The wildfire burning in and around Yosemite National Park is now the fourth-largest in California’s history. Covering nearly 350 square miles, the Rim Fire is threatening the Hetch Hetchy Reservoir, which supplies residents in the San Francisco Bay Area with most of their water and power. It’s a lot like the 2012 High Park Fire—which sent ash and debris into the water supply of Fort Collins. These fires offer lessons on the risks wildfires pose to reservoirs. Dr. Bruce McGurk, a former water manager for Hetch Hetchy and a water consultant, speaks with How On Earth contributor Brian Calvert about the risks and future prospects.