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Dworshak Dam is a concrete gravity dam in the western United States, on the North Fork Clearwater River in Clearwater County, Idaho. The dam is located approximately northeast of Orofino and impounds the Dworshak Reservoir for flood control and hydroelectricity generation. With a height of , Dworshak is the third tallest dam in the United States and the tallest straight-axis concrete dam in the Western Hemisphere. Construction of the dam by the U.S. Army Corps of Engineers (USACE) began in 1966 and was completed in 1973. ==History== The proposal for Dworshak Dam originated in a 1953 USACE survey of the lower Snake River drainage basin for suitable sites to develop reservoir storage and hydroelectric power generation. Dworshak was one of seven dam sites considered on the Snake, Salmon and Clearwater River systems. The final site for Dworshak Dam was chosen at a point on the North Fork of the Clearwater above its confluence with the larger Clearwater River. The project was authorized on October 23, 1962 as Bruce's Eddy Dam; the name was later changed to honor U.S. Senator Henry Dworshak (1894–1962), who was instrumental in gaining congressional approval for the dam project.〔 The dam was controversial from the start because it would block fish migration and its reservoir would flood a large portion of the winter range of elk in the Clearwater River basin. The North Fork of the Clearwater River had an excellent run of steelhead trout and "may have been the finest population of large-size steelhead in the world". However, proponents of the project held that the construction of a fish hatchery would maintain the river's steelhead runs, and that the construction of the dam would render other dam projects in the Clearwater River basin unnecessary. Another big incentive for the project was flood control; supporters of Dworshak cited a devastating flood in 1948, when the Clearwater River reached a peak of – eleven times its normal flow – as further reason for the construction of a large storage dam. While clearing and preparations at the dam site were underway as early as June 1965, actual construction did not begin until early 1966 with the excavation of a diameter diversion tunnel and the creation of a high cofferdam to divert the river around the dam site. Excavation of keyways in the canyon walls to provide future foundations for the concrete commenced in 1966 and continued through 1968. The first bucket of concrete was placed in early 1968, and by May 28, 1969, more than had been poured in the dam. Concrete placement was accomplished by a cableway system supported by three movable towers on the canyon rims, transporting buckets that each had a capacity of of wet concrete. The diversion tunnel was closed on September 27, 1971, allowing the reservoir to begin filling. The last concrete was poured on the dam crest on January 27, 1973. Three generating units of the power plant were also installed in 1973, and by March 1, the reservoir had risen to a sufficient level to allow power generation to begin.〔 The reservoir first reached full capacity on July 3, 1973. After the dam was completed the USACE acquired of land adjacent to the reservoir (later expanded to ) and has since managed it for winter habitat of elk and white-tailed deer.〔 Because Dworshak Dam is too high for a fish ladder to be economically feasible, the USACE constructed the Dworshak National Fish Hatchery (DNFH) at a cost of $21 million to mitigate losses of the North Fork steelhead run caused by Dworshak Dam. The hatchery is located on the small peninsula of land between the North Fork and the main Clearwater River directly above their confluence. Now operated by the U.S. Fish and Wildlife Service, the fish hatchery actually went into operation in April 1969, four years before the dam was completed. The hatchery has a capacity of 6,000 adult fish and releases about 3.4 million juveniles into the river system each year.〔 The Dworshak Dam power station was designed to accommodate six generating units for purposes of peaking power production. However, this form of operation would cause excessively large flow fluctuations on the North Fork Clearwater River, and the main Clearwater River below their confluence at Ahsahka. A dam was to be built on the Clearwater River above Lenore to provide a forebay for smoothing out Dworshak releases, but due to opposition by local residents it was never built.〔 Although it would have been fitted with a fish ladder, the Lenore dam would have affected steelhead and salmon migration to the entire upper Clearwater River and its tributaries, including the South Fork, Middle Fork, Lochsa and Selway Rivers. In addition, the flooding caused by its -long reservoir would require the relocation of the Dworshak fish hatchery. The proposed fifth and sixth units of the plant were deauthorized in 1990, and the fourth unit was deauthorized in 1995.〔 During initial filling of the reservoir, the dam developed several large vertical cracks on the upstream side – some more than long – due to the rapid change of water pressure from the rising reservoir. Workers had to drill diameter drain holes between the cracks and the dam's existing foundation drainage system, or "drainage gallery", at intervals, relieving the pressure on the dam's backside. In June 1980, the dam again experienced leakage problems when a crack opened on its upstream face, sending more than of water spraying across the downstream side of the dam. Seventy drainage holes were drilled into the crack to relieve pressure before the crack was permanently sealed by an aggregate of cement, volcanic ash and sawdust. The cost of the repair exceeded $1 million.〔 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「Dworshak Dam」の詳細全文を読む スポンサード リンク
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