Abstract
Abstract. The different types of geological deposits and rock formations found in alpine watersheds play key roles in regulating the rate and timing of runoff to mountain rivers. Talus and alpine meadows are dominant features in these areas, but scant data exist for their capacity to store and transmit groundwater. To gain further understanding of these processes, we have undertaken a combined geophysical and hydrological study of a small (2100 m2) alpine meadow and surrounding talus within the Lake O'Hara watershed in the Canadian Rockies. Several intersecting ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) profiles and a seismic refraction profile were acquired to map the thickness of the talus and to image the topography of the bedrock basin that underlies the meadow. From analysis of the GPR and seismic profiles, we estimate that the talus deposits are relatively thin (<6 m). Combined interpretations from the GPR and ERT data show that the fine-grained sediment comprising the meadow basin has a total volume of ca. 3300 m3 and has a maximum thickness of ca. 4 m. Annual snow surveys and stream gauging reveal that the total input volume of snowmelt and rainfall to the meadow basin is several times larger than its groundwater storage capacity, giving rise to low total-dissolved species concentrations (14–21 mg/L) within the meadow groundwater. Observations from four piezometers established on the meadow show that the water table fluctuates rapidly in response to spring snowmelt and precipitation events but otherwise maintains a relatively stable depth of 0.3–0.4 m below the meadow surface during summer months. A slug test performed on one of the piezometers indicated that the saturated hydraulic conductivity of the shallow meadow sediments is 2.5×10−7 m/s. We suggest that a bedrock saddle imaged underneath the southern end of the meadow forms a natural constriction to subsurface flow out of the basin and helps to maintain the stable water-table depth.
Highlights
In western regions of Canada and the United States alpine watersheds play an important role in the supply and delivery of freshwater to surrounding lowland areas
We interpret the steep reflections observed on profile GPR1 to be the tails of diffraction hyperbolas that originate from boulders within the small sediment-filled depression beneath the meadow (Fig. 4a)
They are not apparent beneath locations outside the meadow and we suggest that this is a consequence of both strong attenuation effects beneath the meadow and the laterally varying gain functions we use to scale the ground-penetrating radar (GPR) traces
Summary
In western regions of Canada and the United States alpine watersheds play an important role in the supply and delivery of freshwater to surrounding lowland areas. Kattelmann and Elder, 1991; Campbell et al, 1995), a significant component is stored as groundwater within the alpine watersheds that is slowly released during the low-flow period over the remaining nine months of the year (Clow et al, 2003; Hood et al, 2006). Talus and snow avalanche boulder formations are closely related types of slope deposit that occur extensively in alpine watersheds. They represent accumulations of rock debris that
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