Hydroelectric power or hydroelectricity is electrical power which is generated through the energy of falling water. This method of energy generation is viewed as very environmentally friendly by many people, since no waste occurs during energy generation. However, hydroelectric power can have a profound impact on the surrounding environment, leading some people to question the promotion of hydroelectric power as a method of clean energy generation.

People have been harnessing the energy of moving water for centuries. In Ancient Greece and China, waterwheels were installed in rapidly flowing rivers to turn millstones and other equipment. Waterwheels were also used to power saws and other equipment across North America when European colonists began rapidly expanding across this region, and they continue to be used today in some parts of the world.

Most hydroelectric power plants rely on the same basic principles as a historical waterwheel, but they have a few high tech variations. Instead of producing energy which is used directly, as in the case of a waterwheel which causes a millstone to move, a hydroelectric power plant uses the force of the water to push a turbine which in turn powers a generator, creating electricity which can be used on-site or transported to other regions.

Hydroelectric power plants also do not rely on seasonal flows of water, as people did historically with waterwheels. They are usually constructed inside dams, with the dam being used to retain water so that the flow of water through the turbines can be controlled. In periods of peak energy demand, the dam is opened to allow lots of water to flow, generating a great deal of electricity, and in periods of low demand, the dam is kept closed, saving the water for when it is needed. Some dams even have recovery and pumping systems, so water goes back into the reservoir after it has flowed through the dam, allowing it to be used again.

The key environmental problem with hydroelectric power is that damming changes the natural environment dramatically. The flow of a river is fundamentally altered when a dam is installed, posing problems for fish and aquatic plants on both sides of the dam. Dams can also have unintended consequences, as seen in Egypt after the installation of the Aswan Dam. Initially, residents were pleased by the power generated by the dam, and by the control of the Nile's annual flooding. However, they quickly realized that the annual floods had been laden with nutrient-rich silt which made it possible to farm along the Nile, and once the dam was installed, all of that silt collected uselessly in the reservoir, depriving the land downstream of necessary nutrients.

However, there are some arguments in favor of hydroelectric power. Once installed, a hydroelectric power plant does not generate any emissions or waste, making it vastly preferable to something like a coal-fired power plant. The technology of hydroelectric power is also constantly being improved, and sometimes simple measures like fish ladders can mitigate the impact of a dam.