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.

The valley floor consisted of roughly 1,200 acres (490 ha) of meadows fringed by pine forest, through which meandered the Tuolumne River and numerous tributary streams.

[4] A broad, low rocky outcrop situated between Kolana Rock and Hetch Hetchy Dome divided the former meadow in two distinct sections.

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 O'Shaughnessy Dam is near Yosemite's western boundary, but the long, narrow, fingerlike reservoir stretches eastward for about 8 miles (13 km).

[12] During the last glacial period, the Tioga Glacier[13] 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.

[14] At maximum extent, Tioga Glacier may have been 60 mi (97 km) long and up to 4,000 ft (1,200 m) 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.

[15] 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.

[18] Due to its abundant wetlands and stream pools, Hetch Hetchy was notorious among early travelers for becoming infested with mosquitoes in the summertime.

During summer, people of the Miwok and Paiute came to Hetch Hetchy from the Central Valley in the west and the Great Basin in the east.

[20] They hunted, and gathered seeds and edible plants to furnish themselves winter food, trade items, and materials for art and ceremonial objects.

Today, descendants of these people still use milkweed, deergrass, bracken fern, willow, and other plants for a variety of uses including baskets, medicines, and string.

[21] Periodic clearing of the valley provided ample space for the growth of the grasses and shrubs they relied on, as well as additional room for large game animals such as deer to browse.

This was likely because of Hetch Hetchy's narrow outlet, which in years of heavy snowmelt created a bottleneck in the Tuolumne River and the subsequent flooding of the valley floor.

[5] Local legend attributes the modern name Hetch Hetchy to Screech's initial arrival in the valley, during which he observed the Native Americans "cooking a variety of grass covered with edible seeds", which they called "hatch hatchy" or "hatchhatchie".

[31] During this time, the upper Tuolumne River, including Hetch Hetchy Valley, was visited by prospectors attracted by the California Gold Rush.

[31] Its meadows provided abundant feed for "thousands of head of sheep and cattle that entered lean and lank in the spring, but left rolling fat and hardly able to negotiate the precipitous and difficult defiles out of the mountains in the fall.

[21][33] Albert Bierstadt, Charles Dorman Robinson and William Keith were known for their landscapes that drew tourists to the Hetch Hetchy Valley.

[41] In 1906, after a major earthquake and subsequent fire that devastated San Francisco, the inadequacy of the city's water system was made tragically clear.

The U.S. Congress passed and President Woodrow Wilson signed the Raker Act in 1913, which permitted the flooding of the valley under the conditions that power and water derived from the river could only be used for public interests.

Ultimately, San Francisco sold hydropower from the dam to the Pacific Gas and Electric Company (PG&E), which led to decades of legal wrangling and controversy over terms in the Raker Act.

The Great Alaskan Land Fraud and the Pinchot-Ballinger Controversy caused both Richard A. Ballinger and Gifford Pinchot to resign and be fired respectively.

[2] 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.

Those in favor of dam removal have pointed out that many actions by San Francisco since 1913 have been in violation of the Raker Act, which explicitly stated that power and water from Hetch Hetchy could not be sold to private interests.

[69] In 1987, the idea of razing the O'Shaughnessy Dam gained an adherent in Don Hodel, Secretary of the Department of the Interior under President Ronald Reagan.

[71] The dam would not have to be completely removed; rather, it would only be necessary to cut a hole through the base in order to drain the water and restore natural flows of the Tuolumne River.

[citation needed] The George W. Bush administration proposed allocating $7 million to studying the removal of the dam in the 2007 National Park Service budget.

"[83] Opponents of dam removal have pointed out that the flooding of the Hetch Hetchy Valley has also deterred the crowds that overrun other areas of Yosemite National Park.

[85] In November 2012, San Francisco voters soundly rejected Proposition F,[86] which would have required the city to conduct an $8 million study on how the flooded valley could be drained and restored to its former state.