GROVELAND, Calif. -- As a wildfire rages along the remote northwest edge of Yosemite National Park, officials cleared brush and set sprinklers to protect two groves of giant sequoias.
The fire, which began more than a week ago, has also prompted Gov. Jerry Brown to declare a state of emergency for the San Francisco area because of the potential effect on its utilities. The Bay Area draws much of its freshwater from a reservoir not far from the massive wildfire.
The iconic trees can resist fire, but dry conditions and heavy brush are forcing park officials to take extra precautions in the Tuolumne and Merced groves. About three dozen of the giant trees are affected.
"All of the plants and trees in Yosemite are important, but the giant sequoias are incredibly important both for what they are and as symbols of the National Park System," spokesman Scott Gediman said.
The trees grow naturally only on the western slopes of the Sierra Nevada and are among the largest and oldest living things on earth.
The Tuolumne and Merced groves in are in the north end of the park near Crane Flat. While the Rim Fire is still some distance away, park employees and trail crews are not taking any chances.
"We're not looking at them as any kind of immediate threat, but we're taking precautions," Mr. Gediman said.
More than 5,500 homes are threatened and four were destroyed. Voluntary and mandatory evacuations have been ordered.
The Rim Fire started in a remote canyon of the Stanislaus National Forest Aug. 17 and is just 5 percent contained. The cause is under investigation.
The fire held steady overnight at nearly 200 square miles along the park's northern border, but a spokesman for the California Department of Forestry and Fire Protection says firefighters didn't get their usual reprieve from cooler early morning temperatures Saturday.
"This morning we are starting to see fire activity pick up earlier than it has the last several days," Cal Fire spokesman Daniel Berlant said. "Typically, it doesn't really heat up until early afternoon. We could continue to see this fire burn very rapidly today."
The fire has grown so large and is burning dry timber and brush with such ferocity that it has created its own weather pattern, making it difficult to predict in which direction it will move.
"As the smoke column builds up it breaks down and collapses inside of itself, sending downdrafts and gusts that can go in any direction," Mr. Berlant said. "There's a lot of potential for this one to continue to grow."
No rain is expected and humidity levels, which have hovered recently between 20 percent and 25 percent, are expected to stay steady into this week, said Jim Mathews, a National Weather Service meteorologist in Sacramento, Calif. Increased humidity could help firefighting activities.
Wind speeds are expected to rise to 25 mph to 35 mph at higher elevations today and into Monday, as a low-pressure system moves over the area, possibly complicating firefighting efforts, Mr. Mathews said.
Warm temperatures should remain into this week, with highs in the 80s, he said.
The tourist mecca of Yosemite Valley, the part of the park known around the world for such sights as the Half Dome and El Capitan rock formations and waterfalls, remained open, clear of smoke and free from other signs of the fire that remained about 20 miles away.
More than 2,600 firefighters and a half dozen aircraft are battling the blaze.
The fire is burning toward the Hetch Hetchy reservoir, where San Francisco gets 85 percent of its water and power for municipal buildings, the international airport and San Francisco General Hospital.
Officials with the San Francisco Public Utilities Commission are running continuous tests on water quality in the reservoir that is the source of the city's famously pure water.
Deputy general manager Michael Carlin told The Associated Press on Saturday that no problems from falling ash have been detected.
"We've had other fires in the watershed and have procedures in place," he said.
The commission also shut two hydro-electric stations fed by water from the reservoir and cut power to more than 12 miles of lines. The city has been buying power on the open market.
A four-mile stretch of State Route 120, one of three entrances into Yosemite on the west side remains closed. Two other western routes and an eastern route were open.
Los Angeles Times contributed. First Published August 25, 2013 4:00 AM