The National Park Service website is posting alerts for the Rim Fire.

By Sunday afternoon, some 15,000 acres within Yosemite had burned, prompting the evacuation of 74 campsites in the White Wolf area of the park, officials said.

The blaze also crept to within 2 miles of a key reservoir, the Hetch Hetchy, which is the source of 85 percent of San Francisco's water supply, and authorities said they were concerned about ash contamination from the fire.

Hydropower facilities in the area that provide electricity to San Francisco, about 200 miles to the west, also have been threatened.

Two of three power stations that account for all of the city's municipal electricity – for public hospitals, transit, City Hall and airport – have been shut down since last Monday. But the city has so far made up for the loss by purchasing supplemental power from the open market and using some of its reserves.

Governor Jerry Brown declared a state of emergency for San Francisco on Friday due to threats to the city's water supply. He planned to visit the fire zone on Monday to meet with fire managers and firefighters.

The blaze has destroyed about a dozen homes and 1,000 outbuildings, and some 4,500 additional dwellings remained threatened. Residents in the tiny town of Tuolumne on the western edge of the park were evacuated, but evacuation orders have been lifted for the communities of Pine Mountain Lake and Buck Meadows.

The majority of the 1,200-square-mile Yosemite National Park, including the Yosemite Valley area famous for its towering rock formations, waterfalls, meadows and pine forests, remained open to the public.

The cause of the wildfire remained under investigation. More than 2,800 firefighters have been assigned to the blaze, which was burning over steep terrain through scrub, oak and pine, stoked by stiff winds, high temperatures and low humidity.

The blaze in the western Sierra Nevada Mountains was among the fastest-moving of some 50 large wildfires raging across the drought-parched U.S. West. The blazes have strained resources and prompted fire managers to open talks with Pentagon commanders and Canadian officials about possible reinforcements.

(Reporting by Laila Kearney; Writing by Steve Gorman; Editing by Cynthia Johnston and Berenard Orr)