Rivers where European beavers are present store 26% more carbon

The European beaver (Castor fiber) , a species protected by the European Union, has increased its population over the last two decades in Spain and is now on the doorstep of Catalonia. A new study published today in Communications Earth & Environment, led by researcher Lukas Hallberg from the University of Birmingham and in which CREAF participates, shows, for the first time, that the dams built by beavers cause those river areas to store 26% more carbon over the course of the year. This carbon accumulates mainly in sediments and dead wood generated by flooding , and can remain stored for up to three decades. This species also has positive effects on the water cycle , because it infiltrates more and fills aquifers.

The European beaver is a native species that practically disappeared from the European map in the 19th century due to hunting. “Its expansion in Spain, which began in early 2000, has generated controversy due to the possible effects on riverbank forests. But we must take into account that the vegetation we have here is adapted to its presence and with this study we show the benefits that this mammal brings to mitigating climate change”, highlights J osep Barba, CREAF researcher and co-author of the study . There is also concern in the agricultural sector, especially in crops located next to the riverbed, “but almost all the beaver activity is concentrated in the first 20 meters from the riverbank, so the effect tends to be very localized”, continues Barba.

Although the researcher also clarifies that reintroduction should not be done illegally in any case and must always be planned and based on scientific evidence.

The mechanism behind this increase in carbon is that beavers build their dams by cutting branches, bushes and small trees which they then weave together with mud, stones and sediment from the river itself. This structure slows down the speed of the water and creates a flooded area upstream. As the current decreases, organic material such as leaves, plant debris or dead wood accumulates and becomes part of the sediments that contain a lot of carbon . “By slowing down the water, retaining sediment and expanding wetlands, streams become powerful carbon sinks. This pioneering study represents a major opportunity and a breakthrough for future nature-based climate solutions across Europe,” highlights Dr. Joshua Larsen, from the University of Birmingham and one of the lead authors of the study.

The study was conducted over a year in an 800-meter stretch of river in the Rhine basin in Switzerland where beavers have been present since 2010. All carbon fluxes in and out of the system were measured at three points along the river: upstream of the dammed area, within the beaver-modified area, and downstream. Specifically, the team combined flow measurements with automatic sensors, water sampling, and gas chambers to quantify CO₂ and methane emissions. They also analyzed sediments and biomass to calculate carbon storage.

According to the team, beaver dams also transform the river landscape. By slowing down the flow, water stops flowing rapidly downstream and has more time to infiltrate the subsoil. These flooded spaces increase the recharge of underground reserves and improve the quality of the water, which slowly filters through the sediments. “Perhaps with less water visible on the surface at times because the flow is reduced, but with more reserves hidden underground, available in the medium and long term in the event of drought,” explains Barba.

The European beaver nearly disappeared in the 19th century due to hunting for its skin, meat and fat. In the 20th century, several European countries, such as Switzerland, promoted its reintroduction, which allowed the species to gradually recover. In the case of Spain, this happened in 2003, when Belgian activists illegally released a dozen specimens in Navarra.

Initially, there was a perception that it was an invasive species. In this sense, the researcher clarifies that the European beaver should not be confused with the American beaver, which can generate much greater impacts outside its natural area. They are species with very different behaviors and, in the case of the European, it feeds mainly on grasses and shrubs less than five centimeters in diameter and builds smaller dams, usually in secondary or tertiary tributaries, not in large rivers. “With the American beaver we do see images in Patagonia with thousands of hectares of dead trees because it is not native; this would not happen with the European beaver.”

Since 2020 it has been included in the List of Wild Species under Special Protection Regime (LESRPE) in Spain , which prohibits its hunting and capture, except in very exceptional cases authorized by control. Since then, it has expanded throughout the Ebro, Guadalquivir and Tagus basins. There is still no evidence in Catalonia, but it has reached the gates of Mequinenza. So “seeing it in Catalonia is only a matter of time”, concludes Barba.

Article Reference: Hallberg, L., Larsen, A., Ceperley, N., d'Epagnier, R., Brouwers, TF, Schaefli, B., Thurnheer, S., Barba, J., Angst, C., Dennis, M., & Larsen, JR (2026). Beavers can convert stream corridors to persistent carbon sinks. Communications Earth & Environment. DOI: 10.1038/s43247-026-03283-8