Channel incision is a global problem for watersheds - one that is growing in the American west, where wildfire exacerbates channel erosion. Incision lowers water tables, draining aquifers and wetlands. Beaver activity can reduce incision, improving riparian habitat and retaining water, sediment, and nutrients. Unfortunately, it can be difficult to restore beaver to degraded areas. Consequently, man-made Beaver Dam Analogs (BDAs) have become a popular restoration technique. However, adoption of BDAs may be outpacing research on their effectiveness. We report initial results of a Before-After Control-Impact study comparing 5 fire-impacted BDA restoration sites with 5 paired controls and 4 reaches with beaver activity in the Methow and Okanogan watersheds (Washington, USA).
Pre-restoration data show substantial effects of beaver on water storage and riparian condition. Beaver sites had width-to-depth ratios over twice as large as pre-BDA sites, but not significantly different from control sites (p=0.0033). Beaver sites had floodplain ratios over twice as large as both pre-BDA sites and control sites (p=0.0002). 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). We observed dramatically slower water travel times (7x to >400x) through beaver reaches compared to control and planned restoration reaches (p < 0.05).
BDA installation was completed for three reaches during 2022 and the remaining two in 2023. In post-installation data, BDAs were effective in slowing water during low-flow conditions. However, beaver impoundments produced much stronger effects. BDAs also appear to be very effective in capturing sediment, resulting in increased frequency of fine sediment sizes. Our data indicate BDAs are effective in slowing water flow, and storing water and sediment, but that beaver are more effective. Protection of beaver populations is an important goal for protecting water quality in the semi-arid western US.