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.