Dams

In places where the elevation changes of a river are great, dams for hydroelectric plants and other purposes are often built. This disrupts the natural flow of the river, and creates a lake behind the dam. Often the building of dams affects the whole of the river, even the part above the dam, as migrating fish are hindered (see fish ladder) and waterflow is no longer bounded by seasonal changes. Dams are useful in many ways, such as providing HEP, acting as regulator of river flow so as to regulate the occurence of flooding, which is especially important to wet-rice agriculture, and also to improve navigation and transport on the river. Often, dams such as Hoover Dam along Colorado River become famous tourist attractions. However, critics of dams, especially 'Green' advocates, argue that dams remove upper-river biodiversity such as through deforestation and forced migration of rural villages and indigenous tribes. Furthermore, trapping of river sediments behind the dams lead to salination and loss of nutrients for down-water fish. It also raises concern of eathquakes due to instablity of incompetent dams which have to support thousands of tonnes of sediments behind them. One very famous, and problematic, dam is the Aswan High Dam in the Nile.