2. Study Site
The study is conducted at an unnamed headwater stream in a valley called Hathataga (meaning huckleberry) by the local Stoney people (Crawler, Labelle, Mark, & Willie, 1987). Therefore, in this study the stream is called Hathataga Creek. It is located in the Fortress Ski Area (50°49’14” N, 115°12’50” W) in Alberta, on the eastern slopes of the Canadian Rocky Mountains. The stream starts at a set of perennial outlet springs (Gauging Station 1 (GS1); Figure 1B) and flows north until it merges with Galatea Creek. The focus of this study is on an 850 m long reach extending from the outlet springs to a previously installed stream gauging station (GS4; Figure 1B).
A tarn, with no surface water outflows, is located at a higher elevation to the south of the springs, separated by a moraine. The tarn, referred to in this study as Hathataga Lake, forms following snowmelt and dries up by late summer (Christensen, Hayashi, & Bentley, 2020). As a result of the hydraulic gradient, the springs drain the sub-catchment that includes the tarn. This means that in summer months water flows from Hathataga Lake to the springs.
The spring system consists of numerous discrete discharge points that combine to form three spring branches (S1, S2 and S3) that merge just above GS1. The location and presence of these transient discharge points is dependent on the elevation of the water table. It is important to distinguish between lake-headed streams and headwater streams as downstream temperature gradients reflect source water temperature (Mellina, Moore, Hinch, Macdonald, & Pearson, 2002). Hathataga Creek is indirectly lake-headed because there is no surface water connection to Hathataga Lake. Another study at the site determined a travel time from the lake to the spring system of approximately 4 hours, using a tracer test (He, J., 2021). Given the horizontal distance of approximately 100 m the hydraulic conductivity of the moraine is on the order of 1 × 10-2 m s-1.
Elevations in the catchment range from 2034 m.a.s.l at the lower gauging station (GS4) to 2900 m.a.s.l at the southern headwall. The catchment has been divided into four sub-catchments (W1 to W4) (Figure 1A).
The bedrock underlying Hathataga Lake and the study reach consists of recessive shales of the Jurassic Fernie Formation (McMechan, 2012). The ridges between these valleys are made up of Paleozoic carbonates. The Sulphur Mountain Thrust separates the Fernie Formation from the older, more resistant carbonates that make up the headwall in the southern and western part of the study catchment (Figure 1A).
Surficial sediments and the geomorphology of the area can be attributed to the late Pleistocene and Holocene glaciations (Beierle, Smith, & Hills, 2003), which have resulted in the north facing cirque that can be seen today, along with extensive moraine deposits surrounding Hathataga Lake. Freeze-thaw weathering of the carbonate headwall has and continues to contribute to steep talus cones. The coniferous tree line extends to the base of these talus cones.
The outlet springs discharge at the base of the moraine north of the lake. Stream geomorphology changes considerably downstream (Figure 2) (Roesky, 2020). Flow is often turbulent passing through countless pools and riffles. Between the outlet springs and GS2, the western stream banks are shallow with numerous visible bank seeps. The eastern stream banks are typically steeper, incised by 1 to 2 m, with bedrock only occasionally outcropping. The hillslopes are mostly tree covered but some segments are covered in alpine meadow on the western side. Downstream of GS2 the stream gets progressively more incised, by up to 10 m, with increasing bedrock exposure on both sides (Figure 2). Stream meanders also become more frequent.