Washington State has experienced increasingly frequent, and intense wildfire activity. For example, the largest fires on record for Okanogan County have occurred in the past five years, and this region includes the Methow River (MR), a main tributary to the Columbia River from the North Cascades. The MR is also the site of an ongoing beaver reintroduction program, that has transplanted over 300 “problem beavers” to headwater streams over the past decade. Historically beavers were abundant in the area, but populations were decimated by fur trapping in the 1800’s. Previous work documented how dam building by reintroduced beavers can rapidly influence the hydrology, ecology, and biogeochemistry of MR riparian ecosystems. Our project expands upon this research to investigate the effects of beavers in stream sites that burned in recent wildfires.
We studied the streambed sediments in low order, headwater streams in wildfire-burned and unburned areas of the MR. Half each of the burned and unburned sites were also beaver reintroduction locations, with intact dams and ponds. At each site we collected streambed sediments upstream and downstream of the dams, and took 10 cm cores from pond sediments. We sieved all samples to establish grain size distributions. Since fine sand to clay sized sediments can be harmful to aquatic organisms such as macroinvertebrate larvae and salmonids, we combined these size classes. We also studied the mineralogy and angularity of cobbles to understand sediment supply and transport. The pond sediment cores were analyzed for organic carbon content. The stream sites were highly heterogeneous, which complicated interpretations of the relative strength of beaver and wildfire effects on streambed sediments. In general, there were less fine grained sediments downstream versus upstream of beaver dams, and more fine grained sediment in the burned sites, regardless of beaver presence. A higher organic content appears in beaver areas than in no beaver areas, and less organic content appears in burned areas
