6. Conclusions
Hathataga Creek is sourced by a spring complex located downgradient of a
seasonal lake, Hathataga Lake. It represents an example of
groundwater-fed headwater streams in mountain regions. The spring
complex consists of three separate springs having different thermal
regimes, which are strongly influenced by the hydroperiod of Hathataga
Lake. When the lake is dry, the temperatures of the three springs are
equal and controlled by groundwater temperature in the catchment.
However, once the lake forms, following snowmelt, the temperature of the
spring receiving lake-influenced groundwater rises compared to other
springs. During the summer months, lake water level and the lake energy
balance determine the magnitude and temperature of the water flux from
the lake to the springs. Climate change will likely result in changes to
the conditions that control the hydroperiod of the lake, namely the
depth of snowpack, the timing of snowmelt, amounts of summer
precipitation, and lake-atmosphere energy exchange. Therefore,
groundwater-lake water interactions must be considered when predicting
future stream temperatures in these settings.
The energy balance of the stream reach below the headwater springs was
mainly controlled by shortwave and longwave radiation. However, stream
temperature decreased rapidly during snowfall events due to direct entry
of snowfall onto the stream surface and subsequent melting. Stream
temperature models are improved when the latent heat flux associated
with the melting of direct snowfall are included. This process is
especially relevant in winter months and in cold regions where summer
snowfall and hailstorms are common. Tracer tests indicated substantial
effects of hyporheic exchange, but its effects on stream temperature was
negligible implying that energy exchange by hyporheic processes played
insignificant role in this particular setting.
Further case studies should be conducted on both seasonal and perennial
lakes to assess the effect of groundwater-lake water interactions on
stream temperatures downgradient. For the assessment of climate-change
impacts on headwater stream temperatures, it will be beneficial to use
physically-based catchment models including snow accumulation and melt,
energy balances of streams and lakes, and subsurface energy transfer
between streams and lake.