Hydropower can be captured wherever a flow of water falls from a higher level to a lower level. This may occur where a stream runs down a hillside, or a river passes over a waterfall or man-made weir, or where a reservoir discharges water back into the main river.

The vertical fall of the water, known as the head, is essential for hydropower generation. Fast-flowing water on its own does not contain sufficient energy for useful power production, except on a very large scale, such as offshore marine currents. It is generally better to have more head than more flow, since this keeps the equipment smaller.

Fig. 43 – Principle of a hydropower system