Hetch Hetchy is a valley, a reservoir, and a water system in California in the United States. The glacial Hetch Hetchy Valley lies in the northwestern part of Yosemite National Park and is drained by the Tuolumne River. For thousands of years before the arrival of settlers from the United States in the 1850s, the valley was inhabited by Native Americans who practiced subsistence hunting-gathering. During the late 19th century, the valley was renowned for its natural beauty – often compared to that of Yosemite Valley – but also targeted for the development of water supply for irrigation and municipal interests. The controversy over damming Hetch Hetchy became mired in the political issues of the day. The law authorizing the dam passed Congress on December 7, 1913. In 1923, the O'Shaughnessy Dam was completed on the Tuolumne River, flooding the entire valley under the Hetch Hetchy Reservoir. The dam and reservoir are the centerpiece of the Hetch Hetchy Project, which in 1934 began to deliver water west to San Francisco and its client municipalities in the greater San Francisco Bay Area.
Geography
Before damming, the high granite formations produced a valley with an average depth of and a maximum depth of over ; the length of the valley was with a width ranging from . The valley floor consisted of roughly of meadows fringed by pine forest, through which meandered the Tuolumne River and numerous tributary streams. Kolana Rock, at , is a massive rock spire on the south side of the Hetch Hetchy Valley. Hetch Hetchy Dome, at , lies directly north of it. The locations of these two formations roughly correspond with those of Cathedral Rocks and El Capitan seen from Tunnel View in Yosemite Valley. A broad, low rocky outcrop situated between Kolana Rock and Hetch Hetchy Dome divided the former meadow in two distinct sections.
The valley is fed by the Tuolumne River, Falls Creek, Tiltill Creek, Rancheria Creek, and numerous smaller streams which collectively drain a watershed of . In its natural state, the valley floor was marshy and often flooded in the spring when snow melt in the high Sierra cascaded down the Tuolumne River and backed up behind the narrow gorge which is now spanned by O'Shaughnessy Dam. The entire valley is now flooded under an average of water behind the dam, although it occasionally reemerges in droughts, as it did in 1955, 1977, and 1991.
Upstream from the valley lies the Grand Canyon of the Tuolumne, while the smaller Poopenaut Valley is directly downstream from O'Shaughnessy Dam. The Hetch Hetchy Road drops into the valley at the dam, but all points east of there are roadless, and accessible only to hikers and equestrians. The O'Shaughnessy Dam is near Yosemite's western boundary, but the long, narrow, fingerlike reservoir stretches eastward for about .
Wapama Falls, at , and Tueeulala Falls, at – both among the tallest waterfalls in North America – are both located in Hetch Hetchy Valley. Rancheria Falls is located farther southeast, on Rancheria Creek. Formerly, a "small but noisy" waterfall and natural pool existed on the Tuolumne River marked the upper entrance to Hetch Hetchy Valley, informally known as Tuolumne Fall (not to be confused with a similarly named waterfall several miles upriver near Tuolumne Meadows). The waterfall on the Tuolumne is now submerged under Hetch Hetchy Reservoir.
Geology
The Hetch Hetchy Valley began as a V-shaped river canyon cut out by the ancestral Tuolumne River. About one million years ago, the extensive Sherwin glaciation widened, deepened and straightened river valleys along the western slope of the Sierra Nevada, including Hetch Hetchy, Yosemite Valley, and Kings Canyon farther to the south. During the last glacial period, the Tioga Glacier formed from extensive icefields in the upper Tuolumne River watershed; between 110,000 and 10,000 years ago Hetch Hetchy Valley was sculpted into its present shape by repeated advance and retreat of the ice, which also removed extensive talus deposits that may have accumulated in the valley since the Sherwin period. At maximum extent, Tioga Glacier may have been long and up to thick, filling Hetch Hetchy Valley to the brim and spilling over the sides, carving out the present rugged plateau country to the north and southwest. When the glacier retreated for the final time, sediment-laden meltwater deposited thick layers of silt, forming the flat alluvial floodplain of the valley floor.