What potential harm does hydroelectric power do to the environment?

http://www.epa.gov/climatechange/impacts-adaptation/energy.html

Dammed reservoirs are used for multiple purposes, such as agricultural irrigation, flood control, and recreation, so not all wildlife impacts associated with dams can be directly attributed to hydroelectric power. However, hydroelectric facilities can still have a major impact on aquatic ecosystems. For example, though there are a variety of methods to minimize the impact (including fish ladders and in-take screens), fish and other organisms can be injured and killed by turbine blades.

https://media.goboard.io/environmental-impacts-hydroelectric-power.html

https://media.goboard.io/stop-the-biomass-boondoggles