Pollution is defined as the introduction of destructive elements or materials into the environment. As such, even though hydroelectric power is in general less polluting to the environment, the presence of a hydroelectric dam itself is a form of pollution, altering the course of a river and causing serious environmental impacts. The result can affect the entire landscape as well as the plants and animals that live in it.
The most dramatic environmental effect caused by dams is habitat fragmentation. The dam literally cuts off the river environment from its other components. The plants and animals of a river system have evolved with the seasonal changes that occur in river ecosystems such as spring flooding caused by rainfall and winter melt. When floodplains no longer flood due to the presence of a dam, wildlife is impacted. About one-third of the threatened and endangered species in the U.S. relies exclusively on habitats such as floodplains. Removal of habitat increases their risk of extinction.
Hydroelectric dams introduce several types of pollution into the ecosystem, including thermal pollution. Since the normal course of the river is diverted, natural seasonal temperature variations no longer occur. Also, water temperature is altered below the dam, causing temperatures to drop lower than it normally may have been. This factor can affect ectoderms such as reptiles and amphibians. An ectoderm is an organism whose body temperature varies with the environment. If temperatures dip too low, an organism may not be able to undergo metabolism and will soon perish.
Flow and Water Chemistry
The flow of water (or the lack thereof) changes the water chemistry of a river. Sediment drops into the water behind a dam. This increases the cloudiness of water, which can impact filter-feeding animals such as mussels and clams. The risk for elevated salinity levels also exists. Compared to the river it diverted, the surface area of a reservoir lake is many times larger. Thus, with more surface area exposed to sunlight and wind, the rate of evaporation increases, concentrating salts entering the lake from normal decomposition of rocks or from soils.
Factors Affecting Pollution
Many factors play into the environmental effects of a hydroelectric dam. The flow rate and size of the river which is dammed will help determine storage capacity which can in turn affect the surface area of the reservoir lake. The size of the dam can determine the size of the area impacted the dam's presence. Land use in the area around the lake will influence the concentration of pollutants as well as the type of contaminants.
The presence of a dam can further impact fish populations by disrupting natural migrations. Many species will use other parts of the river for spawning. If the isolation were not enough to impact fish, then the surges in water flow from a dam also affect them. A hydroelectric plant may regulate water flow of the dam. During times when energy is in high demand, flow may increase, scouring river beds and destroying their spawning beds.