Dams are becoming ubiquitous on the large rivers of the tropics. They are popular development projects for international aid agencies such as The World Bank. However, they are devastating to rainforests, as they flood large areas of forest, fragment populations of plants and animals, block animal migrations, and inhibit reproduction of both plants and animals. While in 1950, there were only 5700 large dams in the world, now there are 41,000, and they disrupt almost 60% of large river basins (Johnson, Revenga and Echeverria, 2001). Dams are relatively recent innovations in tropical America, being constructed in large numbers only after 1970 when international agencies began to favor and pay for this type of large development project. Dam construction continues apace because of the demand for power in this region. Brazil intends to deliver 50% of its power needs from dams in the Amazon, despite the fact that they produce very limited amounts of electricity (in the range of 250 MW or less). Many dams are ostensibly for flood control, although inundation is a normal state of affairs in areas with monsoons and seasonal forests, and is often essential to maintaining soil fertility, providing food for fish and other animals, and aiding in reproductive migrations of aquatic species.

a. Barriers: Dams affect the aquatic communities of rivers by reducing current flow and by separating the upper and lower parts of rivers with impenetrable barriers. Fish migration is affected, since many neotropical fish migrate long distances, often in complex patterns involving flood plains. Consequently certain species have become locally extinct upstream of dams, such as the dorada, picuda, bagre and patalo above the Betania Dam in Colombia. Similar effects have been seen in fish and shrimp populations on Caribbean islands. Migratory fish have become extinct in the upper Parana River (a tributary of the Plata River) in Brazil/Paraguay, where numerous dams have been built over the past half century. In the Lower Plata basin, catfish are almost extinct, and other fish are declining in population. The population of Chinese paddlefish, endemic to the Chang Jiang River in China, has declined greatly since the Gezhouba Dam was built, as the dam impedes the fishes’ access to breeding sites upstream. This species will doubtless become extinct since it can no longer reproduce. Chinese sturgeon were similarly affected by this dam; they can no longer migrate and the fish is extinct below the dam. Asian river dolphins are endangered by the alteration of rivers. The Indus dolphin population consists of fewer than one thousand animals, the Yangtze dolphin of China, fewer than 200. These are not sustainable population numbers. The Irrawaddy dolphin, an estuary dweller which enters rivers, is gone in many parts of its former habitat, such as the Chao Phya River in Thailand. This dolphin will suffer even more when additional large-scale dams planned for Southeast Asian rivers prevent migration and block access to upstream habitats.

b. Flooding: Dams, by reducing flooding, also adversely affect many fish which depend on flood plains for much of their food supply. These fish may achieve as much as 75% of their growth during the flood season, and juveniles of many species are dependent upon feeding in flood plains. (See the case of the tambaqui, discussed above in Part II, Section F3b). Physical and chemical conditions are also altered in the river waters and reservoirs.

c. Build-ups of organic materials: Organic matter decays slowly in tropical waters, consuming much oxygen. When dams inundate forest areas, the dead plants may not decay for centuries, resulting in oxygen-deprived water and high acidity. During the construction of the Tucurui dam in Brazil, 1750 km2 of forest were flooded to produce 7.6 megawatts of electricity (Wolfe and Prance, 1998). This led to oxygen depletion of the water, and the death of many fish. The water below the Balbina Dam in Brazil, which has a reservoir of the same size, is almost completely deoxygenated (Salati, et al., 1993). It produces only 3% (250 megawatts) of the energy of the Tucurui Dam, and has not produced sufficient energy for the city of Manaus, for which this massive rainforest destruction was planned (Wolfe and Prance, 1998; Salati, et al., 1993). In Surinam, 1% of the total land mass of the country was flooded in building the Brokopondo Dam, producing 30 megawatts. The deoxygenation of the water and release of hydrogen sulfide from decomposing vegetation damaged the dam’s turbines and affected animal and plant life as far as 110 kilometers below the dam site. The same is true for many dams in tropical areas.

d. Alteration of water flow patterns: Dams alter water flow patterns by preventing dry- and wet-season variations, which is disastrous for many organisms that are adapted to these natural cycles. Some fish, for example, will migrate and breed only when water levels reach a certain threshold. Reduced discharge below dams also concentrates pollutants.

e. Water retention: Dams affect the retention of water on land. The natural flow of rivers is replaced by substantial evaporation from reservoirs, changes in the drainage systems of large areas and modification of runoff to the oceans (essential for estuaries and continental shelf fertility) and the timing and volume of these discharges.

f. Introduction of exotic species: Exotic species of fish and crustaceans have been introduced into reservoirs, where they have no natural predators, and where they often drive endemic species to near extinction.