In the upper reaches of the Skykomish River in Washington state, a pioneering team of civil engineers keeps their cool. The relocated beavers boosted water storage and reduced stream temperatures, indicating that such schemes could be an effective tool in mitigating some of the effects of climate change.
Just one year after their arrival, the new recruits reduced the average water temperature by about 2 degrees Celsius and raised water tables by as much as a foot, the researchers report in the July journal Ecosphere. Although researchers have discussed beaver dams as a means of restoring streams and increasing groundwater volumeeffects after a large and specific relocation had been relatively unknown (Serial number: 03/26/21).
“That water storage is so critical during drier periods, because that’s what can keep the ecosystem resistant to drought and fire,” says Emily Fairfax, an ecohydrologist at California Channel Islands State University in Camarillo who was not involved. in the study.
The Skykomish River flows down the west side of Washington’s Cascade Mountains. Climate change is already transforming the region’s hydrology: snow cover is shrinking and snowfall is turning to rain, which drains quickly. The waters are also warming, which is bad news for salmon populations struggling to survive in warm waters.
Beavers are known play with hydrology too (Serial number: 7/27/18). They build dams, ponds, and wetlands, deepening streams for their burrows and shelters (with underwater entrances). Dams slow the speed of water, storing it upstream longer and cooling it as it flows through the ground.
From 2014 to 2016, aquatic ecologist Benjamin Dittbrenner and his colleagues relocated 69 beavers (castor bean) from lowland areas of the state to 13 sites upstream in the Skykomish River watershed, some with relict beaver ponds and others intact. Since beavers are family-oriented, the team moved entire clans to increase the chances of them sticking around.
The researchers also matched singles with potential mates, which seemed to work well: “They weren’t picky at all,” says Dittbrenner of Northeastern University in Boston. Fresh logs and wood clippings got the beavers started in their new neighborhoods.
At the five sites that saw long-term construction, beavers built 14 dams. Thanks to those dams, the volume of surface water (streams, ponds, wetlands) increased about 20 times that of streams without new beaver activity. Meanwhile, below ground, wells at three sites showed that after the dam was built, the amount of groundwater grew to more than double that stored above ground in ponds. Stream temperatures downstream of the dams dropped 2.3 degrees C on average, while streams not subject to beaver adjustments warmed 0.8 degrees C. All of these changes occurred during the first year after relocation.
“We’re hitting restoration goals almost instantly, which is really cool,” says Dittbrenner.
Crucially, the dams lowered temperatures enough to take the streams almost entirely out of the harmful range for salmon during a particularly hot summer. “These fish are also experiencing heat waves within the water system and the beavers are protecting them,” says Fairfax. “That to me was huge.”
The study also found that the small, shallow abandoned beaver ponds were actually heating the streams, perhaps because the cooling system had broken down over time. Targeting these ponds as potential relocation sites could be the most effective way to reduce temperatures, the researchers say. When relocated populations settle and breed, young beavers leaving their homes might seek out those abandoned spots first, Dittbrenner says, since it uses less energy than starting from scratch. “If they find a relic pool, the game is on.”