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Dam removal is the process of removing out-dated, dangerous, or ecologically damaging dams from river systems. There are thousands of out-dated dams in the United States that were built in the 18th and 19th centuries, as well as many more recent ones that have caused such great ecological damage, that they are proposed for removal. Catastrophic dam failures such the 1977 Kelly Barnes Dam, killing 39 students, faculty and staff of the college, the 1976 Teton Dam failure in Idaho, the 1928 St. Francis Dam failure in California, and the 1889 Johnstown Flood remind people of the dangers dams can present. The largest catastrophic failure of a dam was the 1975 Banqiao Dam disaster that killed 26,000 people immediately, resulted in 145,000 dying of disease later, and displacing 11,000,000 residents. In 2014 the film DamNation was released illustrating and advocating the strategy of dam removal. ==Purposes and effects of dams== Many of the dams in the eastern United States were built for water diversion, agriculture, factory watermills, and other purposes that are no longer useful. Because of the age of these dams, over time the risk for catastrophic failure increases. In addition, many of these dams block anadromous fish runs, such as Atlantic salmon and American shad, and prevent important sediments from reaching estuaries. Many dams in the western United States were built for agricultural water diversion in the arid country, with hydroelectric power generation being a very significant side benefit. Among the largest of these water diversion projects is the Columbia Basin Project, which diverts water at the Grand Coulee Dam. The Bureau of Reclamation manages many of these water diversion projects. Some dams in the Pacific Northwest and California block passage for anadromous fish species such as Pacific Salmon and steelhead. Fish ladders and other passage facilities have been largely ineffective in mitigating the negative effects on salmon populations. Bonneville Power Administration manages electricity on 11 dams on the Columbia River and 4 on the Snake River, which were built by the Army Corps of Engineers. In the Desert Southwest, dams can change the nature of the river ecosystem. In the particular case of the Glen Canyon Dam, the originally warm, sediment-filled, muddy water, instead runs cold and clear through the Grand Canyon, which has significant impacts on the downstream ecosystems. Three native fish species have become extinct in the Grand Canyon and others are endangered since the dam was completed, including humpback chub and razorback sucker. Some dam projects, such as those on the Salt River Project in Arizona, eliminate the flow of the river downstream, by diverting the flow into the Arizona Canal system for use in agriculture and urban usage, such that only a dry channel or arroyo heads out across the desert. So much water is taken out of the Colorado River for agriculture, urban use, and evaporation behind the dams, that the river no longer flows into the Gulf of California. While the need for clean, alternative energy sources are important, with so many considerations involved, sometimes it makes sense to evaluate whether the benefits of dams outweigh the costs of safety concerns, ecosystem functions, and management expenses. 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「Dam removal」の詳細全文を読む スポンサード リンク
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