The shores of a key reservoir providing San Francisco\'s water burned relentlessly Wednesday as the giant, wind-whipped Rim fire extended its destructive march. The southern and western shores of Yosemite\'s 117 billion-gallon Hetch Hetchy Reservoir were ablaze, with ash falling on the reservoir\'s famously pure mountain water, which provides 85 percent of the city\'s supply and 2.6 million San Francisco Bay Area residents and businesses south to Silicon Valley. \"It is an emergency,\" San Francisco Public Utilities Commission General Manager Harlan Kelly Jr. told the San Jose (Calif.) Mercury News. \"We are taking it seriously,\" he said. \"We are very concerned.\" The agency owns and operates the reservoir. Water quality has not been compromised, Kelly told the newspaper. And even if the agency has to temporarily shut off Hetch Hetchy water, it has a six-month supply in Bay Area reservoirs and in loans from neighboring water districts, he said. In addition, the agency has been racing more than a billion gallons of clean Hetch Hetchy water to smaller backup reservoirs to supplement potable water availability, an agency spokeswoman told the San Francisco Chronicle. Hetch Hetchy water is drawn from 260 feet below the surface to avoid unwanted debris, spokeswoman Alison Kastama added. But wildfire experts said water problems might come later. \"Now that it\'s there, ensuring water quality and long-term cleanup of that area is going to be critical,\" California Department of Forestry and Fire Protection spokesman Daniel Berlant told the Chronicle. \"The concern is that after the fire is out and the hills are blackened, water runoff in the winter will wash all this debris into the reservoir,\" he said. Kastama said the agency was prepared to handle any additional water debris by filtering the normally crystal-clear water at the agency\'s Bay Area reservoirs about 170 miles away. By late Tuesday, the fire had scorched close to 190,000 acres -- or close to 300 square miles -- making it the seventh-largest wildfire in California history and the largest wildfire in the United States. Firefighters reported moderate progress in their battle, estimating the blaze was 20 percent contained, the same percentage as Monday. At least 31 residences and 80 other structures had burned. More than 4,500 residences remained threatened. Officials said the cost of the firefight swelled to more than $27 million. They said it topped $20 million Monday. Nearly 3,800 firefighters battled the blaze, which started in a remote section of the Stanislaus National Forest in Northern California\'s Sierra Nevada range Aug. 17. The fire\'s cause is not known.