Beaver Science

Beavers are amazing and important creatures. They are ecosystem engineers, building dams that create ponds and wetlands. The cool, deep water of beaver ponds can be vital habitat for fish, including coho salmon. In summer 2016, JCWC volunteers began mapping Johnson Creek’s beaver activity, in hopes of better understanding how to work with beavers to restore the watershed.

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Survey Methods
Data Summary
Map of Beaver Dams
Beaver Video
Background on Beavers
Helpful Links

Survey Methods

2016 beaver surveys took place mainly on one weekend in August and two weekends in September. Twenty-nine volunteers surveyed 13 reaches, recording all beaver dams and other activity signs such as chewed trees and mud slides. Each survey was conducted by 2-3 volunteers.

Reaches were selected based on likelihood of finding beaver activity: we sent volunteers mainly to natural areas owned by Metro and other public agencies, where we guessed the highest density of beavers would be found.

The data were later compiled and used to generate maps, which will soon appear below.

The beaver survey was developed after consulting with several other agencies studying beavers in the Portland metro area. Kate Holleran, Senior Natural Resources Scientist with Metro, was the project’s main scientific advisor. Kate has also been advising beaver surveys conducted by Mount Hood Community College students in 2015 and 2016 on a portion of Johnson Creek. Meanwhile, USGS and Clean Water Services have been surveying beaver activity in the Tualatin River Watershed.

Data Summary

Here are some of the findings from our 2016 surveys:

  • 45 beaver dams were spotted!
  • An average of 3 dams were found per mile of stream.
  • Our surveyors found 11 sites of beaver activity far from dams (>200 ft), which are signs that beavers may be moving through the area along with stopping and building dams. Cool!
  • About half of the beaver dams surveyed were made of large pieces of wood, whereas the other half were made primarily of smaller pieces of wood and twigs.
  • Surveyors documented 6 shrub species and 11 tree species present in dams and chewed nearby. The most common tree species used by beavers was alder; runners-up were willow and dogwood.
Tree species
Big leaf maple
Douglas fir
Red cedar/conifers
Shrub species
Douglas spirea
Vine maple


Map of Beaver Dams and Activity

Click the the photo below to take a fun tour of beaver 2016 beaver dams!



Below is an overview map of beaver activity from the 2016 surveys, followed by closeups of the various map pieces. Click any map or map piece to see a larger version.


Areas surveyed in 2016:

Legend for all maps and pieces:

Closeups of surveyed reaches:

Beaver Video

Check out the awesome video below, taken by Johnson Creek beaver surveyors from City of Gresham:

Beaver Background

Interesting Facts:

  • The beaver is Oregon’s state animal.
  • Beavers are the world’s second-largest rodent. (The first is the capybara.)
  • Beavers can hold their breath for up to 15 minutes!
  • The world’s largest known beaver dam is over half a mile long!


Beavers are native to much of North America, the main exceptions being tundra and desert areas. The North American beaver, Castor canadensis, is one of two beaver species in the world–the other is the Eurasian beaver, Castor fiber. Beavers were abundant throughout their native range prior to European contact, but were nearly extirpated by the 1800s due to the fur trade. They are now making a comeback and populations are generally considered healthy.

Beaver Family Life

In the wild, beavers can live for 12-20 years. They usually live with their parents for the first 2 years of life, helping to build dams, gather food, and raise younger kits before leaving the colony to find a mate. Beavers mate for life, beginning at age 2 or 3. A pair of beavers will have 2-6 kits, in a single litter, each spring. Kits can swim within 24 hours of being born!

Beaver Food

Beavers are herbivores, eating the inner bark of woody plants (trees and shrubs) as well as some plant roots. They prefer certain woody species–for instance willow and alder–over others. When they cut down a tree or shrub, it is sometimes for eating and sometimes for building.

Why do beavers build dams?

Beavers are excellent swimmers, but are vulnerable to predation on land. The ponds created by their dams help them move around more freely and safely. (They do not build ponds to trap fish.)

Beaver Lodges, Canals and Burrows

Helpful Links

Oregon Zoo page on American beavers
Oregon Wild page on beavers
The Beaver Restoration Guidebook