Global Distribution of Reservoirs

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There are nearly 50 000 dams in the world with heights above 15 m- defined as large dams - and an almost innumerable number of small dams built for farm ponds and other tiny impoundments. These dams can retain >6500 km3 of water, which represents >15% of the annual global runoff (Figure 1). The area of former terrestrial habitat inundated by all large (>108 m3) reservoirs in the world is comparable to the area of California or France. The environmental values that were lost as a result of this inundation are only known sporadically. It is not even known how many people were forced to move because of the reservoirs. The estimated number is 40-80 million people. Given that California has a population of approximately 37 million and the French population is 64 million, the magnitude seems accurate.

The project resulting in the highest number of forced resettlement - 1.3 million people - is the Three Gorges Dam on the Chang Jiang (Yangtze) in China. This estimate may increase because landslides in the margins of the filled reservoir threaten populated areas. The relative abundance of various sizes of lakes and reservoirs is discussed elsewhere.

Although dams were probably used much earlier, the first dams for which remains have been found were built about 3000 bc in modern Jordan. Prior to 1950 there were only about 5000 dams in the world, implying that many other dams were built after 1950 (Figure 2). China, which by far has the largest number of dams of the world's nations, shows an even later expansion in this respect. During the revolution in 1949 there were only eight large dams in China, but 50 years later the number had increased to around 22 000. The second most dam-rich country - the United States of America (USA) - lags far behind with only about 6600 dams, followed by India (about 4300 dams), Japan (about 2700 dams), and Spain (about 1200 dams).

The peak in number of dams and reservoirs created per year was reached in the 1970s. Although the rate of dam building has decreased, new dams are continuously being added. A few years ago, on average one new dam was completed every day after an average construction time of 4 years, implying that around 1500 dams were under construction. In 2004, large dams were planned or under construction on 46 of the world's largest rivers, with anywhere from 1 to 49 new dams per basin. Forty of these rivers are in nations not belonging to the Organization for Economic Cooperation and Development (OECD), indicating that future dam development does not depend on strong national economies. Almost half of the new dams are located on just four rivers, i.e., 49 on the Chang Jiang, 29 on the Rio de la Plata in South America, 26 on the Shatt Al Arab in the Middle East, and 25 on the Ganges-Brahmaputra in south Asia. New dams are also planned for several unaffected large river systems, including the Jequitin-honha in South America, and the Ca, Agusan, Rajang, and Salween in Asia. Many of these dams provide a serious threat to many species and habitats. For example, very large hydroelectric reservoirs in the tropics are especially likely to cause global species extinctions although such losses are rarely documented because scientific data are lacking.

Stone Fish World Distribution

Figure 1 There is yet no complete database of the world's water reservoirs. This map shows the location of about 1600 reservoirs from the Global Lakes and Wetlands Database [Lehner and Doll (2004) Development and validation of a global database of lakes, reservoirs and wetlands. Journal of Hydrology 296, 1-22]. The total storage capacity of these reservoirs amounts to approximately 5100 km3. This database will soon be superseded by GRanD (Global Reservoir and Dam Database) - a much more comprehensive database produced under the auspices of the Global Water System Project. Figure credit: Bernhard Lehner.

Figure 1 There is yet no complete database of the world's water reservoirs. This map shows the location of about 1600 reservoirs from the Global Lakes and Wetlands Database [Lehner and Doll (2004) Development and validation of a global database of lakes, reservoirs and wetlands. Journal of Hydrology 296, 1-22]. The total storage capacity of these reservoirs amounts to approximately 5100 km3. This database will soon be superseded by GRanD (Global Reservoir and Dam Database) - a much more comprehensive database produced under the auspices of the Global Water System Project. Figure credit: Bernhard Lehner.

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1940 1950 1960 1970 1980 1990 2000 2010 Year

Figure 2 Global rate of large-dam building 1950-2005. Data are taken from McCully P (2001) Silenced Rivers: The Ecology and Politics of Large Dams. London: Zed Books; WCD (2000) Dams and Development: A New Framework for Decision-Making. London: Earthscan; ScudderT (2005) The Future of Large Dams: Dealing with Social, Environmental, Institutional and Political Costs. London: Earthscan.

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1940 1950 1960 1970 1980 1990 2000 2010 Year

Figure 2 Global rate of large-dam building 1950-2005. Data are taken from McCully P (2001) Silenced Rivers: The Ecology and Politics of Large Dams. London: Zed Books; WCD (2000) Dams and Development: A New Framework for Decision-Making. London: Earthscan; ScudderT (2005) The Future of Large Dams: Dealing with Social, Environmental, Institutional and Political Costs. London: Earthscan.

mainly for use in farmlands. Water in irrigation reservoirs is generally not used for drinking. Thirty to forty percent of the 271 million ha of agricultural land irrigated worldwide rely on reservoirs. In other cases, water is extracted directly from rivers or from groundwater systems.

Hydroelectric Reservoirs

A hydroelectric power station consists of turbines that rely on a gravity flow of water from the dam to turn a turbine to generate electricity. The water can be either released to the river downstream of the dam or pumped back into the reservoir and reused. Generally, hydroelectric dams are built specifically for electricity generation and are not used for drinking or irrigation water. Hydropower provides 19% of the world's total electricity supply, and is used in over 150 countries, with 24 of these countries depending on it for 90% of their supply. The countries producing most hydroelectric energy are Canada, USA, Russia, Brazil, and China.

While new dams and reservoirs are added, some of the old ones are also taken out. In fact, removal of dams and reservoirs has become increasingly common, especially in western societies where old dams are widespread. The reasons for taking down a dam can vary, but one important cause is that a large dam is no longer safe. Another is that the dam is not needed; it might lack owners and be abandoned, or its owners cannot find funds to maintain it. Yet another is that the dam owners fail in the relicensing of the dam, in most cases because the dam has too large an environmental impact. For example, dams that interfere with migration of ecologically important fish stocks are key targets for removal.

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