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| Dams have played an important role in supplying and storing water, particularly for large communities such as cities, and also for [[agriculture]]. However, they are now widely recognized as having many serious negative environmental, social and [[public health]] impacts: | | {{cat header| default.png |Dams| Dams have many serious environmental, social and public health impacts. }} |
| *Trapping of sediment - this reduces [[sediment]] and [[nutrient]] flow down the river, with effects as far away as declining [[marine|offshore]] [[fish]] stocks (as is reported with the [[Three Gorges Dam]]{{wp sup|Three Gorges Dam}} in [[China]]. This sediment also fills the dam, reducing its capacity.
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| *Interference with [[ecosystem]]s including fish stocks. This is particularly the case where indigenous species are adapted to a cycle of wet and dry, as in Australian rivers - dams disrupt the cycle, and indigenous species lose their survival advantage over introduced species.
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| *Traditional farmers whose farming patterns depend on the natural cycle of the river can suffer. For example, in the Nile Valley, traditional farmers received [[irrigation]] and natural [[fertilizer]] from the annual floods, but after the [[Aswan Dam]]{{wp sup|Aswan Dam}} was built, they could not get the water without paying,{{fact}}, and had to pay for fertilizer instead of receiving free nutrient-rich sediment deposited by the flood.
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| *Displacement of local people. Activists (including [[Arundhati Roy]]{{wp sup|Arundhati Roy}}{{fact}})alleged that poor people are frequently thrown off their land with no choice, and little or no compensation.
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| *[[Disease]]s spread by water may be an issue. For example, the use of the Aswan Dam for agriculture removed the traditional dry period, allowing [[snail]]s to survive, spread, and act as intermediate hosts{{fact}} for the [[schistosomiasis]]{{wp sup|schistosomiasis}} [[pathogen]], causing serious health problems to agricultural workers and others who had contact with the water.
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| These factors, and the fact that most suitable sites for dams have been dammed already, mean that other means of [[water supply]] are generally preferred. In some cases, old dams may actually be removed, or at least made to allow more water through (referred to as an [[environmental flow]]{{wp sup|environmental flow}}).
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| Of course, many factors must be considered, and the needs of the community may be such that a dam is required. In this case, design is very important:
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| *Size no larger than necessary. This can be helped by emphasizing [[water efficiency]] in the community.
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| *Use of [[fish ladder]]s{{wp sup|fish ladder}} to minimize ecological impact. These should be designed for the local species.
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| == Interwiki links ==
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| * [[Wikipedia:Dams]]
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| | [[Category:Hydrology]] |
| [[Category:Water supply]] | | [[Category:Water supply]] |