Stream thermal regimes are critical to the stability of freshwater
habitats. There is growing concern that climate change will result in
stream warming due to rising air temperatures, decreased shading in
forested areas due to wildfires, and changes in streamflow. Groundwater
plays an important role in controlling stream temperatures in mountain
headwaters, where it makes up a considerable portion of discharge. This
study investigated the controls on the thermal regime of a headwater
stream, and the surrounding groundwater processes, in a catchment on the
eastern slopes of the Canadian Rocky Mountains. Groundwater discharge to
the headwater spring is partially sourced by a seasonal lake. Spring,
stream, and lake temperature, water level, discharge and chemistry data
were used to build a conceptual model of the system. Meteorological data
was used to set up a stream temperature model. A tracer test was carried
out to estimate hyporheic exchange along the study reach. This study
presents a unique example of an indirectly lake-headed stream i.e.,
where the interaction of groundwater and lake water, and the hydraulic
gradient determine the resulting stream temperature. Energy balance of
the stream is mainly controlled by radiation. Sensible and latent heat
fluxes play a secondary role, but their effects generally cancel out.
Hyporheic exchange is present but plays only a minor role in the energy
balance. During snowfall events, the latent heat associated with melting
of direct snowfall onto the water surface was responsible for rapid
stream cooling. An increase in advective inputs from groundwater and
hillslope pathways did not result in observed cooling of stream water
during rainfall events. The results from this study will assist water
resource and fisheries managers in adapting to stream temperature
changes under a warming climate.