The driving or impelling force of water can be converted into mechanical energy. Grain millers were among the first to harness this type of power several thousand years ago. Millhouses built along a river or stream featured a waterwheel that was propelled by the fall of water; this wheel then rotated a millstone that crushed grain to make flour.

For water to create a force capable of propelling turbines in a hydroelectric plant, the flow (the volume of water running past a given point per second) must be strong enough and the head (how far the water falls) must be high enough. The head is the difference in elevation between a point upstream of a power plant (the water intake) and a point downstream (the tailrace). The higher the head, the farther the water falls and the more force it exerts on the turbine blades. This explains why certain rivers require retaining structures to raise the head.