A giant wildfire raging out of control spread into Yosemite National Park on Friday as authorities urged more evacuations in nearby communities where thousands fled during the week as flames marched through the timbered slopes of the western Sierra Nevada.
The fire, which started a week ago, closed backcountry hiking in the park, but was not threatening the popular Yosemite Valley region.
The fire grew from 99 square miles (255 sq. kilometers) to more than 165 square miles (430 sq. kilometers) overnight and was only 2% contained. Smoke blowing across the Sierra into Nevada forced officials in several counties to cancel outdoor school activities and issue health advisories, especially for people with respiratory problems.
"Most of the fire activity is pushing to the east right into Yosemite," said Daniel Berlant, spokesman for the California Department of Forestry and Fire Protection.
The fire was threatening about 4,500 residences, according to the US forest service. Officials issued voluntary evacuation advisories for two new towns on Friday, spokesman Jerry Snyder said. A mandatory evacuation order remained in effect for part of Pine Mountain Lake, a summer gated community that's a few miles from the fire line.
Already, the blaze has destroyed four homes and 12 outbuildings in several different areas, Snyder said. One firefighter also sustained a heat-related injury.
The Hetch Hetchy reservoir supplies San Francisco with 85% of its water, but the city's Public Utilities Commission said in a statement that water quality has not been affected by the fire and was unlikely to be.
Two of three hydroelectric powerhouses in the area have been shut down because of the fire, the commission said, but the city has been able to make up the difference through buying power on the open market.
The park remained open but the blaze closed a 4-mile (6.5-kilometer) stretch of State Route 120, one of three entrances into Yosemite on the west side. The other western routes and an eastern route were open.
Park fire crews worked to clear brush and other fire fuels from the Merced and Tuolumne Groves of Giant Sequoias as a precaution.
Within the park, the blaze was burning on about 17 square miles (44 sq. kilometers) in a remote area around Lake Eleanor, about 4 miles northwest of Hetch Hetchy reservoir, Yosemite spokeswoman Kari Cobb said.
The fire was more than 20 miles (32 kilometers) from Yosemite Valley and skies there were "crystal clear," Cobb said.
The spectacular valley carved by glaciers offers visitors such iconic sights as the Half Dome and El Capitan rock formations and Bridalveil and Yosemite falls.