Hydroelectric energy is obtained from the kinetic energy of a mass of water that is converted into mechanical energy (by the rotation of an axis) through hydraulic turbines, which in turn drive alternators that generate electricity, as in hydroelectric power plants.
Generally, hydroelectric plants are classified as run-of-river power stations or power stations with regularization. Run-of-river power stations are characterized by limited storage capacity and flow regulation. The plants with regularization have storage capacity and can regulate, up to certain limits, the water flows.
This type of energy use of river water has become, since the beginning of the 20th century, one of the privileged ways of producing electricity. In fact, in the 1950s and 1960s, in Portugal, almost all consumption was provided by hydroelectric power stations, using only thermal power stations to support in periods of drought.