bosses scrambled to send more water towards the metropolitan area before it became tainted as nearly 3,700 firefighters battled the 230-square-mile blaze, the biggest wildfire on record in California's Sierra Nevada.
They reported modest progress, saying the fire was 15% contained.
Experts monitored the Hetch Hetchy Reservoir for clarity and used a massive new £3 billion gravity-operated pipeline system to move water quickly to reservoirs closer to San Francisco. The Hetch Hetchy supplies water to 2.6 million people in the Bay area, 150 miles away.
"We're taking advantage that the water we're receiving is still of good quality," said Harlan Kelly, general manager of the city's Public Utilities Commission. "We're bringing down as much water as possible and replenishing all of the local reservoirs."
At the same time, officials gave assurances that they had a six-month supply of water in reservoirs near the Bay area.
So far the ash that has been raining on to the Hetch Hetchy has not sunk as far as the intake valves, which are about halfway down the 300ft O'Shaughnessy Dam. Utility officials said that the ash is non-toxic but that the city will begin filtering water for customers if problems are detected. That could cost more.
On Monday the fire was still several miles away from the steep granite canyon where the reservoir is nestled, but several spot fires were burning closer, and firefighters were protecting hydroelectric transmission lines and other utility buildings. "Obviously we're paying close attention to the city's water supply," said Glen Stratton, an operations chief on the fire suppression team.
Power generation at the reservoir was shut down last week so that firefighters would not be endangered by live wires. San Francisco is buying replacement power from other sources to run City Hall and other municipal buildings.
The fire has swept through steep Sierra Nevada river canyons and stands of thick oak and pine, closing in on Tuolumne City and other mountain communities. It has confounded ground crews with its 300ft walls of flame and the way it has jumped from treetop to treetop. The US Forest Service said the fire was threatening about 4,500 structures and destroyed at least 23.