The threats of destructive dams are numerous and varied, and includes direct impacts to the biological, chemical and physical properties of rivers and “stream-side” environments.
Dams have led to the extinction of many fish and other aquatic species, the disappearance of birds in floodplains, huge losses of forest, wetland and farmland, erosion of coastal deltas, and many other unmitigable impacts. The dam wall blocks fish migrations, which separate spawning habitats from rearing habitats.
Another significant impact is the transformation of free-flowing river ecosystem into an artificial slack-water reservoir. Changes in temperature, chemical composition, dissolved oxygen levels and the physical properties of a reservoir are often not suitable to the aquatic plants and animals that evolved with a given river system. Indeed, reservoirs often host non-native and invasive species (e.g. snails, algae, predatory fish) that further undermine the river’s natural plants and animals.
A dam also traps sediment, which is critical for maintaining physical processes and habitats downstream of the dam such as deltas, floodplains and coastal wetlands. When a river is deprived of its sediment load, it seeks to recapture it by eroding the downstream riverbed and banks. Altering the riverbed also reduces habitat for fish that spawn in river bottoms, and for invertebrates and lower groundwater tables along a river reducing accesible water to plants.
In aggregate, dammed rivers have also impacted processes in the broader biosphere. Most reservoirs, especially those in the tropics, are significant contributors to greenhouse gas emissions. Recent studies on the Congo River have demonstrated that the sediment and nutrient flow from the Congo drives biological processes far into the Atlantic Ocean, including serving as a carbon sink for atmospheric greenhouse gases.
As other examples Belo Monte Dam in central Amazon will also affect biodiversity over an extensive area. The rich flooded forests of the Big Bend and middle Xingu would no longer receive seasonal floodwaters. Besides affecting endemic and migratory fish species, it would seriously affect aquatic and land fauna, including endangered species such as the white-cheeked spider monkey and black-bearded saki monkey. Threatened turtle species downstream would lose their breeding grounds.
Likewise Ilisu Dam project in Turkey threatens the intact riparian habitat with its 5 Key Biodiversity Areas along the Tigris River and the globally endangered species such as the Egyptian Vulture, Euphrates Soft-shelled Turtle, and the Leopard (Mesopotamian) Barbel. The dam would also affect other important habitats along the Tigris River, which flows through Mesopotamia all the way to the marshes of Basra in Iraq.