BEAVER REMOVAL IMPACTS ON LOW FLOW HYDROLOGY

Many northern coldwater fisheries are at risk from increasing temperatures, particularly in rivers with limited surface-groundwater exchange like the Knife River in northeastern Minnesota, USA. With limited groundwater storage, storage in beaver impoundments may be increasingly important in maintaining summer low flows, but there have been few studies on the impacts of beaver dams on low flow hydrology in this region. To help fill this gap, we conducted a two-year before-after-control-impact study on four paired watersheds in the Knife River to assess the impacts of beaver dam removal on low flow hydrology and temperature.

Current fisheries management in the Knife River involves removal of beaver dams on mainstem designated trout streams to ensure fish passage for spawning Rainbow Trout (aka steelhead). Because beaver dams can also influence hydrology and temperature, resource managers need to know if actions intended to help steelhead may be having unintended negative consequences. We monitored four paired subbasins in the Knife River watershed over the course of two years. All study basins had active beavers in 2018. In 2019, beavers were removed and dams notched in one of the two subbasins in each pair. Streams, ponds, and groundwater levels were gaged over both years and results compared between paired basins. Results show that the impact of beaver dam removal on low flow and temperature is complex. The only statistically significant trend in low flows showed an increase in time spent at low flow following dam removal relative to control basins. In upland basins, groundwater levels dropped following dam removal relative to control basins. In addition, three of the four basins showed an increase in temperature following beaver dam removal relative to control basins. These combined results indicate that beaver dam removal could be detrimental to temperature and flow conditions for coldwater trout species. While the focus of this study is on northeastern Minnesota, the findings are relevant regionally and nationally for coldwater fisheries management in other northern streams with low baseflows and isolated groundwater inputs.