Effects of beaver dams and ponds on the transport and deposition of suspended sediment in urban streams of the Tualatin River Basin, northwestern Oregon

Micelis C. Doyle; Cassandra D. Smith; Krista L. Jones; Alexandria E. Costello

doi:10.3133/sir20255039c

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Effects of beaver dams and ponds on the transport and deposition of suspended sediment in urban streams of the Tualatin River Basin, northwestern Oregon

Micelis C. Doyle·Cassandra D. Smith·Krista L. Jones·Alexandria E. Costello

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Abstract

Significant Findings This study investigated the effects of natural beaver dams and ponds on sediment transport and deposition in two urban beaver-affected reaches in the Tualatin River Basin, northwestern Oregon. Data were collected during 2016–17 from Fanno Creek at Greenway Park (between SW Hall Boulevard and SW Pearson Court) and Bronson Creek (between NW Laidlaw Road and NW Kaiser Road); each study reach contained multiple beaver dams. Continuous turbidity, discrete suspended-sediment samples, and streamflow measurements were collected during storms and baseflow periods to calculate suspended-sediment loads (SSLs) and to compare differences in SSLs upstream and downstream from the two beaver-affected reaches. Turbidity was measured continuously upstream, within, and downstream from these reaches to evaluate seasonal and longitudinal turbidity patterns and fluctuations. The volume and mass of sediment deposited in a large pond along the Fanno Creek study reach were also estimated. Study results include: Instantaneous SSLs at the upstream reach boundaries often were greater at Bronson Creek compared to Fanno Creek during storm events. These differences were likely caused by the greater mean basin slope upstream from the Bronson Creek reach and its proximity to the headwaters in the Tualatin Mountains. In contrast, the frequency of sediment transport events was greater at the upstream boundary of the Fanno Creek study reach than at the upstream boundary of the Bronson Creek study reach, likely because Fanno Creek has a larger upstream contributing area and more frequent streamflow runoff events typical of urban streams. During water year 2017, approximately 140 metric tons (t) of sediment (11 percent of the total incoming load) were deposited in the Fanno Creek study reach, and 1,420 t of sediment (89 percent of the total incoming load) were deposited in the Bronson Creek study reach. Longitudinal reductions in turbidity were observed in both study reaches but varied among the two reaches. Turbidity reductions along the Fanno Creek study reach were associated with sediment deposition and storage in the south pond, a large floodplain pond formed by a beaver dam. Turbidity decreased gradually along the Bronson Creek study reach, indicating that sediment was deposited between the upstream and downstream boundaries. Differences in decreases in SSLs and turbidity among the two study reaches were attributed to reach characteristics, such as beaver dam characteristics, stream-floodplain hydraulic connections, and the type and density of the vegetation. The eastern part of the south pond in the Fanno Creek study reach trapped approximately 1,100 cubic meters of sediment (estimated 250 t) from 2012 to 2016 (the period between when beaver dams were first built in the reach and when topographic surveys were collected for this study). This amount is equivalent to about one seventh of the mean annual SSL moving through Fanno Creek as estimated by a previous study at the downstream U.S. Geological Survey streamgage at Fanno Creek at Durham Road (14206950). Beaver dams and ponds in urban streams can reduce SSLs and trap sediment. Consideration of reach-specific factors influencing the effects of beaver dams on sediment transport could help create realistic expectations for restoration programs about this potential benefit associated with beaver activity.

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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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Micelis C. Doyle, Cassandra D. Smith, Krista L. Jones, Alexandria E. Costello. 2025-09-30. Effects of beaver dams and ponds on the transport and deposition of suspended sediment in urban streams of the Tualatin River Basin, northwestern Oregon. https://doi.org/10.3133/sir20255039c

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