Run-of-river and small hydro: Impact on the biosphere
No big reservoir, but many small weirs still chop rivers up
What this tab covers Effects on human, animal, plant, fungal and microbial life and on whole ecosystems: in the short term (construction and early years), medium term (operating life) and long term (decommissioning and legacy).Small and run-of-river schemes avoid large reservoirs, so they displace few people and emit little methane. But they still interrupt rivers, and many small plants on one river can fragment it as much as a large dam. Early monitoring of in-stream turbines found no fish injuries.
Deaths per TWh, hydropower excluding Banqiao 0.0411 Life-cycle emissions, hydropower, g CO2e/kWh 6-147 (UNECE); median 24 (IPCC AR5)98 Long rivers still free-flowing 37%12Effects over time
| Life affected | Short term | Medium term | Long term |
|---|---|---|---|
| Humans611 | Little displacement; local construction jobs. | Electricity for remote communities; one in-stream turbine cut a village's diesel use by one-third while running. Hydropower death rate excluding the Banqiao disaster is 0.04 per TWh. | Small structures pose little failure risk to large populations. |
| Animals612 | Disturbance of riverbed during construction. | Weirs block fish migration and reduce flow in the bypassed reach. At the RivGen turbine, cameras showed no obvious injuries to fish, and adult salmon monitoring was dropped in 2022 after years with no documented collisions. | Cumulative fragmentation: only 37% of rivers over 1,000 km remain free-flowing, with dams the leading cause. |
| Plants | Small clearings for intakes, pipes and powerhouses. | Riverside vegetation in bypassed reaches can change with lower flows. | Little evidence of effect. |
| Fungi and soil life | Little evidence of effect. | Little evidence of effect. | Little evidence of effect. |
| Ecosystems812 | Local disturbance only. | Low life-cycle emissions (hydropower median 24 g CO2e/kWh, with run-of-river at the low end). | Many small schemes on one river can add up to large-scale fragmentation. |
How the effects are reduced
Leaving a minimum environmental flow in the bypassed reach, fish passes and screens at intakes, and fish-friendly turbine designs reduce harm. Planning at the scale of a whole river basin, rather than plant by plant, avoids fragmenting rivers with many small weirs. For in-stream turbines, real-time camera monitoring is used; at Igiugig, rotor speed appeared to affect how young salmon passed the device, which suggests a way to manage risk during migration.
Sources
- RivGen Power System, Igiugig, Alaska (project site and monitoring summary) Tethys, Pacific Northwest National Laboratory, 2024
- Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters, Table A.III.2 (life-cycle emissions, min/median/max) IPCC AR5 Working Group III, 2014
- Carbon neutrality in the UNECE region: integrated life-cycle assessment of electricity sources (ECE/ENERGY/141) United Nations Economic Commission for Europe, 2022
- What are the safest and cleanest sources of energy? Our World in Data, 2024
- Mapping the world's free-flowing rivers (Grill et al., Nature 569) Michigan Technological University Digital Commons, 2019