Beaver Science

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

Key Takeaways

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.

Survey Methods

Since 2016, JCWC beaver surveys have taken place in August and September. Volunteers survey stream reaches, recording all beaver dams and other activity signs such as chewed trees and mud slides. Each survey is conducted by 2-3 volunteers.

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

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.

Survey Results

We hoped to return to volunteer-run surveys in 2021, but unfortunately continued concerns over COVID-19 safety meant we again surveyed with staff only. 2021 results:

2020 surveys were conducted by staff due to COVID-19. Findings from our 2020 surveys:

Findings from our 2019 surveys:

Findings from our 2018 surveys:


Click on the map to see the stream reaches visited by volunteers in 2018 and 2019:

Click on the maps to see the dams and lodges found in each reach during 2018 surveys:

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

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

Along with dams, beavers create various other visible structures. They often build lodges for living in. The lodge may be on the bank or on the dam, and looks like a large pile of sticks. Beaver lodges are uncommon in Johnson Creek, where beavers more often seem to live in burrows in the bank. Both lodges and burrows have underwater entrances, to maximize the beavers’ safety when coming and going. Finally, beavers create canals in some slough or wetland areas. These canals help them to move wood they have harvested, since it’s easier to float a log than to drag it!

Helpful Links

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

Beaver: the ultimate Keystone Species