Short answers, pulled from the story.
Hydropower is the use of falling or fast-running water to produce electricity or to power machines. It works by converting the gravitational potential or kinetic energy of a water source into power, most often by spinning a turbine connected to a generator. Hydroelectricity is the largest application of hydropower.
Hydroelectricity generates about 15% of global electricity and provides at least 50% of the total electricity supply for more than 35 countries. In 2021, global installed hydropower capacity reached almost 1400 gigawatts, the highest among all renewable energy technologies.
Hydropower can fail catastrophically, with dam failures causing loss of life, property, and pollution. Dams and reservoirs harm river ecosystems by blocking animal movement, de-oxygenating downstream water, and trapping sediment, and they displace people who live nearby. Rotting underwater vegetation also produces methane equivalent to almost a billion tonnes of carbon dioxide a year.
The French engineer Benoit Fourneyron developed the first hydropower turbine in the 19th century. The device was installed in the commercial plant at Niagara Falls in 1895 and is still operating. Later engineers, including James B. Francis in 1848 and Lester Allan Pelton in the 1870s, advanced turbine design further.
Since ancient times, hydropower from watermills powered irrigation and mechanical devices such as gristmills, sawmills, textile mills, and trip hammers. Evidence of water wheels and watermills dates to the ancient Near East in the 4th century BC, and the Roman Barbegal mill in modern-day France ran 16 water wheels processing up to 28 tons of grain per day.
A dam-and-reservoir plant stores water that is released on demand to spin a turbine, giving it more control over when power is produced. A run-of-river plant uses a barrage without a reservoir and depends on continuous water flow, so it generates more electricity in the rainy season and less in the dry season.
Foreign powers and international organizations used hydropower projects in Africa to influence economic development, including the World Bank with the Kariba and Akosombo Dams and the Soviet Union with the Aswan High Dam. Between 1977 and 1990 the Aswan High Dam's turbines generated one third of Egypt's electricity, and the project triggered a dispute between Sudan and Egypt over sharing the Nile.