The wildfire was spreading mainly to the east, heading toward a key part of San Francisco's water supply, the Hetch Hetchy Reservoir, and also could threaten the area's hydroelectric generators, which provide much of San Francisco's electricity, CNN reported Tuesday.
"There's a lot of concern, and there's a lot of work to be done," U.S. Forest Service spokesman Lee Bentley said.
The Rim fire in Yosemite National Park was 20 percent contained as of Monday night,
Because of the approaching flames, officials powered down the generators and San Francisco was temporarily getting power elsewhere, CNN said.
Officials, however, expressed concern ash from the wildfire would foul the water.
The San Francisco Public Utilities Commission transferred 302 million gallons of water a day from the giant reservoir to other reservoirs, hoping to save the water from contamination, a utility spokesman said.
The Tuolumne County Sheriff's Office issued evacuation advisories for Tuolumne and Ponderosa Hill, the federal fire compilation website InciWeb indicated.
Firefighters used bulldozers to cut firebreaks to halt the flames' spread, while planes and helicopters dropped retardant from above.
California Gov. Jerry Brown toured the area Monday and met with fire officials, firefighters and first responders.
"We have to spend what it takes," Brown said in Tuolumne City days after declaring a state of emergency.
Brown said President Obama called him to express support and offer assistance.
The fire began in the Stanislaus National Forest in Northern California's Sierra Nevada range Aug. 17. Its cause is unknown.
So far, it has destroyed 23 structures, including Berkeley's Tuolumne Camp, a summer retreat about 5 miles outside Yosemite.
About 4,500 structures were being threatened, InciWeb said.
While the Rim fire has consumed at least 12,000 acres in the northwest section of the park it has had little or no direct impact yet on Yosemite Valley, a popular tourist stop and home to many of the cliffs and waterfalls in the park, officials said.
The fire has cost more than $20 million so far, Bentley said.
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