BEAVER DAM IMPACTS TO LOW FLOW HYDROLOGY IN NORTHERN MINNESOTA STREAMS

Limiting conditions for coldwater fish species include warm temperatures and low flows in late summer. In addition, fish need access to spawning gravels, and potadromous fish like steelhead (Oncorhynchus mykiss) that migrate upstream from the Great Lakes for spawning need open access for fish passage. The Knife River watershed, located just north of Duluth, MN, USA, contains 70 percent of the steelhead spawning habitat on Lake Superior’s North Shore. In order to conserve and enhance this important spawning area, restoration and management efforts are directed towards facilitating upstream fish passage of migratory steelhead. One such management practice includes the removal of beavers and beaver dams in qualifying streams. The presence (and removal) of beaver dams causes alterations to the hydrology of the watershed. To better understand beaver dam impacts to North Shore low-flow hydrology, a combined mass balance and dual-isotope approach was used to investigate eight paired sub-watersheds over a two year study period. Using a before-after-control-impact study design, changes to the relationship between control and treatment site discharge were analyzed pre-and post- beaver dam removal. Here, we present the investigation into the temporal and volumetric changes to downstream discharge. We focus primarily on impacts to low flow hydrology, as low flows in late summer can drive fish mortality both through increased temperatures and lack of water. The information presented here will not only help watershed managers on the North Shore better understand the hydrologic implications of dam removal management strategies, but will also provide more information on how water is stored and released in groundwater-limited systems with abundant beaver populations.