When you turn on a tap in San Francisco, much of that water started in a valley in Yosemite National Park that has been underwater for a century.
A valley compared with Yosemite
The glacial Hetch Hetchy Valley sits in the northwest corner of Yosemite and is drained by the Tuolumne River. Before the dam, Wikipedia says the valley floor had about 1,200 acres of meadows ringed by pine forest. Granite landmarks such as Kolana Rock and Hetch Hetchy Dome sit in roughly the same positions as Cathedral Rocks and El Capitan in Yosemite Valley. Native Americans lived in the valley for thousands of years before settlers arrived in the 1850s.
The dam fight
In the late 19th century the valley was famous for its beauty, and it was also targeted as a water supply. The fight over damming it got caught up in the politics of the day. The law authorizing the dam passed Congress on Dec. 7, 1913. O'Shaughnessy Dam was finished on the Tuolumne in 1923, and the valley disappeared under an average of about 300 feet of water. Starting in 1934, the Hetch Hetchy Project began sending water 167 miles west to San Francisco and other Bay Area cities. Wikipedia notes the old valley floor has partly reappeared during droughts, including in 1955, 1977 and 1991.
Visiting
The National Park Service says Hetch Hetchy sits at 3,900 feet and has one of the park's longest hiking seasons. A shoreline trail starts by crossing the dam and going through a tunnel, with views of Tueeulala and Wapama falls. Rancheria Falls is 6.5 miles in, and you can do it as a long day hike or an overnight trip. Swimming and boating are banned in the reservoir to keep the drinking water clean. Pets aren't allowed on the dam or the trails. The park also warns hikers to carry plenty of water and watch for rattlesnakes and poison oak.