Containment of the Rim Fire more than doubled to 15 percent, although it was within a mile of Hetch Hetchy Reservoir, the source of San Francisco's famously pure drinking water, officials said Monday.
"Obviously, it's the water supply of the city of San Francisco, so we're paying a lot of attention to that," said Glen Stratton, an operations section chief on the fire.
The fire, which has grown to 234 square miles in size, also posed a threat to giant sequoias in Yosemite National Park. Crews were using sprinklers and lighting fires to clear brush, though the fire remained several miles from the massive trees, Stratton said.
Another part of the fire that is also burning into the park was not of major concern because it was running into rocks that are not heavily forested, Stratton said.
Gov. Jerry Brown planned to visit a fire base camp on Monday to meet with state and federal emergency officials.
While it has closed some backcountry hiking, the fire has not threatened the Yosemite Valley, such sights as the Half Dome and El Capitan rock formations and Bridalveil and Yosemite falls draw throngs of tourists. Most of the park remained open to visitors. Park spokesman Scott Gediman said Monday morning he was not aware of any additional threats to the park overnight.
The U.S. Forest Service said about 4,500 structures were threatened by the fire. At least 23 structures have been destroyed, though officials have not determined whether they were homes or rural outbuildings.
Firefighters were aided by movement of the blaze into less forested areas and cooler temperatures caused at least in part by the shadow cast by the large plume of smoke from the blaze, said Jerry Snyder, a spokesman for the Stanislaus National Forest.
Inaccessible terrain, strong winds and bone-dry conditions have hampered firefighters' efforts to contain the Rim Fire, which began Aug. 17.
San Francisco gets 85 percent of its water from Hetch Hetchy as well as power for municipal buildings, the international airport and San Francisco General Hospital. The threat to the city's utilities prompted Gov. Jerry Brown to declare a state of emergency for San Francisco.
Despite ash falling like snowflakes on the reservoir and a thick haze of smoke limiting visibility to 100 feet, the quality of the water piped to the city 150 miles away was still good, according to officials with the San Francisco Public Utilities Commission.
The city's hydroelectric power generated by the system has been interrupted by the fire, forcing the utility to spend $600,000 buying power on the open market.