Run-of-river hydro
Hydropower that diverts or uses the natural flow of a river with little or no reservoir storage, so output follows river flow rather than demand.
Run-of-river (RoR) plants use a weir or low dam to divert water through an intake, sometimes via a canal or penstock to gain head, then return it to the river downstream. Storage, if any, is limited to hours of pondage, so generation tracks seasonal hydrology: high during snowmelt or rainy seasons, low in dry periods.
Power follows the general hydropower equation:
with η overall efficiency (0.80–0.90 for large units), ρ = 1,000 kg/m³, g = 9.81 m/s², Q flow (m³/s) and H net head (m). A plant passing 50 m³/s through 20 m of head at 88% efficiency yields about 8.6 MW.
Key characteristics:
- Capacity factor: 40–65% for well-sited plants, depending on the design flow chosen relative to the flow duration curve
- Turbines: Kaplan and bulb units for low-head large rivers, Francis or Pelton for high-head mountain diversions
- Examples: Rhine and Danube cascades, Jirau and Santo Antônio on the Madeira (about 3.5–3.75 GW each), Alpine and Himalayan diversion schemes
RoR avoids most reservoir impacts such as large-scale flooding and methane emissions, though diversion schemes can dewater river stretches and require minimum environmental flows and fish passages. Because output is not dispatchable, RoR behaves more like a variable renewable source than reservoir hydro. Micro and small hydro below 10 MW are almost always run-of-river.
Related terms
All terms →- Hydraulic head
The vertical height difference between the water surface at a hydropower intake and the turbine outlet or tailwater, which together with flow determines available power.
- Kaplan turbine
A propeller-type reaction hydro turbine with adjustable runner blades and wicket gates, suited to low heads of about 2 to 40 m and large flows.
- Francis turbine
A reaction-type hydro turbine with fixed blades in which water enters radially and leaves axially; the most widely installed design, used for medium heads of about 30 to 700 m.
- Pelton turbine
An impulse hydro turbine in which high-velocity water jets strike double-cupped buckets on a wheel; the standard choice for high heads above roughly 250 m.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.
- Pumped hydro storage
Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.