Hydropower generators produce clean electricity, but hydropower does have environmental impacts

Most dams in the United States were built mainly to control floods and to help supply water for cities and irrigation. Although many of these dams have hydroelectric generators, only a small number of dams were built specifically for hydropower generation. Although hydropower generators do not directly produce emissions of air pollutants, dams, reservoirs, and the operation of hydropower electric generators can affect the environment.

Fish ladder at the Bonneville Dam on the Columbia River separating Washington and Oregon
Fish Ladder at the Bonneville Dam on the Columbia River Separating Washington and Oregon.

Source: Stock photography (copyrighted)

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The Safe Harbor Dam on the Susquehanna River in Pennsylvania has elevators that lift migrating shad from the base of the dam to the top of the reservoir.

A dam that creates a reservoir (or a dam that diverts water to a run-of-river hydropower plant) may obstruct fish migration. A reservoir and dam can also change natural water temperatures, water chemistry, river flow characteristics, and silt loads. All of these changes can affect the ecology and the physical characteristics of the river. These changes may have negative impacts on native plants and on animals in and around the river. Reservoirs may cover important natural areas, agricultural land, or archeological sites. A reservoir and the operation of the dam may also result in the relocation of people. The physical impacts of a dam and reservoir, the operation of the dam, and the use of the water can change the environment over a much larger area than the area covered by a reservoir.

Although no new hydropower dams have been built recently in the United States, they are being built in other countries like China. Manufacturing the concrete and steel used to construct these dams requires equipment that may produce emissions. If fossil fuels are used as the energy source to make these materials, then the emissions from the equipment could be associated with the electricity that hydropower facilities generate. However, given the long operating lifetime of a hydropower plant (50 years to 100 years) these emissions are offset by the emissions-free electricity that is generated.

Carbon dioxide and methane may also form in reservoirs and be emitted into the atmosphere. The exact amount of greenhouse gases produced in hydropower reservoirs is uncertain. The emissions from reservoirs in tropical and temperate regions, including the United States, may be equal to or greater than the greenhouse effect of the carbon dioxide emissions from an equivalent amount of electricity generated with fossil fuels. Scientists at Brazil's National Institute for Space Research designed a system to capture methane in a reservoir and burn it to produce electricity.

Fish ladders help salmon reach their spawning grounds

Hydropower turbines kill and injure some of the fish that pass through the turbine. The U.S. Department of Energy has sponsored the research and development of turbines that could reduce fish deaths to lower than 2%, in comparison to fish kills of 5% to 10% for the best existing turbines.

There are many species of fish, such as salmon, that swim from the sea up rivers and streams to reproduce in their spawning grounds in the beds of rivers and streams. Dams block their way. Different approaches to fixing this problem have been used, including the construction of fish ladders and elevators that help fish move through or around dams to the spawning grounds upstream.