Impacts of Hydropower Dams

Hydropower dams cause four general types of impacts to river ecosystems and the species dependent upon them:

• migration blockage and delay,
• habitat changes,
• turbine strike,
• and fish predation.

• Migration blockage and delay: Hydropower dams block the continuous flow of rivers and streams. As diadromous fish attempt to move up and down rivers, hydropower dams physically block their movement. In some cases it is possible to build fishways that allow fish to migrate upstream around a dam.
• Habitat changes: Hydropower facilities can impact habitat by changing water volumes, temperature, depth, and velocity, and by altering dissolved oxygen sediment loads. Downstream of hydropower facilities, the variations in the amount of water released, temperature, and quality is often different than the natural seasonal variations of the river that fish are adapted to. Considerable attention is given to the downstream changes below a dam; however, upstream changes are can be just as significant as reservoirs behind dams offer very different habitat attributes than rivers.
• Turbine strike: Even if fish can migrate through a fishway upstream past a dam, they can suffer injury or death during their downstream migration if they pass through a dam’s turbines.
• Fish predation: Hydropower dams may attract predatory fish, birds, and other animals because of increased concentration of fish due to migratory delays, presence of disoriented or injured fish from turbine strikes, or improved habitat for predators.

With over 1000 FERC-licensed hydropower facilities across the United States, the impacts to diadromous fish and their habitats are substantial.