A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as ground flour or lumber production, or metal shaping (rolling, grinding or wire drawing). A watermill that generates electricity is frequently called a hydroelectric plant.
The ancient Greeks and Romans are thought to be the first to used water to power their mills. In the early 1st century BC, the Greek epigrammatist Antipater of Thessalonica referenced a waterwheel which was effectively used to grind grain and reduce human work.
The Romans built some of the first watermills outside of Greece for grinding flour and spread the technology for constructing watermills throughout the Mediterranean. The picture to the right is a reconstructed watermill in Ayrshire, Scotland.
A watermill works by diverting water from a river or pond to a water wheel, usually along a channel or pipe. The water's force drives or pushes the blades of the wheel (or turbine) which then turns or rotates an axle that drives whatever machinery is attached to it. After turning the waterwheel, the water exits the watermill. Sometimes mills are stacked along a waterway so that water passes through multiple mills turning many wheels.
Waterwheels generate power from water flowing through the wheel which turns gears to perform work -- which may be grinding corn. Today, hydroelectric power plants also harness the power of water to provide electricity throughout the world. Hydroelectricity is a form of hydropower, and is the most widely used form of renewable energy today. Hydroelectric and coal-fired power plants produce electricity in a similar way. A hydroelectric plant uses the power of passing water to turn a propeller or wheel-like device called a turbine. The turbine in turn rotates a shaft in an electric generator to produce electricity.