Impoundment hydropower, also known as reservoir hydropower, is the most common type of hydroelectric power generation. It relies on impoundment facilities, such as reservoirs or large dams, to store water and release it on demand through turbines to generate electricity.

This type of hydropower is conducive to large-scale power generation. Impoundment facilities store large volumes of water, which means they can produce a significant amount of electric power. Because the water is stably stored, impoundment hydropower is considered a reliable and predictable energy source; however, in times of drought, power generation can be affected.

Pumped storage hydropower involves moving water between stations at different elevations. These facilities draw water from a lower elevation to a higher elevation during times of low demand; they release it back down to generate electricity during times of high demand.

This type of hydropower plant works like a battery, storing the energy generated by other power sources (such as solar, wind, and nuclear) for later use. These storage systems provide a way to balance load within the network. The stored energy can be used to meet sudden surges in demand or to compensate when intermittent renewable energy sources (such as wind and solar) aren't producing enough. But finding suitable sites, with bodies of water at different heights, can be challenging.

Commonly known as run-of-river hydropower, diversion-based systems require little to no water storage. Instead, the natural flow and drop of a river is used to generate renewable electricity by diverting it through a canal or penstock and into turbines. Diversion hydropower’s small-scale hydroelectric plants are most likely to be found in mountainous regions, where the natural drop of the river provides a strong flow for power generation.

These hydroelectric facilities typically have a smaller environmental footprint than impoundment facilities because they do not require large reservoirs and have less impact on natural ecosystems. But they also operate on a smaller scale—meaning that they generate less electricity overall. And because they are dependent on the river flow, they can be more variable, especially in regions where rainfall changes by season.

While the types of hydropower described above cover most existing facilities, some other variants are in use. For example, some facilities harness tidal power, by using the rise and fall of ocean tides to generate electricity. And micro-hydropower is a term that refers to smaller systems designed to generate power for a small community, a single home or a remote facility. They can either be run-of-river type or involve a small reservoir.

China is the largest producer of hydropower in the world, with over 356,000 megawatts of installed capacity. The country has invested heavily in hydropower projects, including the Three Gorges Dam, which is the largest hydropower project in the world. The dam, which is located on the Yangtze River and completed in 2012, has a capacity of 22.5 gigawatts (GW).

Hydroelectric power plants and other sources make up about 6% of the electricity that is generated in the United States.² The Grand Coulee Dam on the Columbia River in Washington State is the largest hydropower project in the country, with a generating capacity of approximately 6.8 GW. But perhaps the best-known hydropower project in the world is the Hoover Dam. Located on the border between Arizona and Nevada, it was completed in 1936 and has a capacity of about 2 GW. The dam not only provides electricity for public and private utilities in Nevada, Arizona, and California, but also regulates water flow for irrigation and provides flood control.

Hydropower is an important source of renewable energy in Europe, accounting for more than 12% of the European Union’s electricity generation.³ Norway, for example, generates more than 90% of its total electricity from hydropower.⁴ The biggest hydropower project in Europe in terms of capacity is the Sayano-Shushenskaya Dam in Russia. It is the seventh-largest hydroelectric plant in the world, with a capacity of 6.4 GW.

There are several key hydropower projects on the South American continent. The Itaipu Dam, a joint undertaking between Brazil and Paraguay on the Parana River, is one of the world's largest operational hydroelectric energy producers. It has an installed generation capacity of 14 GW. And Venezuela’s Guri Dam, also known as the Simón Bolívar Hydroelectric Plant, has a capacity of about 10.2 GW and provides as much as 80% of the country’s electricity.

Many countries in Africa are turning to hydropower as a way to meet their growing energy needs. Projects such as the Grand Ethiopian Renaissance Dam and the Inga Dam in the Democratic Republic of Congo have the potential to provide significant amounts of renewable energy to the continent. In Egypt, the Aswan High Dam on the Nile River is one of Africa's largest hydropower projects. Completed in 1970, it has a generating capacity of approximately 2.1 GW. The dam has impacted the country’s agriculture and economy by providing increased water storage for irrigation and producing hydroelectric power.