STREAM THERMAL DYNAMICS: THE INFLUENCE OF BEAVER DAMS IN A NORTHERN MINNESOTA WATERSHED

Beaver dams are known to alter the thermal regime of ponds, streams, and adjacent subsurface waters. Downstream of a dam, stream temperature is influenced by increased exchange with the hyporheic zone, which may cool and buffer the stream’s diel temperature cycles. Concurrently, reduced shading in the beaver forage zone is likely to increase heat flux at the stream-atmosphere boundary. The dynamics of these processes can be analyzed to understand how stream temperature is affected on diel and seasonal timescales, as well as longitudinally at distances downstream from the dam. At two beaver dam-impacted stream sites in the Knife River watershed in Minnesota, USA, we monitored in-stream and shallow subsurface flow and temperature during low-flow summer conditions. We used a dye tracer test, vertical heat transport modelling, and hydraulic gradient measurements to estimate flux through the streambed at multiple locations. Topography, surficial geology, and climate data were also collected throughout the summer from the two sites. A 1D model of longitudinal stream temperature, calibrated to in-stream temperature measurements, is being developed to determine which physical parameters have the greatest influence on stream temperature. The model can be used to demonstrate how these changes persist downstream, as well as how parameters influencing stream temperature vary over the course of the summer. Findings from this research will increase scientific understanding of temperature regime in beaver dam-altered streams, and may be used to support management practices in the Knife River watershed.