Water has been used to produce electricity for about a century.Today, around one fifth of the world’s electricity is produced from hydro power. Large unsustainable hydroelectric power plants with concrete dams and extensive collecting lakes often have very negative effects on the environment, however, requiring the flooding of habitable areas. Smaller ‘run-of-the-river’ power stations, which are turbines powered by one section of running water in a river, can produce electricity in an environmentally friendly way. The main requirement for hydro power is to create an artificial head so that water, diverted through an intake channel or pipe into a turbine, discharges back into the river downstream. Small hydro power is mainly ‘run-of-the-river’ and does not collect significant amounts of stored water, requiring the construction of large dams and reservoirs. There are two broad categories of turbines: impulse turbines (notably the Pelton) in which a jet of water impinges on the runner designed to reverse the direction of the jet and thereby extract momentum from the water.This turbine is suitable for high heads and ‘small’ discharges. Reaction turbines (notably Francis and Kaplan) run full of water and in effect generate hydrodynamic ‘lift’ forces to propel the runner blades. These turbines are suitable for medium to low heads, and medium to large discharges.