The Rim Fire in Yosemite spread to 110,000 acres overnight and threatened power lines and water systems serving the San Francisco area, causing California Governor Jerry Brown to declare a state of emergency. The fire, one of the most ferocious in California history, is just 2 percent contained and is spreading in two directions, threatening both power transmission lines and pumping stations that San Francisco depends on. On Thursday, the Rim Fire tripled in size, a rate of expansion that may be a record for large wildfires.
85 percent of San Francisco's water comes from the Hetch Hetchy reservoir in the Yosemite area. San Francisco has had to shut two two of its three hydroelectric power stations in Tuolumne County. The city has made up the difference by buying power on the open market. It appears unlikely that the quality of water in the reservoir will be affected, but transmission to the city could be affected.
The Rim Fire is one of more than fifty burning west of the Rockies, in one of the worst fire seasons in the western United States in living memory. In Idaho, an uncontrollable fire is raging in Sun Valley, devastating vast areas of wilderness and ending tourism in the region. Many of the fires are located in rough terrain and wilderness areas, and cannot be controlled by overtaxed firefighters.
The outbreak is largely due to the phenomenon known as 'dry lightning,' which is caused when unsettled weather causes lighting strikes without accompanying rainfall. The protracted western drought has left the region so dry that trees, mostly oil-filled conifers, literally explode when ignited. The western drought, which has been under way for four years, is among the most intense on record and shows little sign of breaking.
So far, and area the size of Connecticut has burned in the western US this summer.