Stream temperature variability in headwater beaver dam complexes in relation to hydrologic and environmental factors

Beaver modify hydrological regimes and have the ability to both increase water storage and alter stream temperatures. For these reasons, beaver translocation and the construction of human engineered beaver-dam analogs (BDAs) have become popular tools for climate-change adaptation. However, there is a lack of understanding about how beaver engineering affects stream temperature. The results from previous studies vary considerably and the drivers of the discrepancies remain unclear. We investigated the longitudinal and vertical variation of stream temperature in and around 24 sites consisting of one or more beaver ponds across 17 headwater streams around the Methow and Okanogan river valleys of Washington State (USA). We then explored possible hydrologic and environmental drivers of observed thermal patterns. Despite considerable variation, we found that beaver ponds were often associated with an increase in downstream temperatures. The magnitude of this effect increased with total pond hydraulic height, which was directly related to the number and size of ponds per site. We also found that pond bottoms were cooler than upstream inflow sites for a period of time in the afternoon. Thus, although beaver dam complexes in the Methow River watershed do not consistently cool streams—and in fact often increase downstream temperatures—the bottoms of beaver ponds can create relatively cool water areas during the hottest part of the day. Furthermore, if managers choose to use beaver translocation or BDAs, locations with topography that would result in deeper ponds with more canopy cover may limit negative temperature effects on coldwater aquatic biota.