Headwater stream temperature response to forest harvesting: Do lower flows cause greater warming?

This study addressed two hypotheses regarding the relationship between stream temperature response to shade removal and streamflow: (a) that temperature response increases as flow declines and (b) that the relationship can be complicated by shifts in dominant streamflow sources and pathways during low-flow periods. The study was based on a paired-catchment design in rain-dominated headwater catchments in the southern Coast Mountains of British Columbia, Canada, and focused on the effect of a 50% thinning treatment on daily maximum temperature in June, July and August for three sites within the harvest treatment. At the two upstream sites, the treatment response exhibited a negative relationship with daily mean streamflow, especially for days with high incident solar radiation. This result suggests that the effectiveness of forest practice rules for protecting cold-water habitat may be reduced under future climatic conditions characterized by more frequent extended drought. However, stream temperature response at the most downstream site exhibited a pronounced inverted-U-shaped relationship with streamflow measured at a weir. It is hypothesized that the response at the most downstream temperature logger was controlled by the existence of a stable, relatively cool inflow just upstream of the logger, which represented an increasing fraction of flow as streamflow generated higher up in the catchment declined through time. There was a lack of convergent surface topography upstream of the logger, and it is hypothesized that localized inflow may have been controlled by the topography of the soil-till interface and/or originated as hyporheic discharge. Stream temperature response to forest thinning exhibited a negative relationship with daily mean streamflow at two sites, especially for days with high incident solar radiation, while a third site had an invertedU-shaped relationship. It is hypothesized that the inverted-U-shaped response was controlled by the existence of a stable, relatively cool inflow just upstream of the logger, which represented an increasing fraction of flow as streamflow generated higher up in the catchment declined.image