Last Updated 4:32 p.m. ET
TUOLUMNE CITY, Calif. - San Francisco water authorities are scrambling to fill area reservoirs with water from their source in the Sierra Nevada before ash from a wildfire near Yosemite National Park taints supplies.
Ash from the 234-square-mile fire has been falling on the city's Hetch Hetchy Reservoir in Yosemite, but so far hasn't sunk far enough into the lake to reach intake pumps.
General Manager Harlan Kelly Jr. of the city's Public Utilities Commission said Monday the city has a six-month supply on hand. If ash eventually causes turbidity, the city will begin filtering supplies. He was unsure of the cost.
Crews working to contain what is one of California's largest-ever wildfires gained some ground Monday against the flames, which are also threatening several towns near Yosemite National Park and historic giant sequoias.
Containment of the Rim Fire more than doubled to 15 percent, although it was within a mile of the park's Hetch Hetchy Reservoir, the source of San Francisco's famously pure drinking water, officials said Monday.
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.
The governor planned to visit a fire base camp on Monday to meet with state and federal emergency officials. Brown spoke Sunday to President Barack Obama, who reiterated his commitment to providing needed federal resources, according to the White House.
"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 largest of dozens of western wildfires, the Rim Fire has already burned through 15,000 acres of land in Yosemite alone, and is threatening 4,500 nearby buildings this morning, including homes, vacation cabins and businesses, reported CBS News correspondent Teresa Garcia.
Stanislaus National Forest spokesman Jerry Snyder said crews are being helped by the fire's movement into less forested areas, and by cooler temperatures caused, at least in part, by the shadow cast by the large plume of smoke from the blaze.
One official said more than 3,400 firefighters are facing "every challenge that there can be" in tackling the Rim Fire, battling the blaze from the ground and in the air.
Hundreds of firefighters dug trenches, cleared brush and started back blazes to keep the wildfire out of several mountain hamlets.
"Everyone is heads up 24/7 when they're out here, boots on the ground," fire safety officer Sam Lobese told CBS News. "They're looking up at the trees, watching the smoke column, watching what it's doing. if it's running on top of the trees, it's very difficult to put that out, it's almost impossible."
Along with the challenge of battling the blaze in steep, rugged terrain, the afternoon winds keep igniting stubborn spot fires.
"It's going to take embers, it's going to take burning pine cones a quarter-mile, a half-mile -- the more wind, the further it goes," Lobese said. "I mean, it's problem after problem after problem."
Inaccessible terrain, strong winds and bone-dry conditions have hampered their efforts to contain the Rim Fire, which began Aug. 17 and has grown to become one of the biggest in California history.
Firefighters were hoping to advance on the flames Monday but strong winds were threatening to push the blaze closer to Tuolumne City and nearby communities.
Temperatures are forecasted to remain in the mid to upper 80s all week here -- with no rain in the foreseeable future.
The fire has consumed nearly 150,000 acres in total.
The Hetch Hetchy Reservoir supplies drinking water for 2.6 million people in the San Francisco area. 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 is still good, say officials with the San Francisco Public Utilities Commission.
"We cannot serve that water unless we filter it," said SFPCU Deputy General Manager and Chief Operating Officer Michael Carlin. "We have local supplies that we can fill in and serve everybody so that there will be no interruption of water service to anyone."
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.
Also threatened are California's giant sequoias, among the largest and oldest living things on the planet.
"It's really unthinkable to lose the sequoias," said Tom Medema of the National Park Service. "We celebrate those trees and we want to protect them. They're one of the reasons people come to this place, to see them."
Park employees are continuing their efforts to protect two groves of giant sequoias that are unique to the region by cutting brush and setting sprinklers, said Medema.