Updated 6:20 a.m. ET
GROVELAND, Calif. Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the giant wildfire burning at the edge of Yosemite National Park, chewing up the rugged landscape, forestry experts say.
The fire had ravaged 288 square miles by Tuesday night, making it the biggest in the Sierra's recorded history and one of the largest on record in California.
Containment held steady at 20 percent, but the number of destroyed structures rose to 111, and some 4,500 structures remained threatened. At least 31 residences were among those lost. Firefighters were making stands at Tuolumne City and other mountain communities.
Officials said they expected the blaze to continue to grow overnight.
"We're going to get a handle on it, but we're going to have to give some ground to get some," said Lee Bentley, fire spokesman for the U.S. Forest Service.
The fire approached the main reservoir serving San Francisco, but fears that the inferno could disrupt water or hydroelectric power to the city diminished. On Tuesday, the fire moved into the watershed, which increases the chances of sediment runoff this winter.
Yosemite crews continue to keep water on two groves of giant sequoias less than 10 miles from the fire's front lines.
Reporter Nick Janes of CBS Sacramento, Calif. station KOVR-TV went farther into Yosemite Tuesday than any reporter had since the fire began. One official told him he thinks the Rim Fire could begin the biggest in state history:
The flames have also unleashed a smoky haze that has worsened air quality more than 100 miles away, in Nevada.
The plume from the Rim Fire triggered emergency warnings in the Reno and Carson City areas. Schoolchildren were kept inside for the second time in a week, people went to hospitals complaining of eye and throat irritation and officials urged people to avoid all physical activity outdoors.
"It's five hours away," said 22-year-old bartender Renee Dishman in disbelief after learning that the source of the haze was more than 150 miles away. "I can't run. I can't breathe. It makes me sneeze."
The air quality index briefly surpassed the rare "hazardous" level east of Lake Tahoe before improving slightly. It hovered around the next-most serious stage of "very unhealthy" for all populations in the Reno-Sparks area 30 miles north.
Separately, a fire apparently sparked along a freeway spread to a grassy hillside, jumped into trees and then burned several homes in a crowded Northern California neighborhood.
Public Information Officer Gale Spears says two houses were completely destroyed and three others were damaged in the city of Fairfield, about 40 miles northeast of San Francisco. The homes were near Interstate 80.
Spears says fire crews worked quickly to protect dozens of threatened structures including houses and a nearby apartment complex. At least 50 homes were evacuated.
The Yosemite-area blaze was just 40 acres when it was discovered near a road in Stanislaus National Forest on Aug. 17, but firefighters had no chance of stopping it in the early days.
Fueled by thick forest floor vegetation in steep river canyons, it exploded to 10,000 acres 36 hours later, then to 54,000 acres and 105,620 acres within the next two days. On its 11th day, it had surpassed 179,400 acres, becoming the seventh-largest California wildfire in records dating to 1932.
Federal forest ecologists say historic policies of fire suppression to protect Sierra timber interests left a century's worth of fuel in the fire's path.
"That's called making the woodpile bigger," said Hugh Safford, an ecologist with the U.S. Forest Service in California.
Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years, forest ecologists have warned that Western wildfires will only get worse.
"Every year, the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious," said Safford. "People can deny it all they want, but it's happening. Every year, the fuels are a little bit drier."
The Rim Fire's exponential growth slowed only after hitting areas that had burned in the past two decades, and Safford says that shows the utility of prescribed and natural burns that clear brush and allow wildfires to move rapidly without killing trees.
"If you look at the Sierra Nevada as a whole, by far the largest portion hasn't seen a fire since the 1910s and 1920s, which is very unnatural," said Safford, who has authored several papers on the increasing wildlife severity across California's mountain ranges. "This one isn't stopping for a while."
Since a 1988 fire impacted nearly one third of Yellowstone National Park, forestry officials have begun rethinking suppression policies. Yosemite has adopted an aggressive plan of prescribed burns while allowing backcountry fires caused by lightning strikes to burn unimpeded as long as they don't threaten park facilities.
"Yosemite is one of the biggest experimental landscapes for prescribed fire and it's going to pay off," Safford said. "The Rim Fire is starting to hit all those old fire scars."
The 350-mile-long Sierra Nevada is a unique mountain system in the U.S. with its Mediterranean climate, which means four to six months of drought every summer. California's mountain flora is designed to burn and even flourish and regenerate healthier after a fast-moving fire.
Instead, the Rim Fire is killing everything in its path. The understory ignites trees, and wind is sweeping the fire from treetop to treetop in 300-foot walls of flame.
Scientists also expect the impact on wildlife to be severe. The fire has encompassed nearly the entire migratory range of deer in the region, and the burning treetops likely displaced many of the remaining 300 members of a subset of Great Gray Owl along the Yosemite border, said Daniel Applebee of the California Department of Fish and Wildlife.
"Because their population is so small, any loss is significant," Applebee said.
The fire also cut through habitat of the Pacific fisher, a weasel-like animal that is listed for state and federal protections. The fire has fragmented its range, likely leaving it nowhere to expand, Applebee said.
The Rim Fire is the first of any ecological significance in about a decade in the area stretching from the Sequoia National Forest south of Yosemite to north of Lake Tahoe, said Chad Hanson, a forest ecologist and environmental activist who has published a number of papers on the significance and increasing rarity of post-fire habitat in the Sierra Nevada.
Eventually, the forest will come back.
"Because we are in such tremendous deficit of this post-fire habitat type, especially in this area, the Rim Fire is a good thing ecologically," Hanson said. "This is not destruction, this is ecological restoration."