The construction and implementation of hydropower plants involves considerable environmental costs: relocation of populations; loss of land; dramatic changes in species composition; unsurmountable obstacles to migrating fish, such as salmon; and a multitude of other changes in the ecological balance. In addition, the functioning of the dam is not hazard- free: marginal cultivators denude the watershed of forest cover which eventually leads to erosion of the cultivable soil and siltation of the dam reservoir; and waterborne diseases and water weeds breed easily in the reservoir.
In the 1880s there were an estimated 16 million wild Pacific salmon in the Columbia and Snake River System of north western USA. In 1993, there are perhaps 100,000 left. Roughly one third of all species of fish are imperilled in the USA and dams are the main causes.
Artificial lakes constructed to provide hydroelectric power contribute to climate change, in the same way as many natural wetlands are significant producers of methane and carbon dioxide. Per unit of energy some hydroelectric reservoirs might be significant sources of greenhouse gases, comparable to coal-fired power stations.
Hydropower plants not only provide reliable, efficient and clean energy, they also permit flood control and prevention, irrigation, increased fish production and recreational facilities.