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FACTS ABOUT WATER AND DAMS The first dams were constructed some 5,000 years ago, but the commissioning of large dams peaked between the 1960s and the 1980s particularly in China, the United States, the former USSR and India. However, some 300 dams over 60 metres high were listed as under construction in 1999, and authorities are claiming that many more will be needed in the future to meet the burgeoning demand for water. According to the World Commission on Dams there was a world total of 47,655 large dams in 1998 and an estimated 800,000 smaller ones. A large dam by the definition of the International Commission on Large Dams (ICOLD) has a height of more than 15 metres, or has a dam of above 5 metres holding a reservoir volume of more than 3 million cubic metres (Mm3). These include some dams that have been constructed to increase the capacity of existing lakes, for example the Owen Falls Dam on the Nile below Lake Victoria. Cosgrove and Rijsberman (2000) maintain that a further 150 km3 of storage will be required by 2025 to support irrigation alone and 200 km3 more to replace the current overconsumption of groundwater. Most rivers carry sediment in the form of suspended load and bed load, in some cases the latter is charged with metals and other toxic materials. This sediment load is adjusted to the flow regime of the river over time, and changes to this regime accompanied by increases or decreases in the load can cause problems downstream. These include the progressive reduction of reservoir volumes by siltation, the scouring of river channels and the deposition of sediment in them, threatening flood protection measures, fisheries and other forms of aquatic life. River diversions, including dams, can produce some of these effects on sediment, but in addition they may alter the chemical and biological characteristics of rivers, to the detriment of native species. Recent studies in Ethiopia using community-based incidence surveys revealed a 7.3-fold increase of malaria incidence associated with the presence of microdams. The study sites were all at altitudes where malaria transmission is seasonal (in association with the rains). The increase was more pronounced for dams below 1,900 metres of altitude, and less above that altitude. In addition, observed trends in incidence suggest that dams increase the established pattern of transmission throughout the year, which leads to greatly increased levels of malaria at the end of the transmission season. In the Mobile Bay drainage basin in the United States, dam construction has had a catastrophic impact on what was probably the most diverse freshwater snail fauna in the world. Nine families and about 120 species were known from the drainage basin. At least 38 species are believed to have become extinct in the 1930s and 1940s following extensive dam construction in the basin: the system now has 33 major hydroelectric dams and many smaller impoundments, as well as locks and flood-control structures.
Information from: Source: UNESCO Water Portal, June 2005
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