Low-head hydro power refers to the development of hydroelectric power where the head is typically less than 20 metres, although precise definitions vary. Head is the vertical height measured between the hydro intake water level and the water level at the point of discharge. Using only a low head drop in a river or tidal flows to create electricity may provide a renewable energy source that will have a minimal impact on the environment. Since the generated power is a function of the head these systems are typically classed as small-scale hydropower, which have an installed capacity of less than 5MW.