The drinking water provided to San Francisco and many nearby communities is among the purest in the world. Most of this water comes from Hetch Hetchy reservoir, which is located more than 200 miles away in the Sierras and is fed by melted snow via the Tuolumne River. The system for delivering water from Hetch Hetchy is almost entirely gravity-fed, requiring almost no fossil fuel consumption to move the water from the mountains to the tap. Take an exclusive tour of this unique system with the San Francisco Public Utilities Commission (SFPUC) and see how it is designed to work in wet years and drought years.