Hydropower is clean, renewable and the lowest-cost way to generate electricity, but hydropower also requires damming of rivers, which destroys wildlife and changes the natural cycle of the river flooding. It can also cause changes in the composition of water.
Hydropower requires a dam to be constructed in a valley. Eventually, a reservoir lake fills behind the dam. Channels guide the reservoir water into the dam to turbines that spin rotors on generators. The generators produce electricity that is delivered directly to the power grid. As more electricity is required, additional generators can be brought online if they are available.
Dams prevent or reduce the ability of fish and other wildlife to traverse the river. While some steps can be taken to ease the problem, such as fish ladders to help fish swim upstream, it is widely recognized that dams are harmful to salmon and other animals. Salmon in particular may not be able to swim upstream to spawn.
Once constructed, the energy source of water is clean and cheap to acquire as it’s replenished by snow and ice upland from the dam. Droughts can reduce or prevent a dam’s ability to produce electricity, but most dams were built in areas that have sufficient rain or snow upstream and are able to produce reliable and clean energy.