Crews working to contain one of California’s largest ever wildfires had help from the blaze itself as they made advances overnight against the flames threatening San Francisco’s water supply, several towns near Yosemite National Park, and historic giant sequoias.

Stanislaus National Forest spokesman Jerry Snyder said containment of the Rim Fire was at 15 percent on Monday morning, up from 7 percent the previous night.

The fire did continue to grow, however, and is now 234 square miles in size — covering more ground than the city of Chicago.

Snyder said crews are being helped by the fire’s movement 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.

Inaccessible terrain, strong winds and bone-dry conditions have hampered firefighters’ efforts to contain the Rim Fire, which began Aug. 17.

It has blackened a remote wilderness area of Yosemite and is edging closer to the Hetch Hetchy Reservoir, the source of San Francisco’s famously pure drinking water, park spokesman Tom Medema said. 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 is still good, say 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.

While it has closed some backcountry hiking, the fire was not threatening the Yosemite Valley, home to iconic sights such as the Half Dome and El Capitan rock formations and Bridalveil and Yosemite falls. Most of the park remained open to visitors. Park spokesman Scott Gediman said on Monday morning he was not aware of any additional threats to the park overnight.

Park employees are working to protect two groves of giant sequoias that are unique to the region by cutting brush and setting sprinklers, Medema said. The U.S. Forest Service says about 4,500 structures are threatened by the Rim Fire. Berlant said 23 structures were destroyed, though officials have not determined whether they were homes or rural outbuildings

On Sunday, crews worked furiously to hold a line near Ponderosa Hills and Twain Hart, miles ahead of the blaze. But officials warned that the fire was so hot it could send sparks more than a mile and a half out that could start new hot spots.

“We’re facing difficult conditions and extremely challenging weather,” said Bjorn Frederickson, a spokesman for the U.S. Forest Service.

(AP)