As of midday, the eastern flank of the so-called Rim Fire had burned to within a mile of Yosemite's Hetch Hetchy reservoir on the Tuolumne River, raising concerns about the possibility of ash contaminating the sprawling artificial lake.
On Saturday, flames had been no closer than 4 miles from the reservoir, which supplies 85 percent of the water consumed by 2.6 million people in San Francisco and several surrounding communities 200 miles to the west.
Reservoir samples on Monday showed that water quality so far remained healthy, said Suzanne Gautier, a spokeswoman for the San Francisco Public Utilities Commission.
But the fire has already damaged two of the three hydropower generating stations linked to the Hetch Hetchy reservoir that supply electricity for all of San Francisco's public facilities, such as hospitals and firehouses.
Gautier said the city for now was continuing to draw on reserve power stored for emergencies and purchasing additional electricity on the open market to make up for the difference.
Despite threats to water and power, firefighters have made headway against the blaze, though rugged terrain and the fire's remote location were complicating efforts to subdue the flames, fire managers said.
"There are places where they can't get in," said Mike Ferris, a spokesman for the National Interagency Fire Center in Boise, Idaho. "It's just inaccessible."
As of Monday morning, fire crews working with hand tools and backed by bulldozers and water-dropping helicopters had carved containment lines around 15 percent of the blaze's perimeter, more than double Sunday's figure, though the fire's footprint continued to grow.
OVER 200 MILES SCORCHED
According to the latest estimates, the Rim Fire has charred nearly 150,000 acres or 234 square miles - the size of the city of Chicago - since it erupted on August 17, most of that in the Stanislaus National Forest west of Yosemite.
That ranks the blaze as the largest wildfire in California since August 2009, when the so-called Station Fire ravaged nearly 145,000 acres of the Angeles National Park north of Los Angeles, and one of the 20 biggest in state history, according to the California Department of Forestry and Fire Protection.
The largest California blaze on record is the 2003 Cedar Fire, which scorched more than 273,000 acres in San Diego County and destroyed more than 2,800 structures.
Some 15,000 acres within Yosemite had burned by Sunday afternoon, forcing the closure of the main road leading into the park from the San Francisco Bay area and prompting the evacuation of 74 campsites in the park's White Wolf area, officials said.
But 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 National Park Service website is posting alerts for the Rim Fire. (here)
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 and other communities near the western edge of the park were evacuated, but evacuation orders have been lifted for the enclaves of Pine Mountain Lake and Buck Meadows.
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 fire have strained resources and prompted fire managers to open talks with Pentagon commanders and Canadian officials about possible reinforcements.