Run-of-river hydropower generates electricity from the water a river carries at any given time. A low weir or intake diverts part of the flow into a channel or pipe, called a penstock, which drops it to a turbine downstream. The water then returns to the same river. Unlike a reservoir scheme, there is little or no storage, so output rises and falls with the river.
How it compares
| Type | Water storage | Output |
|---|---|---|
| Run-of-river | None, or a small pond behind a weir | Follows rainfall and river flow |
| Reservoir (storage) hydro | Large lake behind a dam | Can be held back and released when needed |
| Pumped storage | Two reservoirs at different heights | Stores grid electricity rather than generating new power overall |
The power from a scheme depends on two things: the head, or vertical drop, and the flow of water. Upland burns with a steep fall can generate useful power from modest flows, while lowland rivers need much larger volumes. Many sites reuse old mill leats and weirs.
Environmental points
Because a run-of-river scheme takes water out of a stretch of river, regulators set how much must be left to protect fish and other wildlife, and many schemes need fish passes or screens. A licence from the environmental regulator is normally required to take and return the water. Without a large reservoir, run-of-river schemes avoid flooding valleys, which is the main impact of big dams.
UK picture
DESNZ puts UK hydro capacity at 1.9 GW at the end of 2025, of which 0.42 GW is classed as small scale; these figures cover all hydro except pumped storage, not only run-of-river. Scotland holds 1.67 GW, almost nine-tenths of the total. Hydro generated 5.4 TWh in 2025, 6% less than in 2024, in line with lower rainfall, and very little new capacity has been added in recent years.
Read more: Hydropower: how it works and its role in the UK.