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in dollars) | owner = U.S. Bureau of Reclamation | dam_type = Concrete thick arch-gravity | dam_height = 〔 | dam_height_thalweg = | dam_height_foundation= | dam_length = 〔 | dam_width_crest = 〔 | dam_width_base = 〔 | dam_volume = 〔 | dam_elevation_crest = 〔 | dam_crosses = Colorado River | spillway_count = | spillway_type = Twin concrete tunnels, controlled by double radial gates | spillway_capacity = 〔 | res_name = Lake Powell | res_capacity_total = | res_capacity_active = | res_capacity_inactive= | res_catchment = | res_surface = | res_elevation = (max) | res_max_depth = | res_max_length = | res_max_width = | plant_hydraulic_head = (max)〔 | res_tidal_range = | plant_operator = | plant_commission = 1964〔 | plant_decommission = | plant_type = | plant_turbines = 5× 165 MW, 3× 157 MW Francis〔 | plant_capacity = 1296 MW〔 | plant_annual_gen = 3.46 billion KWh〔 | website = | extra = }} Glen Canyon Dam is a concrete arch dam on the Colorado River in northern Arizona in the United States, near the town of Page. The dam was built to provide hydroelectricity and flow regulation from the upper Colorado River Basin to the lower. Its reservoir is called Lake Powell, and is the second-largest artificial lake in the country, extending upriver well into Utah. The dam is named for Glen Canyon, a colorful series of gorges, most of which now lies under the reservoir. The dam was proposed in the 1950s as part of the Colorado River Storage Project, a U.S. Bureau of Reclamation (USBR) federal water project that would develop reservoir storage on the upper Colorado River and several of its major tributaries. The project's main purpose was to provide water storage to ensure the delivery of sufficient water to the lower basin during years of drought, so as to allow the upper basin to better utilize its allocation of river flow as designated in the 1922 Colorado River Compact. However, problems arose when the USBR proposed to build dams in the federally protected Echo Park canyon in Utah. After extensive policy disputes and legal challenges with environmental organizations such as the Sierra Club, they settled for a high dam at Glen Canyon. Construction of Glen Canyon Dam started in 1956 and was not finished until 1966. When the reservoir filled, the dam began to deliver a regulated flow of water downstream and a supply of electricity to the region. In 1983, major floods nearly led to the dam's collapse, but disaster was averted by a close margin. By suppressing floods and other factors that once characterized the Colorado, the dam has led to major physical and ecological changes in the lower river. Controversy continues over the effects both positive and negative of the dam, which has also been antagonized in many literary works. ==Background== In 1922, six U.S. states signed the Colorado River Compact to officially allocate the flow of the Colorado River and its tributaries. Each half of the Colorado River Basin – the upper basin, comprising Colorado, New Mexico, Utah and Wyoming – and the lower basin, with California and Nevada – was allotted of water annually, and a treaty between the U.S. and Mexico was signed in 1944 allocating to the latter country. The third lower basin state, Arizona, did not ratify the Compact until 1944 because it was concerned that California might seek to appropriate a portion of its share before it could be put to use. The total, annually, was believed to represent the river's flow at the time as measured at Lees Ferry, Arizona (the official dividing point of the upper and lower basins), downstream of present-day Glen Canyon Dam, while in fact it turned out to be the result of one of the Southwest's climatic shifts during the past 800 years. The dependable natural flow past Lees Ferry is now believed to be about . Annual discharge from the Colorado River and its tributaries ranges from , and 10-year averages may fluctuate as much as .〔 In addition, the Colorado carries a heavy silt load that led to difficulties for the irrigation interests in the lower Colorado River Valley that were a primary benefactor of the treaties. The general consensus among inhabitants of the Colorado River basin and government officials was that a high dam had to be built on the Colorado to provide flood control and carry-over water storage for times of drought. Possible locations for this dam were debated for years, and in fact the Bureau of Reclamation's first study for a dam at Glen Canyon was made in 1924, in addition to studies for locations at Black and Boulder Canyons lower on the Colorado, below Grand Canyon.〔Rogers, p. 9〕 However, these studies found that the lower Colorado sites had stronger foundation rock which might result in less reservoir seepage. The Glen Canyon site, furthermore, was so remote that delivering supplies and transporting workers there would be infeasible at the time. The initial need for a reservoir was realized in 1936 with the completion of Hoover Dam in Black Canyon, marking the first time man held "control" of the Colorado.〔Stevens, pp. 15-18〕 However, even with Lake Mead's mammoth storage capacity, it was not able to handle the worst floods or droughts, and was filling with sediment at a rate that would render it useless in a few hundred years. But most importantly, Hoover only controlled the lower portion of the river. The upper basin states, whose rivers flowed wild and free, had no way to ensure they could utilize their water allotment in dry years because of the lack of sufficient storage. Arizona also had qualms over the chosen site of Hoover Dam, because it was located in a relatively inaccessible northwestern corner of the state and was too far to provide water to the Gila River Valley, its major population center. A dam at Glen Canyon, just upstream of Lee's Ferry, would both be located entirely within that state and provide much of the power needed to pump water from the Colorado River to Phoenix and Tucson. Finally, the Glen Canyon dam would provide flow regulation between Lee's Ferry and Lake Mead which would make it more economically feasible for the USBR to go ahead with even more ambitious plans to construct hydroelectric generating facilities in the Grand Canyon as part of the Pacific Southwest Water Plan (see Bridge Canyon Dam). This lack of water surcharge or insurance for the upper Colorado River basin led to a demand for what would later become the Colorado River Storage Project. The general outline of this project was for a dam on the Colorado River at Glen Canyon, several other dams on the Gunnison and San Juan tributaries of the Colorado, and a pair of dams to be built on the Green River, the Colorado's major upper tributary, at Echo Park and Split Mountain. The two Green River dams would have submerged more than of canyons in the federally protected Dinosaur National Monument, a move abhorred by environmentalists who did not want to see a repeat of the 1924 O'Shaughnessy Dam controversy, when a dam was built in a scenic valley in Yosemite National Park. Led by David Brower, the environmental organization Sierra Club fought a protracted battle against the Bureau of Reclamation, on the basis that "building the dam would not only destroy a unique wilderness area, but would set a terrible precedent for exploiting resources in America's national parks and monuments".〔Billington, Jackson and Melosi, p. 398〕 In the mid-1950s, the USBR agreed not to build the two dams – an act widely hailed as a major victory for the American environmentalist movement – but in exchange for a dam at Flaming Gorge, upriver from Echo Park, and increasing the size of the proposed dam at Glen Canyon to replace the storage that would have been provided by the Echo Park dam on the Green River.〔Billington and Jackson, p. 337〕 In fact, Brower and the Sierra Club supported the expansion of the dam at Glen Canyon. The only qualm that the environmentalists had about the proposed Glen Canyon Dam was that high elevations of its reservoir would extend into Rainbow Bridge National Monument, and a proposal to build a barrier to keep water out of the monument was fought over and litigated for years until it was permanently shelved in 1973.〔Martin, pp. 304–307〕 The Colorado River Storage Project was authorized in April 1956, and groundbreaking of Glen Canyon Dam began in October of the same year.〔Rogers, p. 16〕 A common misconception is that the environmentalists were given a choice between damming Echo Park and damming Glen Canyon, but the USBR "had always planned to build a dam at Glen Canyon, regardless of the outcome of the Echo Park debate".〔Rogers, p. 13〕 In 1963, when construction on the dam was well underway, the Sierra Club published a book on Glen Canyon, ''The Place No One Knew'', featuring photographs by Eliot Porter, and lamenting the loss of the scenic gorge before most of the American public had a chance to visit, or indeed know it. Brower had visited Glen Canyon shortly after the decision to build the dam, and "realized once he arrived that this was not a place for a reservoir". Though little known to most Americans before Porter's book, Glen Canyon had been visited by a handful of hikers and boaters (John Wesley Powell for whom the reservoir is named, leader of the Powell Geographic Expedition of 1869, among them had explored the canyon pre-dam), and some had even been interviewed by Brower. As said to Brower by writer Wallace Stegner, who had been to the canyon in 1947, "Echo doesn't hold a candle to Glen."〔Rogers, p. 14〕 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「Glen Canyon Dam」の詳細全文を読む スポンサード リンク
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