Most hydroelectric stations use either the natural drop of a river, such as a waterfall or rapids, or a dam built across a river to raise the water level and provide the drop needed to create a driving force.
Water at the higher level is collected in the forebay. It flows through the station's intake into a pipe, called a penstock, which carries it down to a turbine. The turbine is a type of water wheel that is connected to a generator. As the water flows down the penstock the water pressure increases. It is this pressure and flow that causes the turbine to revolve which in turn spins a generator.
Inside the generator are large electromagnets attached to a rotor located within a coil of copper wires called the stator. As the generator rotor spins the magnets a flow of electrons is created in the coils of the stator.
This produces electricity that can be stepped up in voltage through the station’s transformers and sent across transmissions lines. The falling water, having served its purpose, exits the generating station through what is called the tailrace, where it rejoins the main stream of the river.