A raging wildfire in Yosemite National Park has rained ash on the reservoir that provides the main source of San Francisco's drinking water, renowned for its purity.
Utility bosses rushed to transport 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.
The California Department of Forestry and Fire Protection reported modest progress, saying the fire was 20 per cent contained by Monday evening. Mandatory evacuations have been ordered in some areas.
However, the fire still threatens to destroy 4,500 structures and could damage power and water supplies in the area, NBC news have reported.
The cause of the fire, which has so far destroyed 23 structures, has not yet been determined.
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."
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 authorities said the ash is non-toxic but the city will begin filtering water for customers if problems are detected.
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 imperiled by live wires. San Francisco is buying replacement power from other sources to run City Hall and other municipal buildings.
Park chiefs cleared brush and set sprinklers on two groves of giant sequoias that were seven to 10 miles away from the fire's front lines, said park spokesman Scott Gediman. While sequoias have a chemical in their bark to help them resist fire, they can be damaged when flames move through slowly.
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.
Rugged terrain, strong winds and bone-dry conditions have hampered firefighters' efforts to contain the blaze, which began on 17 August.
It has been at least 17 years since fire ravaged the northernmost stretch of Yosemite currently under siege.
Additional reporting by agencies
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