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 metres from 1940-1990, increased to about 45 metres 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 most large 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. And a dramatic increase in floods on the Rhine River has been attributed to increased urbanisation, engineering, and the walling off of the river from its floodplain (Revenga et al., 1998).

River fragmentation - the interruption of a river's natural flow by dams, inter-basin transfers or water withdrawal - is an indicator of the degree to which rivers have been modified by man (Ward and Stanford, 1989, and Dynesius and Nilsson, 1994, as cited in Revenga et al., 2000). A fragmentation analysis carried out by the University of Umea and the World Resources Institute showed that, of 227 rivers assessed, 37% were strongly affected by fragmentation and altered flows, 23% were moderately affected, and 40% were unaffected.

• Strongly or moderately fragmented systems accounted for nearly 90% of the total water volume flowing through the rivers analysed.
• Strongly fragmented river systems are defined as "rivers with less than a quarter of their main channel remaining without dams, where the largest tributary has at least one dam, as well as rivers where the annual flow pattern has changed substantially." Fragmented rivers are only considered unaffected if their main channel has no dams or, if their tributaries have been dammed, the total river discharge has only declined by less than 2% (Revenga et al., 2000).
• The combined length of rivers altered for shipping increased from less than 9 000 km in 1900 to more than 500 000 km in 1997 (Naiman et al., 1995, as cited in Revenga et al., 2000).
• The only remaining large free-flowing rivers in the world are found in the tundra regions of North America and Russia, and in smaller coastal basins in Africa and Latin America.
• Considerable parts of large rivers in the tropics, such as the Amazon, the Orinoco and the Congo, remain basically unaffected. China's Yangtze River will become strongly affected with the completion of the Three Gorges dam project (Revenga et al., 2000).

The last three decades have seen several inland ecosystems (e.g. the Aral Sea, Lake Chad, the Mesopotamian Marshlands) decline in size and function.