Small hydropower: “Run-of-the-river” (ROR) hydroelectricity

Run-of-river hydropower is a method of producingrenewable hydraulicenergy that uses the natural flow of a river to generate electricity, without the need to build a large dam and create a reservoir.

How run-of-river power plants work

The operation of run-of-river hydroelectric power plants is based on the use of the kinetic force of water flowing naturally along a river. Here are the main stages in the process of generating electricity in a run-of-river power plant:

  1. Water capture : Part of the river’s flow is captured and directed into a headrace or penstock that carries the water to the hydropower plant. The rest of the water continues to flow in the river, maintaining its natural flow.
  2. Turbines : Captured water is conveyed to turbines, usually Kaplan or Francis turbines, specially designed to operate at low to moderate flows and heads. The force of moving water turns the turbines, transforming the water’s kinetic energy into mechanical energy.
  3. Generator : The rotation of the turbines drives an electrical generator coupled to them. The generator converts mechanical energy into electrical energy by means of electromagnetic induction. The electricity produced is then fed into the power grid.
  4. Return of water to the river: After passing through the turbines, the water is returned to the river via a tailrace. This minimizes the impact on the local ecosystem and maintains the river’s natural flow.

Run-of-river hydroelectric plants are relatively simple to operate, with fewer components and infrastructure than large dams. They are also generally easier to build and maintain, and have less environmental impact. Nevertheless, energy production depends on river flow and can therefore vary according to climatic and seasonal conditions.

How to calculate production potential

To calculate the production potential of a run-of-river hydroelectric plant, several parameters need to be taken into account, including water flow and head (the difference in height between the water entering and leaving the plant). Available power (in watts) can be estimated using the following formula: Power = η × ρ × g × Q × H where :

  • η is the plant efficiency (usually between 0.8 and 0.9)
  • ρ is the density of the water (approx. 1000 kg/m³)
  • g is the acceleration due to gravity (9.81 m/s²)
  • Q is the water flow rate (in m³/s)
  • H is the head (in metres)

Concrete example: the Coo-Trois-Ponts power plant in Belgium

Let’s take the example of the Coo-Trois-Ponts run-of-river hydropower plant in Belgium. This plant uses water from the Amblève, a river in the Walloon region of Belgium.

The Coo-Trois-Ponts power plant has an installed capacity of 1,164 MW. To calculate the production potential, we need to know the river’s flow rate and head. The Amblève has an average flow of around 30 m³/s. The effective head of the power plant is around 250 metres. However, to preserve aquatic fauna, a certain instream flow must be maintained. Let’s assume that the local authorities require an instream flow equivalent to 20% of the mean flow, i.e. around 6 m³/s. The usable flow for power generation would then be 30 m³/s – 6 m³/s = 24 m³/s.

To estimate energy production, we can use the following formula: Power (kW) = Usable flow (m³/s) × Head (m) × Efficiency × Gravity acceleration (9.81 m/s²).

Assuming a plant efficiency of 90%, we obtain : Power (kW) = 24 m³/s × 250 m × 0.9 × 9.81 m/s² ≈ 53,154 kW, or approximately 53.15 MW. To calculate total output, simply multiply this by the number of operating hours.

Advantages and disadvantages of run-of-river power plants


  1. Low-carbon renewable energy (REC): Run-of-river hydropower uses the power of water to generate electricity, making it a renewable and sustainable energy source.
  2. Low greenhouse gas emissions: Run-of-river power plants have a relatively low environmental impact in terms of greenhouse gas emissions, as they do not directly produce CO2 when generating electricity.
  3. Production flexibility: Run-of-river plants are generally more flexible than large hydroelectric plants with dams, as they can adapt quickly to variations in electricity demand and fluctuations in river flow.
  4. Reduced environmental impact: Compared to hydroelectric dams, run-of-river power plants generally have a lower environmental impact, as they do not require the construction of large dams and the creation of vast reservoirs, which can disrupt local ecosystems.


  1. Dependence on river flow: Electricity production from run-of-river power plants is highly dependent on river flow, which can vary according to weather conditions and seasons. This makes their energy production less predictable and less constant than that of dam power plants.
  2. High initial cost: Building a run-of-river hydropower plant can require a significant initial investment, although operating and maintenance costs are generally low.
  3. Impact on aquatic fauna: Although run-of-river power plants have a lower environmental impact than dams, they can nevertheless affect aquatic fauna by modifying water flow and creating obstacles to fish migration. Fish passage devices often have to be installed to mitigate these impacts.
  4. Limited energy production: Run-of-river hydroelectric plants generally have a lower energy production capacity than dammed plants, as they depend on the natural flow of the river and cannot store water to produce electricity on demand.

How much of a river’s flow can be used without affecting wildlife?

There is no single answer to this question, as the proportion of a stream’s flow that can be used without harming wildlife depends on many factors, including the type and sensitivity of the species present, the geography and characteristics of the watercourse, seasonal variations in flow and regulatory requirements.

However, a general rule often adopted to ensure the protection of aquatic fauna is to maintain an instream flow, also known as ecological flow or minimum flow. This instream flow is the part of the river’s natural flow that must be left free of any abstraction to ensure the preservation of aquatic ecosystems.

Instream flows are generally defined by legislation or local regulations, and can vary according to the specific characteristics of the watercourse and the region concerned. For example, in some regions, 10% to 30% of the natural flow rate may be required to preserve flora and fauna. In other cases, instream flows may be determined according to specific thresholds for different periods of the year, taking into account species life cycles and seasonal flow variations.

It is important to note that the impact of a run-of-river hydroelectric plant on wildlife also depends on the design of the facility and the environmental management measures put in place, such as fish passage devices (fish ladders, fish passes) or habitat restoration.

Run-of-river power plants around the world

Here’s an overview of run-of-river hydropower production by country and some notable installations:

  1. The United States is a major producer of run-of-river hydropower. There are several run-of-river power plants, notably on the Columbia River. The Dalles hydroelectric plant in Oregon is one of the largest of its kind in the United States, with a capacity of 1,878 MW.
  2. Canada has a vast hydroelectric resource and operates numerous run-of-river power plants. One of the largest is the La Romaine-1 power station in Quebec, with a capacity of 270 MW.
  3. Germany is another European country that benefits from run-of-river hydropower. The Rheinfelden power station on the Rhine has a capacity of 100 MW.
  4. France has numerous run-of-river power plants, particularly in mountainous regions. The Bort-les-Orgues power station on the Dordogne has a capacity of 235 MW.
  5. Norway, with its many rivers and fjords, is a major producer of run-of-river hydroelectricity. The Tonstad power station in southern Norway has a capacity of 960 MW.
  6. Brazil is another country that exploits run-of-river hydropower, mainly in the Amazon basin. The Santo Antônio power plant, located on the Madeira River, has a capacity of 3,568 MW.
  7. China also has run-of-river hydroelectric power plants. The Longyangxia power station on the Yellow River has a capacity of 1,280 MW.