Hydrologic Responses to Beaver Dam Removal in the Knife River Watershed

Abstract

The hydrologic changes undergone during beaver dam construction and removal are profound and important to the health of aquatic organisms. The alteration of low-flows post-beaver dam removal was addressed here through a paired watershed study in the Knife River watershed on the north shore of Lake Superior, MN. Stream gages and piezometers measuring shallow groundwater levels were monitored for two summers to determine impacts of beaver dam removal on groundwater levels and volumetric discharge downstream. Water samples were taken for isotopic analysis and analyzed for differences in source-water contributions before and after beaver dam removal. A new procedure for analyzing changes to low-flow hydrology was created by combining three common methods of defining a low-flow threshold. This new procedure was used to determine the change in time each stream spent under low-flow conditions; removing the beaver dam was found to increase the amount of time spent in these conditions in all watershed pairs except one upland pair. Watershed characteristics including topography, substrate, and channel slope all played roles in controlling the amount of surface and subsurface storage in each sub-watershed. The upland watersheds comprised of low-order, low gradient, alluvial channels had significant decreases in groundwater levels post-beaver dam removal. The lower watersheds with smaller subsurface storage capacity did not have any significant differences in groundwater levels post-beaver dam removal. Patterns of groundwater contribution to the streams were also analyzed, but data were limited to snapshots in time.