Yosemite National Park, California -- The numbers are staggering and the prospects are absolutely scary as a massive California wildfire menaces Yosemite National Park.
The Rim Fire, which has devoured nearly 161,000 acres, is also threatening San Francisco's key water and power sources.
"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 wildfire, which was 20% contained Monday night, was spreading primarily to the east and threatened to grow amid extremely dry conditions and hot weather.
As many as 20 helicopters and air tankers were aiding the efforts of 3,600 firefighters.
The fire continued to spread Monday toward a key part of San Francisco's water supply: the Hetch Hetchy Reservoir, which lies within Yosemite and is just east of the flames.
Northwest of the reservoir, rocky terrain could stop the fire's growth there, Bentley said.
The fire also could threaten the area's hydroelectric generators, which provide much of San Francisco's electricity. Because of the approaching flames, officials shut down the generators, and the city -- more than 120 miles to the west -- temporarily is getting power from elsewhere.
Yosemite: "A national treasure"
A top priority is stopping the fire from spreading further in Yosemite National Park.
"The park is a national treasure," said U.S. Forest Service spokeswoman Vickie Wright, "so no matter what it takes, we're going to do everything in our power to protect that park."
Yosemite, with hundreds of campground sites and lodging units, had nearly 4 million visitors last year, the National Park Service said. The park typically has 15,000 visitors on a busy summer weekend.
While the Rim Fire has consumed at least 12,000 acres in the northwest section of the park, so far it has had little or no direct impact on Yosemite Valley, a popular spot for tourists and home to many of the famous cliffs and waterfalls in the park.
About 4,500 structures, many of them vacation homes, were under threat, according to InciWeb, a federal website that collects information from agencies such as the U.S. Forest Service and Bureau of Land Management. The fire so far has cost more than $20 million, Bentley said.
The inferno threatened the Yosemite gateway communities of Groveland and Pine Mountain Lake just outside the Stanislaus National Forest.
"Business is slow, very slow," said Corinna Loh, owner of the Iron Door Saloon in Groveland.
Her normal season is Memorial Day to Labor Day.
"This is time we manage to save up money to make it through the winter, so it's really scary for all of us," she said, sitting among empty tables.
The Tuolumne County Sheriff's Office, meanwhile, issued evacuation advisories for the town of Tuolumne and nearby Ponderosa Hill, InciWeb said. It was not clear how many residents were covered by the evacuation advisory.
The fire has destroyed 11 homes and 12 outbuildings, as well as a Yosemite campground owned by the city of Berkeley, officials said.
Authorities say the Rim Fire started on August 17. The cause is under investigation.