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

Abstract

Due to their ability to modify their environments, translocation of beaver, along with the construction of human engineered beaver dam analogs (BDAs), have become popular approaches to both stream restoration and climate-change adaptation. However, there is a lack of understanding about how beaver engineering affects aquatic ecosystems. Reported findings of how beaver dam complexes alter one of the most fundamental and important factors, stream temperature, vary considerably, and the drivers of the discrepancy remain unclear. I investigated the longitudinal and vertical variation of stream temperature in and around 24 beaver ponds across 17 headwater streams in the Methow Valley of Washington State (USA). In addition, I explored possible hydrologic and environmental drivers of observed thermal patterns. Despite considerable variation, I found that on average beaver ponds were associated with an increase in downstream temperatures. The magnitude of this effect increased with total pond hydraulic height, which reflected the number and size of ponds per site. I also found that pond bottoms were cooler than upstream reference reaches for a period of time in the afternoon. Thus, although beaver dam complexes in the Methow watershed do not consistently cool streams—and in fact cause increases in temperatures downstream—the bottom of beaver ponds can create cool water areas during the hottest part of the day. Furthermore, if managers choose to use beaver translocation or BDAs, locations where topography would result in deeper ponds, and where canopy cover of ponds would be maximized may limit harmful temperature effects.