In 2015, California's Scott River Watershed Council built 2 artificial beaver dams to restore wetlands. 11 years later, juvenile coho survival rose from 8% to 60% in restored habitat

Artificial beaver dams California: In northern California, artificial beaver dams helped recreate lost wetland habitat, while juvenile coho salmon survival in restored habitat rose from 8% to 60%.

The Scott River valley was once home to so many beavers that it was known as Beaver Valley. Their dams created an enormous riverine wetland that provided habitat for juvenile coho salmon and other wildlife. They also created cold, slow-moving habitat favored by young coho.

That changed after European fur trappers arrived in the 1830s. Thousands of beavers were trapped and killed, and much of the wetland habitat created by their dams was lost.

Two artificial dams were built in 2015

With the beavers all but gone, the nonprofit Scott River Watershed Council built two artificial dams on Sugar Creek, a tributary of the Scott River, in 2015.

Wooden posts were hammered into the streambed and interwoven with willow and conifer branches. The gaps were then plugged with gravel, straw and mud.

Three years later, several more dams were built on another tributary, French Creek.

Some of the remaining beavers eventually adopted the structures, repairing, modifying and sometimes expanding them.

Charnna Gilmore of the Scott River Watershed Council said, “When beavers do adopt a structure, their work is far superior to what we can accomplish,” said Charnna Gilmore of the Scott River Watershed Council, as quoted by Discover Wildlife.

Artificial dams created thousands of square metres of habitat

The structures created around 9,000 square metres of new habitat, with the capacity to support more than 8,500 young salmon.

Compared with areas where no new dams were built, water in the restored habitats stayed cooler. This helped avoid temperatures known to stress the fish and slow their growth.

The difference was particularly clear in juvenile coho survival in French Creek.

Before the dams were built, juvenile coho survival was 8%. Afterward, the survival rate rose to 60%.

More salmon returned to the Scott River

Coho salmon spend their first summer in freshwater before heading out to sea. They later return to streams to breed.

Two years after the first dams were built, more salmon were returning to the Scott River than to any of the other rivers monitored in the study.

While salmon numbers elsewhere remained low, returns to the Scott River stayed healthy even during severe drought.

The study was published in Frontiers in Ecology and Evolution. Fish biologist Michael Pollock of NOAA’s Northwest Fisheries Science Center described the results as “mind blowing.”

The project also underscored the role beavers can play in stream ecosystems.

Pollock said, “My take is that the artificial dams have helped people remember what the beaver can do and how integral they are to the health of stream ecosystems,” adding that, “But beavers can only thrive if landowners allow them to,” as quoted by Discover Wildlife.