Elizabeth Weise, USA TODAY

The nation's largest active forest fire continued to burn at the edge of Yosemite National Park Monday, reaching 234 square miles and prompting multiple mandatory and advisory evacuations in the area to the northwest of the park.

At least 4,500 structures are threatened, according to the California Department of Forestry and Fire Protection.

The fire was about 20 miles from the iconic Yosemite Valley, home to Half Dome, the Ahwahnee Hotel and Curry Village. "There is no imminent threat of any kind," said Tom Medema, a park spokesman.

Overall, the fire was 15% contained as of Monday at 7 a.m. local time, a recorded message at the incident command center said. The blaze has burned 23 structures. Mandatory evacuations were ordered for residents in some areas in the path of the fire while other towns, including Tuolumne and Mi-Wuk Village, were under advisory evacuations.

Highway 120, one of the main roads into Yosemite National Park, was closed to all inbound and outbound traffic.

Larry Brown lives in Sonora, Calif., more than 10 miles from the fire. "It's about a half-mile visibility here because of the smoke. Everything smells like smoke. When I open the car door, it smells inside," he said.

Brown is a ham radio operator with the Tuolumne County Amateur Radio Electronics Society which is manning phone lines at the Sonora community information line for those affected by the fire.

He and others have been impressed "as always," he says, with the work of the firefighters. He says people who live in the Sierra realize that fire is a natural part of the landscape.

"This is part of the ecosystem," he says. "We do burn in here every so often. There are plants and things within the canyons that are reliant on the fire to open their seeds, that have adapted to that environment."

The Rim fire is one of several burning in the west. It's a mile away from the Hetch Hetchy reservoir, San Francisco's main source of water, but the water supply is safe, said Charles Sheehan, a spokesman for the San Francisco Public Utilities Commission.

The reservoir supplies water to 2.6 million people in the San Francisco Bay area, pumping approximately 260 million gallons of famously pure water down 160 miles of gravity-fed pipes every day.

"There's no change to water quality," Sheehan said. "We have instruments monitoring the water constantly." The turbidity, a measure of particles in the water, is 0.2, which is normal, he said. If the turbidity were to reach 5.0, the utility would switch to suppliers of water from other nearby reservoirs that it has emergency contracts with.

The water facilities are not in great danger because they are relatively fireproof. "The O'Shaughnessy dam is made of concrete, the water facilities are concrete and they're underground, as are the steel pipes that carry the water," Sheehan said.

One saving grace is that the reservoir sits in a granite basin, one of the reasons the site was chosen for damming in the first place. "That makes it an ideal place to store water, and it's also a natural fire break," Sheehan said.

Firefighters are on site at the reservoir and are in frequent contact with the utility to update on the status of the dam and reservoir, he said.

A secondary concern is the power generated by the dam, used for some municipal buildings in the city, including San Francisco International Airport and San Francisco General Hospital.

The system has three powerhouses, two of which were taken offline Saturday because of fire damage, Sheehan said. Twelve to 14 miles of power transmission line were also taken out of operation. "There's been some damage, we don't know how much," Sheehan said.

Even if the powerhouses were all shut down, there would be no loss of power in San Francisco. The city's electrical infrastructure is linked to the main electrical grid, and it would simply purchase power to supplement the lost transmissions. Since the fire began Aug. 19, the city has purchased about $600,000 in supplemental power, the Public Utilities Commission website said.

California Gov. Jerry Brown has declared a state of emergency for San Francisco because of the possible threat to the city's water and electrical infrastructure.

The weather is not likely to be much help to firefighters. "Breezy, southwest winds are expected on Monday with 15- to 30-mph gusts in the higher terrain," said Weather Channel meteorologist Chris Dolce. "This will tend to blow the smoke to the north and east of the fire."

Dry conditions are expected through Tuesday, but an isolated storm in the highest terrain of the Sierra Nevada is possible, he said.

Rainfall would be beneficial, but the lightning and gusty winds accompanying any thunderstorm could hinder firefighting efforts, AccuWeather said.

Contributing: Doyle Rice

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