The Hetch Hetchy Project provides 80% of the water supply for 2.6 million people in the City and County of San Francisco and several surrounding municipalities in the greater San Francisco Bay Area. ^[1] The project is operated by the San Francisco Public Utilities Commission. The city must pay a lease of $30,000 per year for the use of the Hetch Hetchy Valley, which sits on federal land. ^{[2] [3]} The aqueduct delivers an average of 265,000 acre⋅ft (327,000 dam³) of water each year, or 31,900,000 cu ft (900,000 m³) per day, to residents of San Francisco and San Mateo, Santa Clara and Alameda Counties. ^[4]

Route

The O'Shaughnessy Dam impounds water in the Hetch Hetchy Valley, forming the Hetch Hetchy Reservoir, the origin of the Hetch Hetchy Project. As completed, O'Shaughnessy Dam is 910 feet (280 m) long, spanning the valley at its narrow outlet. ^[5] The dam contains 675,000 cu yd (516,000 m³) of concrete. The Hetch Hetchy Reservoir created by the dam has a capacity of 360,400 acre⋅ft (0.4445 km³), with a maximum area of 1,972 acres (798 ha) and a maximum depth of 306 feet (93 m). ^[5] From Hetch Hetchy Reservoir, the water flows through the Canyon and Mountain Tunnels to Kirkwood and Moccasin Powerhouses, which have capacities of 124 and 110 megawatts, respectively. ^[6] An additional hydroelectric system comprising Cherry Lake, Lake Eleanor and the Holm Powerhouse is also part of the Hetch Hetchy Project, adding another 169 megawatts of generating capacity. ^[6] The entire system produces about 1.7 billion kilowatt hours per year, enough to meet 20% of San Francisco's electricity needs. ^{[6] [7]}

After passing through the powerhouses, Hetch Hetchy water flows into the 167 mi (269 km) Hetch Hetchy Aqueduct which travels across the Central Valley. Upon reaching the Bay Area near the city of Fremont, the aqueduct splits into four pipelines. These are called Bay Division Pipelines (BDPL) 1, 2, 3, and 4, with nominal pipeline diameters of 60, 66, 78, and 96 inches (1.5, 1.7, 2.0 and 2.4 m, respectively). ^[8] All four pipelines cross the Hayward fault. Pipelines 1 and 2 cross the San Francisco Bay to the south of the Dumbarton Bridge, while pipelines 3 and 4 run to the south of the bay. In the Bay Area, Hetch Hetchy water is stored in local facilities including Calaveras Reservoir, Crystal Springs Reservoir, and San Antonio Reservoir. ^[9] Pipelines 3 and 4 end at the Pulgas Water Temple, a small park that contains classical architectural elements which celebrate the water delivery. ^[10]

Water from Hetch Hetchy is some of the cleanest municipal water in the United States; San Francisco is one of six U.S. cities not required by law to filter its tap water, although the water is disinfected by ozonation and, since 2011, exposure to UV. ^[11] The water quality is high because of the unique geology of the upper Tuolumne River drainage basin, which consists mostly of bare granite; as a result, the rivers feeding Hetch Hetchy Reservoir have extremely low loads of sediments and nutrients. The watershed is also strictly protected, so swimming and boating are prohibited at the reservoir (although fishing is permitted at the reservoir and in the rivers which feed it). ^[12]

References

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