Artificial beaver dams have been constructed in northern California to recreate lost wetlands, resulting in improved survival rates for threatened coho salmon. The findings, published in Frontiers in Ecology and Evolution, indicate that juvenile coho salmon survival rates increased from 8% to 60% after the installation of these dams.
Historically, the Scott River valley in northern California, known as Beaver Valley, was home to numerous beavers whose dams created extensive riverine wetlands that provided habitat for juvenile coho salmon and other wildlife. However, the arrival of European fur trappers in the 1830s led to the trapping and killing of thousands of beavers, resulting in the loss of much of the habitat that supported coho salmon.
In 2015, the Scott River Watershed Council built two artificial dams on Sugar Creek, a tributary of the Scott River. These structures were created using wooden posts, willow and conifer branches, and materials such as gravel, straw, and mud to fill gaps. Three years later, additional dams were constructed on another tributary, French Creek. The remaining beavers contributed to the maintenance and enhancement of these artificial structures. Charnna Gilmore from the Scott River Watershed Council noted that when beavers adopt a structure, their modifications are often superior to human efforts.
The artificial dams created approximately 9,000 square meters of new habitat, capable of supporting over 8,500 young salmon. Compared to areas without new dams, the water in the restored habitats remained cooler, which is beneficial for the fish's growth.
Coho salmon typically spend their first summer in freshwater before migrating to the ocean and returning to spawn. Two years after the initial dams were built, more salmon were returning to the Scott River than to any other monitored rivers, maintaining healthy returns even during periods of severe drought.
Pollock remarked that while the artificial dams have raised awareness about the ecological role of beavers, their populations can only thrive if landowners permit their presence.