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Stream Network Capacity to Support Beaver Dams in the Tualatin River Basin, Northwestern Oregon

Scientific Investigations Report 2025-5039-A
Prepared in cooperation with Clean Water Services
By: , , , and 
https://doi.org/10.3133/sir20255039A

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Significant Findings 

Beaver dams can help streams connect to their floodplains. These floodplain connections can expand the range of available aquatic habitats and aid in the restoration of stream and floodplain function and processes. American beavers (Castor canadensis) occupy a wide variety of aquatic habitats; however, their ability to build dams, the agent of stream and floodplain change, is constrained in large part by three physical variables—local vegetation, topography, and hydrology.

These three physical variables are combined in the Beaver Restoration Assessment Tool (BRAT), a geographic information system-based utility that uses a Fuzzy Inference System (FIS) to estimate the capacity of each reach within a stream network to support beaver dams. In this study, version 1.0 of BRAT was adapted and applied to the entire perennial stream network of Tualatin River Basin in northwestern Oregon. Beaver-dam locations in the Tualatin River Basin were compiled to (1) define the distribution of dams in the basin during 2013–16 and (2) provide necessary data for calibrating and validating BRAT predictions. BRAT was calibrated to the current known distribution of dams, as compiled in the inventory. The input FIS equations of the original BRAT model were adjusted to account for local topographic conditions; specifically, the low gradient of many streams in the basin, although subsequent updates to BRAT may obviate the need for these changes.

Results from this modified BRAT model reasonably simulated the dam inventory. Results show that beavers can currently build the greatest density of dams, defined as number of dams per kilometer of stream, in the higher-gradient forested streams of the basin, whereas they can build the fewest number of dams per kilometer in urban streams along the lower-gradient valley floor. Estimated dam density was generally 5-15 dams per kilometer (km) for forested streams and 2-4 dams/km for urban streams. Improving riparian vegetation along urban streams may allow beavers to build on average four additional dams per kilometer compared to current conditions. Results from this study may help inform local stream and stormwater management by (1) identifying stream reaches with the most potential to support beaver dams, (2) determining the likely factors limiting potential for dam building, and (3) identifying potential areas where dam building may affect human infrastructure.

Suggested Citation

White, J.S., Smith, C.D., Jones, K.L., and Rounds, S.A., 2025, Stream network capacity to support beaver dams in the Tualatin River Basin, northwestern Oregon, chap. A of Jones, K.L., and Smith, C.D., eds., Beavers in the Tualatin River Basin, northwestern Oregon: U.S. Geological Survey Scientific Investigations Report 2025–5039–A, 20 p., https://doi.org/10.3133/sir20255039A.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Preface
  • Acknowledgments
  • Significant Findings
  • Introduction
  • Methods
  • Results and Discussion
  • Summary
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Stream network capacity to support beaver dams in the Tualatin River Basin, northwestern Oregon
Series title Scientific Investigations Report
Series number 2025-5039
Chapter A
DOI 10.3133/sir20255039A
Publication Date September 30, 2025
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Oregon Water Science Center
Description Report: viii, 20 p.; 2 Data Releases
Country United States
State Oregon
Other Geospatial Tualatin River Basin
Online Only (Y/N) Y
Additional publication details