California's Hetch Hetchy aqueduct, completed in 1936, supplies about 85% of the water used in the San Francisco Bay area. Originating over 160 miles away, at Yosemite's Hetch Hetchy reservoir, and working chiefly by gravity, a 21 mile section of two pipelines crosses the South Bay via a series of above- and under- ground tunnels and bridges just south of the Dumbarton Bridge. This section, near the end of its useful life, has recently been augmented by a deep tunnel.