The construction of large dams - defined as those with walls at least 15 metres high - has increased significantly over the past 50 years. The average height of new dams, estimated at 30-34 m from 1940-1990, increased to about 45 m in the 1990s, due largely to construction trends in Asia. The average area and volume of freshwater reservoirs have also steadily increased, rising to about 50 km2 between 1945 and1970, declining through the 1980s to 17 km2, and increasing again in the 1990s to about 23 km2 (WCD, 2000). By 1997 there were more than 45,000 large dams worldwide, 22,100 of them in China. Other nations with many large dams include the United States (with 6,390 large dams), India (with more than 4,000), and Spain and Japan (with 1,000-1,200 each) (WCD, 2000).
The countries with the greatest number of large dams under construction, in order of significance, are: Turkey, China, Japan, Iraq, Iran, Greece, Romania and Spain, and countries in the Parana basin in South America. The river basins with the largest dams under construction are: the Yangtze with 38, the Tigris and Euphrates, with 19 each, and the Danube, with 11 (Revenga et al., 2000). Damming and flood control can have negative impacts, such as declining fish catches, loss of freshwater biodiversity, increases in the frequency and severity of floods, loss of soil nutrients on floodplains, and increases in diseases such as schistosomiasis and malaria. In Egypt, for example, the massive Aswan Dam has caused the fertile Nile Delta to shrink, with 30 of 47 commercially exploited fish species becoming economically or biologically extinct. On the Mississippi River, the rising frequency and severity of flooding - attributed to local flood control structures - have reduced the river’s ability to support native flora and fauna, while a dramatic increase in floods on the River Rhine has been attributed to increased urbanization, engineering, and the walling off of the river from its floodplain (Revenga et al., 1998).