In a world in which it seems that nearly every natural ecosystem is under stress, freshwater ecosystems – the diverse communities found in lakes, rivers, and wetlands – may be the most endangered of all. Some 34 percent of fish species, mostly from fresh water, are threatened with extinction, according to the latest tally of the World Conservation Union (IUCN), which tracks threats to the world’s biodiversity . Freshwater ecosystems have lost a greater proportion of their species and habitat than ecosystems on land or in the oceans; in addition, they are probably in greater danger of further losses from dams, pollution, overfishing, and other threats .
In extent, freshwater ecosystems are quite limited, covering only about 1 percent of the Earth’s surface. Yet, they are highly diverse and contain a disproportionately large number of the world’s species. The Amazon River, for instance, is home to more than 3,000 fish species. Lake Victoria in Africa has – or had before recent depredations – as many as 350 species of a single family of fish (cichlids) . The Mississippi River in North America contains almost 300 freshwater mussel species . In all, more than 40 percent of the world’s fish species  and some 12 percent of animal species in general  reside in freshwater habitats, with many of these species restricted to extremely small areas and therefore quite vulnerable to disturbance.
The majority of the world’s population lives near and depends on freshwater environments, with most inland cities lying adjacent to a river or lake . In addition to being biologically rich, freshwater systems play a vital role in the lives of many people, providing a source of water, food, and employment. About 6 percent of the world’s fish catch, or 7 million metric tons per year,  come from rivers and lakes, as well as the bulk of the world’s irrigation water. Rivers and lakes are also crucial as transportation and shipping routes, as power sources, and, unfortunately, as waste sinks. All of these human uses take their toll on freshwater ecosystems.
Dams and channelization remain the two most pervasive threats to freshwater ecosystems today, with dramatic effects on species abundance and diversity. Since 1970, when Egypt’s Aswan Dam came into operation, the number of commercially harvested fish species on the Nile has dropped by almost two thirds, and the sardine catch in the Mediterranean has fallen by more than 80 percent . On the Rhine River, more than 100 years of channelization and riverside development have cut the river off from 90 percent of its original flood plains, and the native salmon run has nearly disappeared .
The scale and extent of these human impacts on freshwater systems have risen precipitously in recent years. In 1950, there were 5,270 large dams; today, there are more than 36,500 . (See River Habitats Have Been Heavily Altered.) Meanwhile, the number of waterways altered for navigation has grown from fewer than 9,000 in 1900 to almost 500,000 today, with a consequent decline in their viability as habitat .
|River habitats have been heavily altered|
|Waterways altered for navigation, 1680-1980|
|Source: Janet N. Abramowitz, “Imperiled Waters, Impoverished Future: The Decline of Freshwater Ecosystems,” Worldwatch Paper No. 128 (Worldwatch Institute, Washington, D.C., 1996), p. 13.|
Given their benefits to shipping, agriculture, and power production, dams and channelization projects remain important components of national development strategies, even though their environmental impacts are well known. In Southeast Asia, dozens of dams are being planned along most of the length of the Mekong River and its tributaries. The Mekong River Basin has few dams so far and retains one of the world’s richest troves of freshwater biodiversity. Estimates of the number of fish species in the basin run to 500 and higher, and the annual fish catch on the Mekong and its tributaries is a vital part of the local food supply  . Experience with one of the few dams in the basin shows how vulnerable this resource can be. After the Pak Mun Dam was built in the early 1990s on Thailand’s Mun River, a Mekong tributary, all 150 fish species that had inhabited the river virtually disappeared .
In South America, the Hidrovia—or Water Highway—project will create a 3,400-kilometer shipping corridor, opening landlocked Paraguay and Bolivia, as well as parts of the Brazilian interior, to river trade. The project involves dredging, widening, and straightening large sections of the Paraguay and Paraná rivers to allow large convoys of barges to pass into the continent’s interior. Unfortunately, the Paraguay River runs through the Gran Pantanal, the world’s largest and most pristine wetland. Dredging and channelization through the area could lower the water level considerably, imperiling the 600 species of fish, 650 varieties of birds, 80 types of mammals, and more than 90,000 varieties of plants that inhabit the Pantanal  .
1. The World Conservation Union (IUCN), 1996 IUCN Red List of Threatened Animals (IUCN, Gland, Switzerland, 1996), pp. Intro-24.
2. Don E. McAllister, Andrew L. Hamilton, and Brian Harvey, “Global Freshwater Biodiversity: Striving for the Integrity of Freshwater Ecosystems,” Sea Wind, Vol. 11, No. 3 (1997), p. 7.
3. Janet Abramovitz, “Imperiled Waters, Impoverished Future: The Decline of Freshwater Ecosystems,” Worldwatch Paper No. 128 (Worldwatch Institute, Washington, D.C., 1996), p. 59.
4. Ibid., p. 5.
5. Peter B. Moyle and Robert A. Leidy, “Loss of Biodiversity in Aquatic Ecosystems: Evidence from Fish Faunas,” in Conservation Biology, the Theory and Practice of Nature Conservation, Preservation, and Management, Peggy L. Fiedler and Subodh K. Jain, eds. (Chapman and Hall, New York, 1992), p. 130.
6. Peter H. Gleick, ed., Water in Crisis: A Guide to the World’s Freshwater Resources (Oxford University Press, New York, 1993), p. 5.
7. Op. cit. 5, p. 130.
8. Food and Agriculture Organization of the United Nations (FAO), The State of World Fisheries and Aquaculture, 1996 (FAO, Rome, 1997), p. 5.
9. Sandra Postel, “Dividing the Waters: Food Security, Ecosystem Health, and the New Politics of Scarcity,” Worldwatch Paper No. 132 (Worldwatch Institute, Washington, D.C., 1996), p. 29.
10. Op. cit. 3, p. 15.
11. Op. cit. 3, p. 13.
12. Op. cit. 3, p. 13.
13. Maurice Kottelat and Tony Whitten, “Freshwater Biodiversity in Asia With Special Reference to Fish,” World Bank Technical Paper No. 343 (The World Bank, Washington, D.C., 1996), pp. 7, 20.
14. Denis D. Gray, “Dam Builders Eye Mekong: Poverty, Energy, Thirst May Tame Mighty River,” Washington Times (May 26, 1997), p. A14.
15. Op. cit. 3, pp. 29-30.
16. Op. cit. 3, p. 21.