Small hydropower plant on river with low flow. (Source: © bubutu / stock.adobe.com)

The amount of water that passes through a section of river per unit time. We can also use flow to describe the movement of a fluid in a pipe. For hydropower, flow, besides head, is a very important parameter indicating what type of turbine is most suitable for a given river and how much electricity it is likely to produce. For smaller rivers with low flows, e.g., a cross-flow or Pelton turbine is suitable; for flows varying throughout the year, a Kaplan turbine with adjustable blades is an option; and for high flows, a Francis turbine can be used.