SAN FRANCISCO - The wildfire threatening Yosemite National Park grew Tuesday to become California's seventh biggest ever, as firefighters battled to protect San Francisco's main water supply.
The so-called Rim Fire now covers some 180,000 acres (73,000 hectares) - an area bigger than Chicago - and was still only 20 per cent contained, albeit up from 15 per cent on Monday.
A main road into Yosemite, one of America's top tourist destinations, remains closed as more than 3,700 firefighters battle the flames. "It's burning its way into the record books," Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection, or Cal Fire, told the Los Angeles Times.
The blaze has forced the closure of multiple roads, campgrounds and other facilities in the area, and has also threatened a number of groves of giant sequoia trees, some of the biggest and oldest living organisms on the planet.
It remains more than 15 miles away from the majestic Yosemite Valley area at the heart of the park, visited by millions of tourists every year to see natural wonders including the Half Dome and El Capitan rock formations. But ash from the inferno has reached the reservoir that supplies San Francisco's drinking water.
The Hetch Hetchy Reservoir is the main source of fresh water for 2.6 million people living in the San Francisco Bay Area, located some 200 miles (320 kilometers) to the west.
"Water quality is not currently impacted by the Rim Fire," the San Francisco Public Utilities Commission said in the latest update on its website.
"We are monitoring water quality every minute of each day to ensure its quality and safety."