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The Marble Canyon Dam, also known as the Redwall Dam, was a never-built dam on the Colorado River in Arizona. The dam was proposed to impound a relatively small reservoir in the central portion of Marble Canyon to develop hydroelectic power. Plans centered on two sites between miles 30 and 40 in the canyon. At one point a tunnel was proposed to a site just outside Grand Canyon National Park to develop the site's full power generation potential, reducing the Colorado River to a trickle through the park. Although first proposed in the 1920s to generate hydroelectricity, work did not begin until the dam was incorporated as part of the U.S. Bureau of Reclamation (USBR)'s Pacific Southwest Water Plan in the 1940s for its Central Arizona component. Together with Bridge Canyon Dam, located at the lower end of the Grand Canyon, it would have provided the hydroelectric power necessary to lift Colorado River water from Lake Havasu to central Arizona's farms and cities, including Phoenix and Tucson. The two dams would have operated as "cash register" facilities to provide funds for future reclamation projects through the sale of cheap hydropower. After a series of studies and site investigations, the dam was abandoned as a project in order to facilitate legislation creating the Central Arizona Project. The dam sites were incorporated into Marble Canyon National Monument in 1968, which was absorbed into Grand Canyon National Park in 1975. ==Project description== First proposed in the 1940s by the United States Bureau of Reclamation, the location in Marble Canyon was regarded as one of the most remote and difficult-to-access dam sites in the United States, about below the rim of the canyon. The proposed dam was of moderate size, about high, using a thin-arch concrete design and retaining about 363,000 acre-feet.〔 To keep the small reservoir free of silt, silt retention dams were needed on the tributary canyons. The retention dam in Paria Canyon was planned to be tall, and only wide at the base and wide at the crest, retaining 100 years of silt deposits. Despite these measures, the Marble Canyon reservoir was projected to have a lifespan of 104 years before filling with silt. One appealing aspect of a Marble Canyon damsite was the potential to develop the Grand Canyon's hydroelectric potential without building a dam or reservoir in Grand Canyon National Park by diverting about 90% of the Colorado River's waters at Marble Canyon into a tunnel, in diameter and capable of carrying about , to a site in the western part of the canyon in what was then Kaibab National Forest. The plan would provide about of hydraulic head for power generation. The power plant would produce about 6.5 billion kilowatt hours (KWh) per year – almost twice that of Hoover Dam. The tunnel would allow the hydroelectric system to effectively bypass Grand Canyon National Park, avoiding the construction of more dams within the park between Marble and Bridge.〔(【引用サイトリンク】title=National Park Service: A Survey of the Recreational Resources of the Colorado River Basin (Chapter 7) )〕 A flow of would be released from Marble Canyon at all times – a "scenic trickle" – for wildlife and recreational purposes in the Grand Canyon. This water would be released through a 22 megawatt power station at the base of the dam, with an annual energy production of 164 million kilowatt hours.〔(【引用サイトリンク】title=National Park Service: A Survey of the Recreational Resources of the Colorado River Basin (Chapter 7) )〕 Together, the two dams would utilize most of the elevation drop between Lee's Ferry and Lake Mead, which represents one of the biggest untapped hydroelectric potentials in the United States. A number of accessory lakes or pools were projected in the adjacent Deer Creek drainage, or possibly at a dam on Kanab Creek. The power station at Deer Creek would have been at the head of the reservoir backed up by the never-built Bridge Canyon Dam. By the 1960s the tunnel project had been dropped in favor of a 600 MW power station at the dam's base, reducing the project's costs considerably.〔〔 The two dams would be operated as "cash register" power plants, meaning that sale of hydroelectricity would pay for their construction cost as well as provide funds for future reclamation projects. The power thus produced would be vital for pumping along the Central Arizona Project (CAP), which would lift water from the Colorado River near Lake Havasu to central and southern Arizona. The pumped water would reach as far as Tucson, which sits about higher than the Colorado River. The Bureau had even grander visions, too: eventually, this generated revenue would be necessary to fund water import projects from the Columbia River system to the Colorado River, which continues to face shortages due to over-appropriation of its flow. Thus, "hydropower at Bridge and Marble Canyon Dams was viewed as an instrument and not as a major goal in itself". 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「Marble Canyon Dam」の詳細全文を読む スポンサード リンク
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