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. Analysis indicates that:
- Strongly or moderately fragmented systems accounted for nearly 90% of the total water volume flowing through the rivers analyzed.
- 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, and the Mesopotamian Marshlands) decline in size and function.