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. We report 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 widths 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 through beaver reaches compared to control and planned restoration reaches, during both low and high flow conditions (p < 0.05).
BDA installation was completed for 3 reaches during 2022 and the remaining 2 in 2023. In post-installation data, BDAs were effective in slowing water during both low-flow conditions and high-flow conditions (paired t-test of differences between pre and post restoration travel times, p <0.05). However, beaver impoundments slowed water much more, under both low and high flow conditions. BDAs were very effective in capturing sediment, resulting in increased frequency of fine sediment sizes (Mann-Whitney test, p <0.001). We are currently analyzing data on floodplain topography and riparian vegetation post-restoration. Our data indicate BDAs are effective in slowing water flow, and storing water and sediment, but that beaver are more effective in storing water. Protection of beaver populations is an important goal for protecting water quality in the semi-arid western US.