Hydroelectric power uses height differences and large volumes of water to spin a turbine’s blades and create electricity. Dams can be massive, such as Three Gorges Dam, or smaller, such as those found in mountain rivers.
Dams are highly efficient in converting mechanical energy into electrical energy, efficiencies reaching almost 90%.
Merits of hydroelectric energy are high efficiency, low emissions, relative reliability and additional benefits (flood control, water storage, irrigation). Drawbacks are generation limits almost reached in developed countries, high environmental footprint, disturbance of water ecosystems, siltation (which might limit the life of a dam to 100 years) and unsolved questions on end-of-life dams (Webber 2014; Sheldon 2013)