Hydropower plants harness the kinetic energy of flowing water to power machinery or make electricity. There are typically three different types of hydropower facilities: impoundment, diversion, and pumped storage. The most common is impoundment in a dam that stores the water in a reservoir. Water flowing through the dam spins a turbine, which uses a generator to make electricity and sends it out to the grid.
A diversion facility does not use a dam but instead diverts a portion of the river. Pumped storage requires the pumping of water from a lower reservoir to a higher reservoir so the water can be released when electrical demands are high.
Because water is not destroyed in the process of creating energy, it is considered a renewable resource. It doesn't produce pollutants like fossil fuels, and the dams are often used for flood control. Unfortunately, because hydropower relies on the natural water cycle, energy production can be impacted during dry seasons or times of drought.
Hydroelectric plants also interfere with the natural flow of rivers and everything that moves along with them, including spawning fish and river and tributary flows. Another consideration with dams is “thermal pollution,” because the water released on the other side of the turbine is often warmer than the water entering. This can have a negative effect on aquatic life that is suited to narrow temperature ranges.
Some companies and power utilities are incorporating the use of renewable resources into their conventional systems. ConEdison Solutions now includes a combination of wind and hydropower, offering customers an alternative to traditional power. By signing up for green power, customers receive a rebate and don't have to pay sales tax on the delivery of their power.
The National Geographic Society uses Africa's Zambezi River and the floodplain created by the Cahora Bassa Dam in a hydropower educational program. Students in grades nine through twelve are asked to consider the benefits, including the electricity produced for villages and irrigation water for farms, along with detrimental effects to the elephant population and teak trees.
On the downside, dams can also infuse the water with nitrogen and dissolved oxygen, causing problems with fish that are comparable to what divers call “the bends.” At deeper depths, the gases are absorbed by the blood; when the fish surface, however, the bubbles come out of the blood and form on the eyes, skin, and gills with deadly results.
Fish ladders and elevators can be installed on the upriver side of the dam, allowing safe passage for the fish upstream. This is particularly important when fish are migrating upstream to spawn. In some situations, fish are trapped and hauled upriver by truck. This is usually seen as a temporary measure and not relied on to protect fish populations long-term. Other concerns include the creation of floodplains in otherwise dry areas and the displacement of wildlife.