Dams hold the promise of a nearly constant supply of electrical power with no associated emissions of greenhouse gases or toxic contaminants. They have some advantages compared to other ways of generating electricity, such as the ability to rapidly change the amount of electricity being generated, which reduces the amount of energy lost.
Unfortunately, dams have negative effects on the environment. When filling the reservoir any land located in the area where the reservoir will be, such as farm land, houses, or cities can be destroyed. The flooding will also destroy all the existing vegetation and animal habitat in that area.
Dams also interfere with the natural river dynamics, changing the natural flow patterns such as creating a more consistent flow. A dam traps sediment carried by the rivers and streams flowing into the reservoir, and the reservoir will eventually fill up with sediment. This blockage deprives the downstream river of sediment and associated nutrients. This can cause erosion and drastically change the downstream habitat, as clear, cold water from the depths of the reservoir replaces the warmer, muddy water that flowed down the river before the construction of the dam.
Dams are also lethal for migratory fish, such as salmon. Adult fish are blocked from migrating to upstream spawning areas. Juvenile fish die if they go through hydroelectric turbines.