North American Beaver (Castor canadensis)

American Beaver


Beaver were likely to be prevalent in the Sierra Nevada, but they were extirpated from the region by trappers early enough that they were missed entirely by field biologist and zoologist Joseph Grinnell when he led surveys of the Sierra Nevada region between 1914 and 1920. Because of this, it was thought that beaver were not native to the Sierra Nevada until recent evidence confirmed their long residency here. Beaver have also been historically perceived by game managers and other wildlife officials as pest species. It is now well recognized that beaver are a species of great importance and benefit to aquatic and riparian ecosystems.

Image right: American Beaver, Photo by "Steve from Washington, DC, USA" [CC BY-SA 2.0 (] via Wikimedia Commons.

Throughout North America, beaver were nearly driven to extinction by the pursuit of the species for the fur trade, particularly by the Hudson Bay Company. Fortunately, beaver are a resilient species and with the implementation of regulated hunting and trapping, along with reintroduction efforts, beavers have made a remarkable recovery throughout most of their range. We are featuring it here because beaver have not made a full comeback in the Sierra Nevada and continue to be threatened by misguided and outdated management policies that include killing beavers by the U.S. Forest Service in the Tahoe Basin Management Unit. Beaver are not only native to the Sierra Nevada, they are hugely important ecosystem architects of aquatic habitats and are considered keystone species upon which the health and diversity of riparian ecosystems are dependent.


Beaver alter the structure and function of streams by cutting trees and building dams, activities that result in development of bog, meadow, and marsh wetlands. The result of their activities is increased biodiversity at multiple levels and scales. Beaver activities contribute to enhanced resources for a variety of species including birds associated with riparian and meadow habitat, waterfowl, and fish including trout and salmon. Beaver dams have been shown to improve water quality downstream from their dams. Studies demonstrate that beaver dams are rarely an impediment to migrating salmonids and may actually be an important association necessary for recovery of salmonid species. There are effective flow devices that can be installed in beaver dams that permit fish passage where it is found necessary.

Beaver diets include the leaves, buds, and inner bark of growing trees. While aspen and poplar are their preferred foods, they also eat willow, maple, alder, oak, occasionally pine and spruce. They also eat cattails and other types of aquatic vegetation, especially in the early spring. Despite their diet and the use of trees for dam building, aerial surveys of riparian habitats have shown that riparian vegetation is densest in areas with beaver dam presence.

Beavers are the largest rodents in North America, and, along with the European beaver, the second largest rodent in the world. Adult beavers typically weigh between 24 to 71 pounds, averaging approximately 44 pounds. The beaver has a large, flat tail ranging from 7.9 to 13.8 inches in length. Its body, minus the tail, ranges in length from 29 to 35 inches. The beaver uses its flat tail to slap the water to warn of danger, and the tail also acts as fat storage to help the beaver to maintain warmth in its aquatic habitat.

The beaver is a monogamous species. Breeding occurs in late winter and litters are born in the early spring. Litter size is typically 2-4 but can range from 1 to 9.


The North American beaver is one of two species of beaver world wide (the other species is the European beaver, Castor fiber, which is native to the Eurasian continent). Beaver were once found in virtually all aquatic ecosystems in North America, from the Arctic tundra to the deserts in northern Mexico. Almost every northern temperate ecosystem that had trees or shrubs growing along streams also once had beaver dams.

Beginning in the early 17th century, tens of thousands of beaver were killed annually to supply the demand for beaver fur, primarily for hats and coats. The disappearance of beaver in the east is thought to be a prime reason for the western expansion, as trappers moved further west in search of new populations of beaver. As a result, the beaver was nearly extinct in North America by 1900. Wetlands simply dried up in the absence of beaver damming activities. It has been estimated that 195,000 to 260,000 square kilometers of wetlands in the U.S. were converted to dry land as a result of the loss of beaver.

Although beaver have occurred historically in every other North American mountain range from the Arctic Circle to northern Mexico, there are few historical references to beaver in the Sierra Nevada, and confirmation of their occupation in the region remained controversial until recently. In 1988, previously buried beaver dams were discovered in the Feather River watershed in eastern Plumas County in an incised stream channel 60 miles north of Truckee. Using radiocarbon dating methods, researchers determined that the beaver dams dated back to AD 580, and were more or less continuously used until the onset of ranching settlements in the area around 1850 and coinciding with the peak of beaver trapping for the fur trade. Restoration of beaver in the Sierra Nevada may be necessary to adequately restore Sierran meadows and other wetlands and restore habitat for salmonids and other aquatic and riparian species that coevolved in association with beaver.

Beaver populations currently extant in the Sierra Nevada are descended from reintroduction efforts in the last century, although the current populations in the region may also include genetic mixing from beaver migration from valley stream populations into the mountains, as well as from the Cascades south into the Sierra Nevada.


The Sierra Forest Legacy coalition and partners have asked the Forest Service to include the following changes to the draft forest plans for the Sierra and Sequoia National Forests to help conserve and benefit beaver:

Desired Conditions

  • Riparian ecosystem composition, structure, and function is restored and enhanced by
    beaver habitat.
  • Beaver habitat (including wetlands and riparian areas), which benefit and enhance
    groundwater, surface water, and floodplain and riparian complexity, is present
    forestwide in suitable areas.
  • The presence of beavers and the persistence of beaver habitat, contributes to channel
    recovery and floodplain function.

Potential Management Approaches

  • Conduct a beaver restoration assessment across the plan area and to evaluate locations
    where beavers can help improve instream flows and attenuate late summer flows.
  • Evaluate opportunities to support expansion of beavers from known locations.
  • Cooperate with federal, tribal, and state governments and other stakeholders to identify
    potential stream areas for beaver reintroduction.


Beaver are not rated by any conservation organization or official agency as a sensitive species or threatened species of conservation concern. It is legal to trap or kill beaver with a permit and no bag limit in 42 of 58 counties in California.

Scientific Research

Baker, B. W., and E. P. Hill. 2003. Beaver (Castor canadensis). Pages 288-310 in G. A. Feldhamer, B. C. Thompson, and J. A. Chapman, editors. Wild Mammals of North America: Biology, Management, and Conservation. Second Edition. The Johns Hopkins University Press, Baltimore, Maryland, USA.(585 KB PDF)

James, Charles D., and Richard B. Lanman. 2012. Novel physical evidence that beaver historically were native to the Sierra Nevada. California Fish and Game 98(2):129-132; 2012. (481 KB PDF)

Fountain, S.M. 2014. Ranchers’ friend and farmers’ foe: Reshaping nature with beaver reintroduction in California. Environmental History. 19(2):239–269 (742 KB PDF)

Lanman, R.B., Perryman, H. Dolman, B., and C.D. James. 2012. The historical range of beaver in the Sierra Nevada: a review of the evidence. California Fish and Game 98(2):65-80. (522 KB PDF)

Lundquist, Kate and Brock Dolman (Occidental Arts and Ecology Center Water Institute). 2018. Beaver in California: Creating a Culture of Stewardship. (9.4 NB PDF)

Lundquist, Kate and Brock Dolman (Occidental Arts and Ecology Center Water Institute). 2018. Beaver Restoration Feasibility Assessment for the North Fork Kern River Drainage. Report to California Trout. (6 MB PDF).

Muskopf, Sarah. 2007. The effect of beaver (Castor canadensis) dam removal on total phosphorus concentration in Taylor Creek and Wetland, South Lake Tahoe. Thesis, California Humboldt State University, Natural Resources. (2 MB PDF).

Naiman, R.J.; Johnston, C.A., and J.C. Kelley. 1988. Alteration of North American Streams by Beaver BioScience. 38(11): 753–762. (2.6 MB PDF)

Pollock, M.M., G.M. Lewallen, K. Woodruff, C.E. Jordan and J.M. Castro (Editors) 2017. The Beaver Restoration Guidebook: Working with Beaver to Restore Streams, Wetlands, and Floodplains. Version 2.0. United States Fish and Wildlife Service, Portland, Oregon. 219 pp. Online at: (10 MB PDF)

Wright, J.P.; Jones, C.G.; Flecker, A.S. 2002. An ecosystem engineer, the beaver, increases species richness at the landscape scale. Oecologia. 132(1): 96–101. (130 KB PDF)

Supporting Resources

Go to Worth a Dam ( to read more about the ecological benefits provided by beaver.

California Department of Fish and Game Natural History Information --This California state website contains rather limited and old information but is a good basic background composite for the species. Choose from a drop-down list to select the animal you are interested in.

Return to Top