Yosemite wildfire threatening San Francisco's water and power supply, as well as giant sequoias A massive wildfire racing through the Yosemite wilderness - fueled by high winds - is threatening San Francisco's fresh water and power supply as well as California's iconic giant sequoias. It's one of several fires statewide being fought by more than 8,000 firefighters across nearly 400 square miles. The fire has consumed approximately 225 square miles of picturesque forests. Officials estimate containment at just 7 percent. "This fire has continued to pose every challenge that there can be [in] a fire: inaccessible terrain, strong winds, dry conditions," Daniel Berlant of the California Department of Forestry and Fire Protection said on Sunday. The fire continues burning in the remote wilderness area of Yosemite, but park spokesman Tom Medena told the Associated Press it's edging closer to the Hetch Hetchy Reservoir, the source of 85 percent of San Francisco's famously pure drinking water, as well as power for a number of key city buildings, including the airport. The city has issued assurances that the water quality remains good, but the San Francisco Public Utilities Commission has shut two hydro-electric stations fed by water from the reservoir and cut power to more than 12 miles of lines. The city has been buying $600,000 worth of power on the open market to ensure San Francisco doesn't go dark. Meanwhile, park officials are clearing brush and setting sprinklers to protect two groves of giant sequoias. The iconic trees can resist fire, but dry conditions and heavy brush are forcing extra precautions to be taken in the Tuolumne and Merced groves. "All of the plants and trees in Yosemite are important, but the giant sequoias are incredibly important both for what they are and as symbols of the National Park System," park spokesman Scott Gediman told The Associated Press on Saturday. Fire officials are using bulldozers to clear contingency lines on the Rim Fire's north side to protect the towns of Tuolumne City, Ponderosa Hills and Twain Hart. The lines are being cut a mile ahead of the fire in locations where fire officials hope they will help protect the communities should the fire jump containment lines. Firefighters were hoping to advance on the flames Monday but strong winds, gusting up to 50 miles per hour in some places, were threatening push the blaze closer to Tuolumne City and nearby communities. "Winds are increasing, so it's going to be very challenging," said Bjorn Frederickson, a spokesman for the U.S. Forest Service. The Forest Service confirmed late Sunday to KTVU.com that the fire had burned through the Berkeley Tuolumne Family Campsite, which was owned by the city of Berkeley, Calif., and had been in operation since 1922. Firefighters were unable to immediately assess the damage, and it was not clear if any structures survived. The camp had been evacuated Tuesday and no injuries were reported. "It's a very difficult firefight," Berlant said. Frederickson added that the fire is slowing down a bit, but still growing. The high winds and movement of the fire from bone-dry brush on the ground to 100-foot oak and pine treetops have created dire conditions. "A crown fire is much more difficult to fight," Berlant told The Associated Press on Sunday. Our firefighters are on the ground having to spray up. The blaze sweeping across steep, rugged river canyons quickly has become one of the biggest in California history, thanks in part to extremely dry conditions caused by a lack of snow and rainfall this year. Investigators are trying to determine how it started Aug. 17, days before lightning storms swept through the region and sparked other, smaller blazes. The fire is the most critical of a dozen burning across California, officials say. More than 12 helicopters and a half-dozen fixed wing tankers are dropping water and retardant from the air and 2,800 firefighters are on the ground. Statewide, more than 8,300 firefighters are battling nearly 400 square miles of fires. Many air districts have issued health advisories as smoke settles over Northern California. On Saturday, organizers cancelled the 24th annual Lake in the Sky Air Show at Lake Tahoe because of poor visibility. 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. At the nearby Black Oak Casino in Tuolumne City on Sunday, the slot machines were quiet as emergency workers took over nearly all of the resort's 148 hotel rooms. "The casino is empty," said casino employee Jessie Dean. Technically, the casino is open but there's nobody there. As thick smoke portends the fire's fast approach, the area has been cleared of everyone but locals and emergency workers. Dean lives on the reservation of the Tuolumne Band of Me-Wuk Indians and left her four children at relatives' homes in the Central Valley. But the tourist mecca of Yosemite Valley, the part of the park known around the world for such sights as the Half Dome and El Capitan rock formations and waterfalls, remained open, clear of smoke and free from other signs of the fire that remained about 20 miles away.