GROVELAND, Calif. — The Rim fire spread deeper into Yosemite National Park on Tuesday with flames racing unimpeded to the east even as firefighters shored up defenses for communities on the western edges of the blaze. 
 
 The fire was 20% contained by Tuesday evening, with almost all of the containment coming on the fire's southwest edge. On the east, the fire has a relatively flat, clear path farther into Yosemite and the 3,700 firefighters battling the blaze have fewer options to control it. 
 
 "They're in scouting mode," Dick Fleishman of the U.S. Forest Service said of fire crews. "There's not a lot of real good areas to get out in there and do a lot of work." 
 
 The blaze has destroyed 111 buildings, including 31 residences, and is now the seventh-largest fire in state history, having spread across 281 square miles. 
 
 "It's burning its way into the record books," said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection. 
 
 On Tuesday, firefighters bolstered defensive perimeters to the south and west — using bulldozers to clear brush and vegetation from strips of land up to the Tuolumne River to protect homes near California 108, Berlant said. 
 
 "We've burned back down the mountain so that if the fire makes the river, jumps the river, that side of the hill is already burned," he said. "We're coming around the corner, catching the western portion of the fire and we'll continue to pinch it off as it goes up to the northern flank." 
 
 Officials consider about 4,500 homes north of the fire and two groves of giant sequoias and other historic landmarks still in danger. 
 
 The Stanislaus National Forest is taking the brunt of the blaze, with the Groveland Ranger District making up most of the southern flank. The region has been hit hard by fires in the past, the most significant in 1987, which claimed the life of a firefighter. 
 
 This week's fire has brought sorrow among the district's employees, who not only recall the past devastation but also begrudge the current damage. The fire burned though an area that had a pending $1-million timber sale, said Maggie Dowd, district ranger in the Groveland Ranger District. 
 
 "The economic impacts are real, but we haven't begun to estimate them yet," Dowd said Tuesday from her office in a building shrouded in smoke. 
 
 So far, the fire has destroyed two federal campgrounds that had been recently upgraded and a day-use area at Rainbow Pools, Dowd said. At least one historic structure — a cabin that had also been recently refurbished — was destroyed and Forest Service crews are still assessing damage to other structures. 
 
 Smoke from the fire is starting to drift into the park's northern areas, but it's unavoidable at the command post. 
 
 The blaze is a "campaign fire," crews say, meaning that everyone who joins in the effort knows they are going to be there for days, weeks or even months. 
 
 Firefighters who returned to camp to rest looked like raccoons, their faces striped with soot and ash. The command post is big enough to feed and house the thousands of firefighters who are working in shifts, with about half a dozen tractor trailers with showers standing by. 
 
 As the blaze stretched into its 11th day, officials received visual confirmation of ash on the surface of Hetch Hetchy reservoir, which supplies water to 2.6 million people in the Bay Area. But officials monitoring water quality have seen no change, said Tyrone Jue, spokesman for the San Francisco Public Utilities Commission. 
 
 Hetch Hetchy supplies are drawn from the reservoir at a depth of 260 feet, so there is little chance of contamination, Jue said. More problematic than ash on the surface of the water is the potential for runoff to enter the reservoir when rain comes later in the season, he said. 
 
 Gov. Jerry Brown issued a state of emergency for the city and county of San Francisco on Friday because of the threat to the reservoir. 
 
 Following standard procedure when the reservoir is threatened, additional water is being pumped to reservoirs in Alameda and San Mateo counties, which also supply Hetch Hetchy customers, Jue said. The amount of water being transferred has been increased from 275 million gallons a day to 302 million gallons as a precaution, the utility's commission said. 
 
 Crews have also repaired one of two hydroelectric power stations shut down by the fire, and they are in the process of assessing the damage to the second unit, Jue said. 
 
 Supplemental power has cost about $600,000 since the two power stations were taken offline Aug. 17, the day the Rim fire started. 
 
 joseph.serna@latimes.com 
 
 julie.cart@latimes.com 
 
 samantha.schaefer@latimes.com 
 
 Times staff writer Diana Marcum contributed to this report. ||||| — The power of social media to disseminate information almost immediately is both a help and a hinderance to those working the Rim fire. 
 
 "One of the biggest problems we have had is social media," Tuolumne Utilities District spokeswoman Lisa Westbrook said last week. The Rim fire has been burning in Tuolumne and Mariposa counties, spreading into Yosemite National Park as it grew to nearly 150,000 acres. 
 
 Westbrook said people are posting fire information on Facebook about where the blaze is heading or which neighborhoods are being evacuated, and the information is wrong. 
 
 Deputies and firefighters have to spend valuable time getting the correct information out. 
 
 Tuolumne County Sheriff's Sgt. Scott Johnson agreed. 
 
 "Early on in the fire it was put out that Groveland firehouse burned down," he said, adding dryly, "that would be a surprise to the firefighters sleeping there." 
 
 Another rumor that spread nearly as fast as the wildfire itself was that there had been an arson arrest in the case. The cause of the fire, which began the afternoon of Aug. 17, remains under investigation. 
 
 "The problem is there's no way of verifying that information," Johnson said. "If people didn't exaggerate things and actually gave out factual information it would be a very good tool." 
 
 There are official Twitter feeds, Facebook pages and websites associated with the agencies fighting the fire, and Johnson said those are good places to look for information. 
 
 "The problem is, a lot of times it's just 'Mary Smith saw ...' " he said. 
 
 On the positive side, a lot of people have used social media to bring help to those affected by the fire. 
 
 Facebook is rife with residents of neighboring communities gathering supplies or posting offers of homes for displaced pets or livestock. And the Groveland Facebook page has been turned into a repository for updates, questions and concerned sentiments for those whose homes are threatened. 
 
 A group from the Oakdale Saddle Club started a drive for baby wipes, sports drinks, water and other items that could be of use to Saddle Club members in Tuolumne County, as well as other residents. 
 
 "We're not an organization, we're just a group of people that got together to do this," said Jennifer Jordan. 
 
 It really took off when they put together a Facebook page, organizer Cortney Napier said: "That's the power of Facebook." 
 
 Fliers spread over social media 
 
 Volunteers set up drop sites where they collect donations and load them in a horse trailer to bring them up the hill. 
 
 "It's been really great," said Amanda Jones, another organizer. "Once we put the fliers on Facebook, people started sharing it and it went from there." 
 
 Most of the donations have come from the area, with people from as far away as Los Banos leaving items at the Yogurt Station in Oakdale, a drop spot for the effort. But it goes farther afield than that  a woman from Texas called Jordan, wanting to contribute. 
 
 "We're just doing what we can to help," Jones said. "People that we never met before, they're just coming out of the woodwork, wanting to know, 'What can I do?' " 
 
 Bee staff writer Kevin Valine contributed to this report. 
 
 Breaking News Editor Patty Guerra can be reached at pguerra@modbee.comor (209) 578-2343. Followher on Twitter, @pattyguerra.
– The wildfire threatening Yosemite National Park is throwing so much smoke into the air that it is significantly hurting the air quality in Nevada—more than 100 miles away. Reno and Carson City issued emergency warnings over the smoke from the Rim Fire, and school children were kept inside for the second time in a week to avoid it, the AP reports. The air quality index even briefly passed the rare "hazardous" level. "It's five hours away," says one shocked resident. "I can't run. I can't breathe. It makes me sneeze." The fire is about 20% contained, but almost all of that progress has come at its southwest edge, the LA Times reports. On the east, it has a relatively flat, direct path deeper into Yosemite. "There's not a lot of real good areas to get out in there and do a lot of work," says one US Forest Service official. Meanwhile, state officials are urging residents to be wary of social media, where false rumors are spreading like, well, wildfire. Recently, for instance, "news" spread that the Groveland firehouse had burned down, which "would be a surprise to the firefighters sleeping there," a local sergeant tells the Modesto Bee.