Some hydro schemes have dams and reservoirs, ours do not. A dam raises the water level and hence the pressure at the turbine, as well as storing water for times of high energy demand. Since they have no storage mechanism, run-of-river hydro schemes operate according to the flow rate present at any one time. Hydro schemes without storage operate more consistently than wind and wave power devices.

Power output depends on the pressure (or head) at the turbine and the water flow rate. Large rivers tend to exist in lowland areas where heads are small and are therefore less well suited to financially viable projects . The size and hence cost of the equipment needed to handle large volumes of water is great.