One of the biggest disadvantages of hydroelectric energy is the major disturbance that damming a river causes to the surrounding people, animals and habitat. Another problem is that dams and hydroelectric power plants take many years to build and are incredibly expensive, although they do have only minimal costs once completed. Water must be abundant in the area, and maintenance standards must be strict.
The ability of a hydroelectric power plant to create electricity depends entirely on water flow, so it requires a location with regular precipitation. If enough rain doesn't fall each year to continually fill the reservoir created by the dam, the hydroelectric power plant cannot continuously generate electricity.
When an area is flooded after a dam is built, the trees and other plant life in that area quickly die and begin to decompose. When this happens, methane and carbon dioxide are released into the reservoir and eventually into the atmosphere. The dam also causes another problem when the water picks up added nitrogen flowing through it, which can be toxic to fish and result in the death of marine populations.
Dams also pose a major risk if they break or are destroyed, which would send out a massive wall of water and destroy anything downstream.