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O'Shaughnessy Dam is a high concrete arch-gravity dam in Tuolumne County, California, in the United States. It impounds the Tuolumne River at the lower end of Hetch Hetchy Valley in Yosemite National Park, about east of San Francisco. The dam and reservoir are the source for the Hetch Hetchy Aqueduct, which provides water for over 2 million people in San Francisco and other municipalities of the west Bay Area. The dam is named for engineer Michael O'Shaughnessy, who oversaw its construction. Although San Francisco had sought Tuolumne River water as early as the 1890s, this project did not move forward until the disastrous earthquake and fire of 1906, which underscored the insufficiency of the existing water supply. The Hetch Hetchy Valley – then compared to Yosemite Valley for its scenic beauty – was chosen for its water quality and hydroelectric potential, but the location within the national park generated controversy. An act of Congress was required to circumvent federal protection of the Tuolumne River, with the reasoning that public land should be developed for the public benefit. Construction of the dam started in 1919 and was finished in 1923, with the first water delivered in 1934 after numerous delays. From 1935–38 the dam was raised to increase its capacity for water supply and power generation. The dam, aqueduct and appurtenant hydroelectric systems are collectively known as the Hetch Hetchy Project. Deriving from a largely wild and pristine area of the Sierra Nevada, the Hetch Hetchy supply is some of the cleanest municipal water in the US, requiring only primary filtration and disinfection. Hetch Hetchy represented the first great environmental controversy in the US, and debate over the dam and reservoir continues today. Preservationist groups such as the Sierra Club lobby for the restoration of the valley, while others argue that leaving the dam in place would be the better economic and environmental decision. ==Background== In the late 19th century, the city of San Francisco was rapidly outgrowing its limited water supply, which depended on intermittent local springs and streams. The city looked east to the Sierra Nevada, where snowmelt fed the headwaters for many of California's largest rivers. In 1890, San Francisco mayor James D. Phelan proposed to build a dam and aqueduct on the Tuolumne River, one of the largest southern Sierra rivers, as a way to increase and stabilize the city's water supply. In 1900, a United States Geological Survey (USGS) report also described the Tuolumne River as "the best source of sustainable water for San Francisco". Although Phelan managed to secure water rights for the Tuolumne River in 1901, his appeals to the federal government for development of the Hetch Hetchy Valley were unsuccessful. But when the 1906 San Francisco earthquake and subsequent fire razed the city, the dangerous inadequacies of the city's water supply system were brought to national light. Out of fourteen potential water sources considered by the city – which included Lake Tahoe, the Eel River, and tributaries of the Sacramento and San Joaquin Rivers – Hetch Hetchy was considered superior for its excellent dam site, abundant sediment-free water, lower cost and hydroelectric potential. At the time, Hetch Hetchy was an isolated, seldom visited subalpine valley, visited intermittently by gold seekers and sheepherders. However, since 1890, Hetch Hetchy Valley and the surrounding lands had been part of Yosemite National Park and thus off-limits to utility development, let alone at the grand scale proposed by the city. Even though the valley was not well known to the general public, organizations such as the Sierra Club treasured it for its spectacular beauty, often compared to that of Yosemite Valley itself. Led by naturalist and mountaineer John Muir, the Sierra Club adamantly opposed the city of San Francisco as it sought permission from the federal government to build a dam in the valley.〔 In 1908 Secretary of the Interior James R. Garfield responded to San Francisco's appeal, granting the city rights to development at Hetch Hetchy, stating that "Hetch Hetchy was not unique, a lake would be even more beautiful than its meadow floor and the hydroelectric power generated could eventually pay for the costs of construction." One of the strongest supporters of the Hetch Hetchy project was Gifford Pinchot, Chief Forester of the United States Forest Service, who pushed a policy of "conservation through use", promoting the sustainable development of natural resources in the U.S. On December 19, 1913, President Woodrow Wilson signed the Raker Act, which permitted San Francisco's development of the Hetch Hetchy project on the terms that water and power derived from the project could only be used for public utilities, not private profit. Though highly controversial, the bill passed the Senate by a vote of 43 for and 25 against. The consensus was that since Hetch Hetchy lay on public land, it was reasonable for its natural resources to be developed for the public benefit. Muir, the Sierra Club and other groups were outraged by the federal government's permission for development at Hetch Hetchy. However, on December 24, 1914, with construction on the dam barely underway, Muir died, leaving his Sierra Club to fight a protracted battle against the Hetch Hetchy Project over the next ten years.〔 "Dam Hetch Hetchy!" Muir had said – "As well dam for water tanks the peoples' cathedrals and churches, for no holier temple has been consecrated by the heart of man!" The Sierra Club argued that it was not necessary for San Francisco to destroy the valley for its water supply, pointing out the availability of other sites with reasonable proximity – including the Mokelumne River, which the U.S. Army Corps of Engineers reported in 1913 as "a better and cheaper source than Hetch Hetchy". (The Mokelumne was later dammed, in a similar scheme to the Hetch Hetchy project, to provide water to the East Bay.) By this point, however, San Francisco had become "obsessed" with developing Hetch Hetchy, and "dismissed or discarded other rivers and valleys that would have served them better ... as if it was created for their purpose." 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「O'Shaughnessy Dam (California)」の詳細全文を読む スポンサード リンク
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