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.