The Rim fire near Yosemite National Park was 23 percent contained Wednesday and
firefighters got help from a layer of moist air in the region, authorities said.
The fire has burned 187,466 acres and is projected to grow into at least the sixth-largest fire in state history, the Los Angeles Times reported.
In Boise, Idaho, the National Interagency Fire Center said Wednesday it has ordered its three Modular Airborne Fire Fighting System aircraft, operating at the Boise Air Tanker Base, moved to McClellan Air Tanker base near Sacramento to take part in the Rim fire operation.
The deployment came at the request of the U.S. Forest Service.
Firefighters were able to reinforce containment lines and carry out backfire operations overnight, the Times reported.
Nearly 4,100 firefighters are involved in the operation.
The Rim fire has destroyed at least 111 buildings, including 31 homes, and has spread across 281 square miles.
As the giant fire rained ash into a key San Francisco reservoir, a smaller blaze ripped through 40 acres of nearby Fairfield, Calif., officials said.
The fire in Fairfield, about 40 miles north of San Francisco, destroyed two homes and damaged four more, the San Francisco Chronicle reported Tuesday.
About 50 residents were evacuated as 200 firefighters from four counties battled the fire, which began about 4 p.m. in brush next to the freeway, Fairfield Fire Chief Tony Velasquez said.
Shifting winds pushed the fire back toward the highway, saving many homes. The blaze was under control about 2 hours after it started.
The Rim fire has blackened the southern and western shores of the 117 billion-gallon Hetch Hetchy Reservoir, which provides 85 percent of the water supply for San Francisco, 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 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 shut off Hetch Hetchy water temporarily, 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 Chronicle.
Hetch Hetchy water is drawn from 260 feet below the surface to avoid unwanted debris, San Francisco Water, Power and Sewer spokeswoman Alison Kastama said.
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.
Officials said the cost of the firefight swelled to more than $27 million. They said it topped $20 million Monday.
The blaze started in a remote section of the Stanislaus National Forest in Northern California's Sierra Nevada range Aug. 17. The fire's cause is unknown.
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