A wildfire raging along the northwest edge of Yosemite National Park gained strength Saturday morning, threatening the transmission lines that serve San Francisco, as firefighters scrambled to protect nearby mountain communities.

The fire held steady overnight at nearly 200 square miles, 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," said Cal Fire spokesman Daniel Berlant. "Typically, it doesn't really heat up until early afternoon. We could continue to see this fire burn very rapidly today."

The Rim Fire started in a remote canyon of the Stanislaus National Forest a week ago and is just 5 per cent contained. More than 5,500 homes are threatened, four have been destroyed and voluntary and mandatory evacuations are underway.

'There's a lot of potential for this one to continue to grow.'—Cal Fire spokesman Daniel Berlant

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," Berlant said. "There's a lot of potential for this one to continue to grow."

After burning for nearly a week on the edges of Yosemite, the fire moved into the northwest boundary of the park Friday. 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 32 kilometres away.

Dry fuel and hot weather have combined already to make this the 16th largest fire in California's history. More than 2,600 firefighters and a half dozen aircraft are battling the blaze.

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More than 2,000 firefighters have been recruite to battle the blaze. (Max Whittaker/Reuters)

This has been a particularly busy fire season in California and throughout the West because a lack of winter rains and snow have left forests extremely dry. This year so far Cal Fire and the U.S. Department of Forestry have fought 5,700 fires, compared with 4,900 by this date last year.

The fire is burning toward the Hetch Hetchy reservoir, where San Francisco gets 85 per cent of its water and power for municipal buildings, the international airport and San Francisco General Hospital. Gov. Jerry Brown declared a state of emergency because of the threats.

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.

2 hydro-electric stations shut down

Deputy General Manager Michael Carlin told the Associated Press on Saturday that they fear ash could fall into the reservoir and cause a federal water quality violation. No problems were noted by Saturday morning.

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A smoky haze blankets the main downtown casino strip of Reno as health officials raised an air quality alert due to the massive wildfires burning in California. (Scott Sonner/Associated Press)

Carlin said that ash is non-toxic, but it can create a cloudiness that would force the city to enact filtering contingency plans on water stored in reservoirs in the Bay Area.

"So far the fire is still [four to five kilometres]

 below O'Shaughnessy Dam," Carlin said. "We've had other fires in the watershed and have procedures in place."

The commission also shut two hydro-electric stations fed by water from the reservoir and cut power to more than 19 kilometres of lines. Carlin said they didn't know Saturday whether the lines were destroyed.

"The fire has passed through that area and we're trying to get back in today to see what damage has occurred to plan our restoration efforts," Carlin said.

The city has been buying power on the open market.

'No visitor services are being affected'

While smoke is not present in Yosemite Valley, across the Sierra into neighboring Nevada smoke warnings have forced cancellation of some outdoor events.

Park spokeswoman Kari Cobb said that the park had stopped issuing backcountry permits to backpackers and had warned those who already had them to stay out of the area.

'We just have to take it one day at a time.'—Kari Cobb, Yosemite Park spokesman

"Right now there are no closures, and no visitor services are being affected in the park," Cobb said. "We just have to take one day at a time."

But a six-kilometre stretch of State Route 120, one of three entrances into Yosemite on the west side remains closed because fire has burned on both sides. Two other western routes and an eastern route were open.

On Friday Officials issued voluntary evacuation advisories for two towns — Tuolumne City, population 1,800, and Ponderosa Hills, a community of several hundred — which are about eight kilometres from the fire line, Forest Service spokesman Jerry Snyder said.

A mandatory evacuation order remained in effect for part of Pine Mountain Lake, a summer gated community a few miles from the fire.

More homes, businesses and hotels are threatened in nearby Groveland, a community of 600 about eight kilometres from the fire and 40 kilometres from the entrance of Yosemite.

Usually filled with tourists, the streets are now swarming with firefighters, evacuees and news crews, said Doug Edwards, owner of Hotel Charlotte on Main Street.

"We usually book out six months solid with no vacancies and turn away 30-40 people a night. That's all changed," Edwards said. "All we're getting for the next three weeks is cancellations. It's a huge impact on the community in terms of revenue dollars."