PS 34-254 - Baseline data to test Beaver Dam Analogs as a Stream Restoration Tool in fire-affected tributaries of the Methow and Okanogan Watersheds, WA

Abstract: Incised streams are disconnected from their floodplains and no longer store water effectively, leading to diminished ecosystem function, loss of critical riparian and aquatic habitat, and reduced biodiversity. Beaver dams improve incised streams by raising surface and groundwater levels, leading to reconnected floodplains and improved ecosystem function. When beaver reintroduction is not feasible, Beaver Dam Analogs (BDAs) may be used to mitigate damage from stream incision. However, it is unclear how effective BDAs are at mimicking natural beaver dams for wildfire-affected stream restoration. The objective of our project is to assess BDA effectiveness. We hypothesized that beaver sites will have lower channel incision, higher accumulation of fine sediment, higher abundance of wetland species, and higher soil moisture. We used a Before-After-Control-Impact study design to compare five BDA restoration sites with paired control sites and three natural beaver dam complexes. In the summer of 2021, pre-restoration data was collected on 1) channel morphology using a laser level and stadia rod, 2) riparian vegetation using the line-intercept method, and 3) sediment composition using a Wolman pebble count. In the summer of 2022, we recorded floodplain soil moisture after one BDA site was built. Overall, we found that beaver sites had width-to-depth ratios and floodplain widths over twice as large as non-beaver sites indicating they were less incised (Linear Mixed-Effects Model, df=2, p< 0.01 & p< 0.01). Compared to beaver and control sites, pre-BDA had the smallest abundance of wetland species across the landform types. Of the landform types, the floodplain had the highest abundance of wetland species, and the terrace had the highest abundance of upland species (Linear Mixed-Effects Model, df=, p< 0.01 & p< 0.01). Beaver ponds had a higher frequency of fine sediment sizes, whereas non-beaver sites had a wider distribution of sediment sizes (Kruskal-Wallis, df=2, p< 0.01). Beaver dams had the highest values of soil moisture compared to all other treatment types (Linear Mixed-Effects Model, df=3, p< 0.01 ). Soil moisture in non-beaver sites decreased throughout the summer months, whereas in beaver sites it remained high. Our study has shown that beaver dams effectively trap fine sediment and recharge soil moisture in the floodplain, and in wide floodplains, with high soil moisture there is a higher abundance of wetland species. After restoration, BDAs will be expected to become more like beaver sites over time and improve stream quality by establishing wetland species, accumulating fine sediment, and decreasing stream incision.