Presenter/s: Katie Holzer
Symposium Session: 2023 - 03 Water Quality Considered in Restoration Design & Watershed Management
Topics covered: amphibians, beavers, fish passage, instream structure (culvert/bridge/dam), urban, and water quality
Beaver populations have been returning to urban watersheds in the Pacific Northwest, bringing with them both benefits and challenges. To better understand the effects on local watersheds, we conducted several studies of beaver activities within the city of Gresham, Oregon. We looked at pollutant removal efficiencies in a large stormwater treatment wetland with and without beaver activity, assessed the effect of beaver dams on stream temperatures, and documented the physical and biological changes to stream channels near dams. We found that pollutants of concern were generally removed more efficiently when beaver dams were present in the treatment wetlands. This is likely due to the water being filtered through the dams which are repaired after each storm. The effects of dams on stream temperatures varied depending on site characteristics, but all dams created ponds with temperature heterogeneity and stratification. Dams on higher order streams sometimes reduced overall stream temperature by pulling water from the entire stratified water column and increasing hyporheic flow. Within just a few years of beaver activity, several sites experienced increased stream complexity with new gravel bars, side channels, and braided streams. We also found increased macroinvertebrate diversity in the relatively sediment-free cobbles downstream of dams. These findings demonstrate multiple benefits of beavers in urban watersheds. After communicating these benefits to stakeholders, we have been working with beaver experts to adapt our systems to find ways to protect infrastructure while coexisting with beavers through targeted use of technologies such as pond levelers, culvert fencing, and tree protection.