Hydropower can generate electricity without directly emitting air pollutants at the plant, but dams and reservoirs have real drawbacks. The U.S. Energy Information Administration notes that a dam and reservoir may obstruct fish migration and change natural water temperatures, chemistry, river flow, and silt loads—shifts that can harm native plants and animals. Reservoirs can also cover natural areas, farmland, or archaeological sites and may require relocating people.
Hydropower turbines can kill or injure fish that pass through them, and output depends on water supply, so droughts and dry seasons can reduce generation. For a current overview, see the U.S. EIA page on hydropower and the environment.