Run-of-river hydro uses the natural flow of a river as it passes by, with little or no water stored behind a dam. Reservoir hydro holds back a large volume of water behind a dam and releases it when electricity is needed. The difference comes down to storage: reservoirs can be dispatched on demand like a giant battery, while run-of-river output rises and falls with the seasons.
Key takeaways
- Reservoir hydro stores water, so it can ramp up and down to follow demand and back up variable sources like wind and solar.
- Run-of-river hydro has a much smaller physical footprint but produces less in dry months and freeze-up.
- Both affect rivers and fish; reservoirs flood more land and can change ecosystems and communities more dramatically.
- Canada's grid uses both, and many large systems combine big storage reservoirs upstream with run-of-river stations downstream.
How reservoir hydro works
A reservoir (or "storage") station uses a dam to create a lake. Water flows from the lake through large pipes called penstocks, spins turbines, and exits downstream. Because the operator controls how much water goes through the turbines, the plant can generate more in a cold January evening and less on a mild spring night.
That flexibility is the main reason reservoir hydro is so valuable. Quebec, Manitoba, British Columbia and Newfoundland and Labrador all rely on large storage systems. Some reservoirs hold enough water to smooth out not just daily swings but seasonal and even multi-year variations in rainfall and snowmelt.
The trade-off is scale. Creating a reservoir floods valleys, forests and sometimes homes, hunting grounds and burial sites. Flooded vegetation can release methane and carbon dioxide for years, and flooding can raise mercury levels in fish, which matters a great deal for communities that rely on country food. You can read more about these impacts in our post on hydro and Indigenous communities.
How run-of-river hydro works
A run-of-river station diverts part of a river's flow, often through a channel or pipe, to a powerhouse downstream, then returns it to the river. Some designs use a low weir or small "pondage" that stores a few hours of water at most.
Because there is little storage, output follows the river. In British Columbia, for example, many independent run-of-river projects produce most of their power during the spring freshet, when snowmelt swells mountain creeks, and much less in late winter.
Run-of-river projects can still have real impacts. The stretch of river between the intake and the powerhouse (sometimes called the "diversion reach") can carry significantly reduced flow, which affects fish habitat, and access roads and transmission lines disturb the surrounding land. Good projects set minimum flow requirements and monitor fish populations.
Side-by-side comparison
| Feature | Reservoir hydro | Run-of-river hydro |
|---|---|---|
| Water storage | Large: days to years | Little or none: minutes to hours |
| Can follow demand? | Yes, highly flexible | Limited; follows river flow |
| Land flooded | Often large areas | Small |
| Seasonal output | Fairly steady year-round | Peaks with snowmelt and rain |
| Typical size | Hundreds to thousands of MW | Under 1 MW to a few hundred MW |
| Grid role | Baseload, peaking, backup for wind and solar | Energy supply, often paired with storage elsewhere |
| Main environmental concerns | Flooding, methane, mercury, fish passage, displacement | Reduced flow in diversion reach, fish habitat, roads |
Why the grid needs both
Electricity systems need two things: energy (total kilowatt-hours over a year) and capacity (the ability to deliver power at the exact moment it's needed). Run-of-river stations are good at the first and weaker at the second. Reservoir stations do both.
In practice, many Canadian rivers have a mix. A large reservoir sits at the top of a river system, and a chain of stations downstream use the regulated flow, so they behave almost like run-of-river plants while benefiting from the upstream storage. The Niagara River stations in Ontario are a well-known example of mostly run-of-river generation, supported by a small pumped storage facility that shifts water between night and day (see our explainer on pumped storage hydro).
Reservoirs also make it easier to add wind and solar to the grid. When the wind is blowing, a hydro operator can hold water back; when it drops, they release more. That pairing is one reason provinces with big reservoirs are well placed for a low-carbon future.
Which one is "greener"?
There's no single answer. Lifecycle emissions for both are low compared with fossil fuels, though reservoir emissions vary widely depending on how much organic matter was flooded and the climate of the region. Northern reservoirs in Canada generally have lower emissions than tropical ones.
When comparing projects, it helps to look at:
- Area flooded per megawatt. Smaller is generally better.
- Fish passage and minimum flows. Does the project protect spawning runs?
- Who was consulted and who benefits. Projects co-owned with local and Indigenous communities tend to be designed with more care for local impacts.
- What the power displaces. Hydro that replaces coal or gas generation delivers bigger climate gains.
For a broader look at how hydropower fits into Canada's mix, see our hydro overview and the glossary for terms like capacity factor and head.
Frequently asked questions
Is run-of-river hydro better for the environment than dams?
It usually floods far less land, so its footprint is smaller, but it can still significantly reduce flow along the diverted stretch of river. The best choice depends on the specific river, fish species and how well the project is designed and operated.
Why does run-of-river hydro produce less power in winter?
In much of Canada, rivers run lowest in late winter when precipitation is locked up as snow and ice. Without a reservoir to store water from the spring melt, a run-of-river plant can only use what is flowing at the time.
Can a reservoir dam be removed if it causes problems?
Yes, dams have been removed in Canada and elsewhere, usually small or aging ones, to restore fish passage. Removing a large dam is expensive and complex, so most efforts focus on better operating rules, fish ladders and habitat restoration instead.
The bottom line
Reservoir and run-of-river hydro are two tools with different strengths. Reservoirs deliver flexible, on-demand power but reshape landscapes; run-of-river plants tread more lightly but follow the river's moods. Canada's grid is cleaner because it uses both, and understanding the trade-offs helps you judge new projects in your own province.
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