What is hydropower? (with photos)

A dam used in a hydroelectric power plant.

Hydroelectric power (HEP), or hydroelectricity, is electrical energy generated through the energy of flowing water. It has the advantage of using only renewable resources and does not produce harmful waste or by-products and is therefore seen by many as an environmentally friendly alternative to burning fossil fuels which still provide most of the electricity consumed around the world. HEP schemes, however, often encounter considerable opposition on the grounds that they themselves cause major environmental problems.


A ship passing through a lock at the Three Gorges Dam, the largest hydroelectric dam in the world.

People have been harnessing the energy of moving water for centuries. In Ancient Greece and China, water wheels were installed in fast-flowing rivers to turn millstones and other equipment, and they continue to be used today in some parts of the world. These devices are actually using gravity, as water flows from an upper area to a lower one.

Hydropower plants follow the same basic principle, but instead of using kinetic energy directly, the force of the water drives a turbine which, in turn, powers a generator, producing electricity that can be used locally or transported to other regions. As with other turbine-powered methods of generating electricity, the magnets in the turbine move relative to the static conductors. This results in an electric current flowing through the conductors.

The Hoover Dam is one of the largest hydroelectric power plants in the world.

There are several different designs for hydropower plants, but the most common involves building a large dam on a lake at a relatively high elevation to form a water reservoir and allow water to flow from it in a controlled manner down to the generator, which is located at a lower elevation. Water flow can be maximized during periods of high electricity demand and reduced when demand is lower. In most cases, the water is replenished by rainfall, but some power plants may use excess electricity to pump water back into the reservoir at times of low demand.

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The Aswan dam in Egypt has been successful in providing power and controlling the floods of the Nile River downstream.

The main advantages of hydropower are that it is clean and renewable. Because it does not involve burning fossil fuels such as coal, oil and natural gas, it does not produce harmful pollutant emissions such as soot and sulfur dioxide. Equally important, it does not produce carbon dioxide, the “greenhouse gas” believed to be responsible for climate change, through energy generation. It has the advantage over nuclear energy of not generating hazardous waste that presents long-term disposal problems.

HEP is also renewable as it does not use a limited resource. Although coal, oil, natural gas and even uranium eventually run out, water is inexhaustible. Once the plant is built, the water it needs is available free of charge, unlike fuel that has to be extracted from the land. The water supply is continually renewed in most areas by rainfall, which actually means that the Sun is providing the energy as its heat evaporates surface water that falls as rain.


While HEP is clean and renewable, it is not necessarily environmentally friendly. Large-scale projects capable of providing significant amounts of electricity often require the damming of lakes or rivers, with consequent flooding of large areas of land. This can result in the displacement of people and the destruction of habitats. Dramatic changes in river flows can also have a profound impact on wildlife, ecosystems and, in some cases, agriculture. Migrating fish such as salmon can find their routes blocked by dams, although this can be mitigated by installing a fish ladder – a series of steps that fish can negotiate.

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The most suitable sites for hydropower schemes are often of great scenic beauty and may lose some of their value due to new roads, poles and other construction associated with a HPP project. Some projects had unintended consequences. For example, the Aswan dam in Egypt, which was very successful in providing energy, also controlled the annual flooding of the River Nile downstream. At first, this was considered another advantage, but the floods were loaded with nutrient-rich silt that made farming possible along the Nile, and once the dam was installed, all that silt usedlessly accumulated in the reservoir, depriving the land. downstream of the necessary nutrients. Another possible problem, according to some studies, is that decaying vegetation in areas flooded to provide HEP can emit significant amounts of carbon dioxide, offsetting the advantages of clean energy generation.

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