5.1 Lake and spring thermal and hydrogeologic connectivity
The headwater springs of Hathataga Creek is thermally influenced by the seasonal body of water in the lake located upgradient of the springs, resulting in a strong seasonality in the temperature of spring discharge, which in turn affects the stream thermal regime (Figurer 13). The spring has a steady temperature of ~1.3 °C during winter and early spring while the lake is covered by snow (Figure 7). As the water table rises under the lake and forms the surface water body after snowmelt, the lake becomes an effective absorber of shortwave radiation. As a result, the lake water temperature rises, and so as the temperature of S1 spring receiving the water sourced by the lake (Figure 13A). S2 and S3 springs receive less of lake-influenced groundwater but more of the groundwater that bypasses the lake. This results in a temperature contrast between S1 and S2/S3 (Figure 5). As the lake dries out towards the end of the summer (Figure 13B), the spring temperature decreases rapidly due to the loss of warm water from the lake.
The distinct thermal characteristics of lake-headed stream temperatures have previously been described (Dripps & Granger, 2013; Garrett, 2010; Mellina et al., 2002). 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.
The processes taking place in this system have important implications for the stream thermal regime under a changing climate. There have been greater than average regional increases in mean air temperatures on the eastern slopes of the Canadian Rockies. Warming of 2.6 °C has been recorded at the Marmot Creek research basin, located 15 km north of the Hathataga valley, since the 1960s (Harder, Pomeroy, & Westbrook, 2015; Fang & Pomeroy, 2020). Typically, the concern is that this leads to stream warming (Leach & Moore, 2019). Higher summer air temperatures in the Hathataga valley would lead to higher lake temperatures and therefore even greater stream warming. Earlier snowmelt (Stewart, Cayan, & Dettinger, 2005) and a reduced snowpack (Mote, Hamlet, Clark, & Lettenmaier, 2005) have also been shown for the region. Reduced summer streamflows would exacerbate stream warming. However, the duration of the surface water in the lake would be shortened due to a reduced snowpack. This in turn would reduce the time of warm groundwater contribution to the springs, resulting in stream cooling. On the other hand, summer precipitation has been increasing in the Canadian Rockies (Harder et al., 2015). Increased summer precipitation could also impact the lake’s hydroperiod by increasing lake water levels and extending the life of the lake. The frequency of mid-winter melts is also likely to increase into the future (O’Neil, Prowse, Bonsal, & Dibike, 2017). This could cause a lake-spring connection in winter months. Climate change will therefore impact the timing and magnitude of lake water levels, which are an important control on stream temperatures.
This study illustrates the complex and non-linear nature of climate change on surface water-groundwater interactions and the need to further study the effects of both intermittent and non-intermittent alpine lakes on stream thermal regimes.