Stream network capacity to support beaver dams in the Tualatin River Basin, northwestern Oregon

James S. White; Cassandra D. Smith; Krista L. Jones; Stewart A. Rounds

doi:10.3133/sir20255039a

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Stream network capacity to support beaver dams in the Tualatin River Basin, northwestern Oregon

James S. White·Cassandra D. Smith·Krista L. Jones·Stewart A. Rounds

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Abstract

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.

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Source-reported bounding extent: 45.375° to 45.75° latitude; -123.5° to -122.5° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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James S. White, Cassandra D. Smith, Krista L. Jones, Stewart A. Rounds. 2025-09-30. Stream network capacity to support beaver dams in the Tualatin River Basin, northwestern Oregon. https://doi.org/10.3133/sir20255039a

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