Influence of a Waterfall on Summer Stream Temperature

Abstract:

Summer stream temperature generally increases from the headwaters down to the outlets of major rivers, with longitudinal thermal gradients modified by riparian forest cover, groundwater and tributary inflows, the presence of surface water bodies, and potentially other landscape features. The objective of this study was to document downstream changes in summer stream temperature associated with a waterfall. The study focused on Shannon Falls in south coastal British Columbia, which descends 350 m over a horizontal distance of 500 m. Flow is cascading and highly aerated for most of its descent. Stream temperature was recorded at 10-minute intervals above and below the falls, and discharge was recorded 200 m downstream of the falls. Hourly air temperature, relative humidity and wind speed were recorded at Squamish Airport, 12 km from the falls; air temperature and humidity were also recorded upstream of the falls at streamside and at an open site 100 m from the stream. The stream warmed 1 to 4 °C during its descent, with diurnal variations of about 1 °C during fine weather. A full energy balance analysis is not possible due to an inability to quantify the stream's surface area and its complex radiation geometry. Calculations indicate that the conversion of potential energy accounts for 0.8 °C of warming. An index of the combined effects of sensible and latent heat flux explained 70% of the variance in the downstream temperature changes, consistent with the hypothesis that the turbulent exchanges are enhanced in cascading, aerated flow.