Stream temperatures in two shaded reaches below cutblocks and logging roads: downstream cooling linked to subsurface hydrology

This study examined water temperature patterns and their physical controls for two small, clearing-heated streams in shaded reaches downstream of all forestry activity. Field observations were made during July–August 2000 in the central interior of British Columbia, Canada. For both reaches, downstream cooling of up to 4°C had been observed during daytime over distances of ~200 m. Radiative and convective exchanges of energy at heavily shaded sites on both reaches represented a net input of heat during most afternoons and therefore could not explain the observed cooling. In one stream, the greatest downstream cooling occurred when streamflow at the upstream site dropped below about 5 L·s –1 . At those times, temperatures at the downstream site were controlled mainly by local inflow of groundwater, because the warmer water from upstream was lost by infiltration in the upper 150 m of the reach. Warming often occurred in the upper subreach, where cool groundwater did not interact with the channel. At the second stream, creek temperature patterns were comparatively stable. Energy balance estimates from one afternoon suggested that groundwater inflow caused about 40% of the ~3°C gross cooling effect in the daily maximum temperature, whereas bed heat conduction and hyporheic exchange caused about 60%.