Hydropower is obtained from the kinetic energy of a mass of water, which is converted into mechanical energy (by the rotation of a shaft) through hydraulic turbines. These turbines, in turn, drive alternators that generate electrical energy, as happens in hydroelectric power plants. Hydroelectric power plants are generally classified as run-of-river plants or regulated plants. Run-of-river plants are characterized by limited storage capacity and flow regulation. Regulated plants have storage capacity and can regulate water flows up to certain limits. This type of energy use of river water has become, since the beginning of the 20th century, one of the preferred forms of electricity production. In fact, in the 1950s and 60s in Portugal, almost all consumption was ensured by hydroelectric plants, with thermal power plants only used for support during periods of drought.