A dam interferes with the normal flow of sediments downriver. The loss of natural sedimentation results in erosion of the river banks and tends to deepen river channels. The eroded river channels reduce fish habitat and can lead to a drop in the surrounding water table, depriving riverside plants of water. For large-scale dams, the erosion can continue for hundreds of miles and out into the coastal environment. The effect can be especially damaging to estuaries and deltas, which tend to be biologically rich. The interference with natural sedimentation leads to a loss of nutrients and fertility in these coastal areas.
Migratory spawning fish, such as salmon and steelhead, evolved in undivided ecosystems and must travel upriver to reproduce. Dams have been devastating to salmon runs. According to the American Fisheries society, 37 genetically distinct salmon runs have been exterminated due to dams, with many more wild salmon runs critically endangered. While newer dams have installed fish ladders, the salmon often have trouble finding and negotiating the ladders. Reservoirs also add an entirely new and oftentimes large new ecosystem in the middle of the migratory path. Large reservoirs can present problems due to the relative warmth of the water, the lack of flow or the presence of invasive predatory fish species.
By altering the natural flow of rivers, dams introduce many changes to downstream ecosystems, including the elimination of natural floods, changes to water temperature and chemistry and reduction in overall flow due to diversion of water to other uses. The change in flood dynamics can be damaging to downstream flood plains, which depend on regular flooding. Changes in temperature, chemistry and water volume can reduce species populations and disrupt the natural balance of downstream ecosystems.
Throughout the world, the requirements for fresh water and electricity are increasing. At the same time, consciousness of the ecological costs of dams has risen. As a result, in many cases dams are being re-engineered to add fish ladders, to more closely mimic natural water flows and to otherwise be less disruptive to ecosystems above and below. The World Commission on Dams, initiated by the World Bank and the World Conservation Union, was established in 1998 to study the impact of dams and to develop recommendations for best practices in design, planning, construction, operation and decommissioning of dams.