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Creek temperatures in shaded reaches downstream of forestry activities, Central British Columbia Story, Anthony Charles


Removal of streamside vegetation often causes daytime increases in stream temperature, due to greater warming by solar radiation. Several studies have reported creeks subsequently cooling as they flow back into shade, but none have established the causal processes. Fisheries and Oceans Canada observed downstream cooling during several summers in two small streams (catchments of 0.5 - 1.5 km²) that were monitored as part of a multi-year study in the central interior of British Columbia. The purpose of this thesis was to use an energy balance framework to evaluate the physical processes responsible for the observed temperature patterns. Field investigations in July-August, 2000, showed that both streams continued to gain energy from the overlying atmosphere/vegetation on most afternoons as they flowed through the ~200 m forested reaches. This effect only consistently resulted in measurable downstream warming in the upper section of the first reach, where groundwater did not contact the channel. Net inflow of groundwater in the lower section of the reach (~0.02 L s⁻¹ m"1) sometimes caused rapid downstream cooling, depending on the magnitude of streamflow input at the head of the reach. The greatest downstream cooling (up to 4 °C) occurred when the streamflow input was <5 L s⁻¹, because infiltration in the upper 150 m of the reach consumed the warmer streamwater from the upstream catchment. Modest inputs of groundwater (~0.002 L s⁻¹ m⁻¹) occurred throughout the second study reach, causing an estimated 0.5 °C cooling in the average daily creek temperature. Two-way exchanges of energy between the creek and its subsurface, driven by conduction, and possibly hyporheic exchange, were important influences on the daily temperature extremes. Heat appeared to be stored within the riparian zone during daytime and released to the stream at nighttime. Heat storage also affected the mean daily temperatures, apparently depending on the relative strengths of the daytime heat sink and nighttime heat source effects. This study adds to a growing recognition of the high spatial variability in environmental responses to forest management, even within small watersheds. A better understanding of the hydrology-temperature linkage is required for effective management of the thermal effects of forestry.

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