U.S. Wildfire Spreads towards San Francisco Water Supply

A wildfire raging on the edge of the Yosemite National Park on Monday threatened both one of America's premier sites of natural beauty and a reservoir that supplies San Francisco.

The massive blaze, which began nine days ago, has grown to become the 13th largest in California's recorded history, authorities said.

A map posted on InciWeb, an inter-agency fire tracking website, shows the inferno nearing the Hetch Hetchy Reservoir, the main source of fresh water for 2.6 million people living in the San Francisco Bay Area, some 200 miles (320 kilometers) to the west.

Local news media reported early Monday that flames from the Rim Fire were a mere five miles away from the reservoir.

The blaze has scorched 149,780 acres (60,614 hectares) and is 15 percent contained, said Cal Fire, the California Department of Forestry and Fire Protection.

Several evacuation orders and advisories are in effect, it said, adding that 4,500 homes remain threatened and that windy conditions could complicate firefighting efforts.

"Winds again today will pose a challenge with gusts of 26 miles per hour (42 kilometers per hour) out of the south, pushing the fire further to the northeast," it said in an update.

Tyrone Jue with the San Francisco Public Utilities Commission told the San Francisco Chronicle that there have been no interruptions in the water supply, and that there has been no change in the quality of drinking water.

The utility said it also had large reserves of water stored locally, which it will make available if the fire interrupts delivery.

However, the city also relies on hydroelectric power from the same region: two of its three power plants were shut down when the fire swept through, Jue told the Chronicle.

The city kept the power flowing by relying on agreements with other utility companies and buying supplemental power.

One of the plants was still too dangerous to reach on Sunday, but repairs on the other were underway and should be completed by Monday, Jue told the paper.

More than 2,800 firefighters, supported by helicopters and air tankers, are struggling to contain the blaze, which started on August 17 from still unknown causes.

Extremely dry conditions due to a prolonged drought, coupled with inaccessible terrain in the affected area, have exacerbated the severity of the incident.

Governor Jerry Brown earlier declared a state of emergency for San Francisco due to the threat to its water and electricity.

The state's firefighting efforts got a boost when California received federal assistance over the weekend to help mobilize the necessary resources.

But the blaze showed little sign of abating.

According to satellite photos, giant columns of white smoke were drifting north into the neighboring state of Nevada.

The Rim Fire "is expected to continue to exhibit very large fire growth due to extremely dry fuels and inaccessible terrain," Inciweb said.