California's Rim Fire now 20 Percent Contained, but Homes and Resources Still Threatened

California's Rim Fire, which now covers an area larger than Chicago, continues to burn though the Stanislaus National Forest and a portion of Yosemite National Park, but firefighters have more of a handle on the enormous wildfire than at any point to-date.

Relying heavily on fire-fighting airplanes and helicopters, crews made significant gains in battling the blaze, with the latest update from the Incident Information System (Inciweb), which handles California's wildfire information, indicating the wildfire is 20 percent contained, more than double the percent of containment from the day before.

Still, some 4,500 structures are threatened by the blaze, as is the Hetch Hetchy reservoir, which provides water to the San Francisco area some 200 miles away. Ash from the fire raining down on the reservoir has posed an environmental and health concern, though officials in San Francisco said the water quality remains good, NBC News reported. Two of the three hydroelectric power stations on the reservoir were also shut down due to the fire.

The Rim Fire covers an area of about 161,000 acres as of late Monday; it burns within a mile of the Hetch Hetchy reservoir.

Several areas of Yosemite have been closed due to the fire, and a number of popular area campsites have burned, including the Berkeley Tuolumne Family Camp, which has stood as an institution for Bay Area campers since 1922. The camp was completely destroyed Sunday, the San Jose Mercury News reported.

"Our family is feeling pretty devastated because we have so many happy memories there, and it was a place we always look forward to returning," said camper David Kojan about the Tuolumne camp just west of Yosemite.

The Yosemite Valley, home to some of the park's most iconic landmarks such as the Half Dome and El Capitan rock formations, and most other parts of the park remain open to visitors.

The Rim Fire has cost the state of California more than $20 million, the Mercury News reported, adding that President Barack Obama promised federal resources help fight the fire. While the Rim Fire is the largest active wildfire in the nation and the 13th largest in California history, it is one of at least 10 wildfires currently blazing throughout the state at the peak of wildfire season.

About 3,700 firefighters are battling the Rim Fire, which has resulted in two injuries since it ignited on Aug. 17. The cause of the fire remains unknown.