Abstract:Understanding groundwater processes in alpine watersheds is critical to understand the timing of water release and late-season stream flow for both headwater and downstream environments. Moraines and talus features can play an important role in groundwater flow and storage processes in alpine watersheds, but neither process is well understood for these features. We examined the complex hydrogeological environment of a partially ice-cored moraine in the Lake O'Hara watershed in the Canadian Rockies. Electrical resistivity imaging (ERI) and seismic refraction tomography delineated regions of buried ice and frozen and unfrozen moraine material. Seismic refraction data also clearly indicated the depth to bedrock, which varied primarily due to the thickness of the overlying moraine material. Water levels in a lake and several tarns on the moraine responded differently to inputs of rain, snowmelt, and glacier melt, indicating the different degree of hydrological connectivity of these features to the groundwater flow system in the moraine. Such differences reflect the effects of bedrock topography and the location and geometry of buried ice. Ground-penetrating radar images and ERI indicated regions of perched groundwater and focused infiltration. The location of these regions appears to be controlled by buried ice. All geophysical and hydrological data suggest that a relatively thin (<5 m) layer of saturated sediments and/or fractured bedrock likely provides a major flow system within the moraine.

[1] Despite our current understanding of permafrost thaw in subarctic regions in response to rising air temperatures, little is known about the subsurface geometry and distribution of discontinuous permafrost bodies in peat-covered, wetland-dominated terrains and their responses to rising temperature. Using electrical resistivity tomography, ground-penetrating radar profiling, and thermal-conduction modeling, we show how the land cover distributions influence thawing of discontinuous permafrost at a study site in the Northwest Territories, Canada. Permafrost bodies in this region occur under forested peat plateaus and have thicknesses of 5-13 m. Our geophysical data reveal different stages of thaw resulting from disturbances within the active layer: from widening and deepening of differential thaw features under small frost-table depressions to complete thaw of permafrost under an isolated bog. By using two-dimensional geometric constraints derived from our geophysics profiles and meteorological data, we model seasonal and interannual changes to permafrost distribution in response to contemporary climatic conditions and changes in land cover. Modeling results show that in this environment (1) differences in land cover have a strong influence on subsurface thermal gradients such that lateral thaw dominates over vertical thaw and (2) in accordance with field observations, thaw-induced subsidence and flooding at the lateral margins of peat plateaus represents a positive feedback that leads to enhanced warming along the margins of peat plateaus and subsequent lateral heat conduction. Based on our analysis, we suggest that subsurface energy transfer processes (and feedbacks) at scales of 1-100 m have a strong influence on overall permafrost degradation rates at much larger scales.Citation: McClymont, A. F., M. Hayashi, L. R. Bentley, and B. S. Christensen (2013), Geophysical imaging and thermal modeling of subsurface morphology and thaw evolution of discontinuous permafrost,

Three-dimensional ground-penetrating radar ͑GPR͒ data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these data are invariably based on subjective analyses of reflection patterns. Such analyses are heavily dependent on interpreter expertise and experience. Using data acquired across gravel units overlying the Alpine Fault Zone in New Zealand, we demonstrate the utility of various geometric attributes in reducing the subjectivity of 3D GPR data analysis. We use a coherence-based technique to compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrix ͑GLCM͒ method to compute the texture-based energy, entropy, homogeneity, and contrast attributes. A selection of the GPR attribute volumes allows us to highlight key aspects of the fault zone and observe important features not apparent in the standard images. This selection also provides information that improves our understanding of gravel deposition and tectonic structures at the study site.Anew depositional/structural model largely based on the results of our analysis of GPR attributes includes four distinct gravel units deposited in three phases and a well-defined fault trace. This fault trace coincides with a zone of stratal disruption and shearing bound on one side by upward-tilted to synclinally folded stratified gravels and on the other side by moderately dipping stratified alluvial-fan gravels that could have been affected by lateral fault drag. When used in tandem, the coherence-and texture-based attribute volumes can significantly improve the efficiency and quality of 3D GPR interpretation, especially for complex data collected across active fault zones.

Abstract:Alpine watersheds are the source region of some of the largest rivers in North America and elsewhere. Understanding of hydrological processes in alpine watersheds is important for understanding the response of river basins to meteorological forcing. Talus units in alpine watersheds have been suggested in the literature as potential reservoirs of groundwater, but relatively little is known about hydrological processes in talus. To develop conceptual understanding of alpine talus and determine its storage capacity and hydraulic properties, we investigated a talus unit in the Lake O'Hara watershed in the Canadian Rockies using ground-penetrating radar, electrical resistivity tomography, measurements of talus discharge, tracer tests, and isotopic hydrograph separation. The study talus, consisting mainly of quartzite and carbonate rock fragments, had very high hydraulic conductivity (0Ð01-0Ð03 m s 1 ) and fast hydrograph recession (exponential decay coefficient of 1 d 1 ), suggesting that its storage capacity is limited to a time scale of less than a week. Groundwater flow through the talus occurs in a relatively thin (0Ð01-0Ð1 m) saturated zone at the base of the talus, which appears to have discrete flow paths rather than a single continuous sheet. A late-lying snowpack, located at the top of the talus and cliff ledges above, sustains baseflow discharging from the talus, which provides moisture to alpine meadows downstream. Although this study indicates limited storage capacity of talus, further research is required to examine the storage and transmission characteristics of talus consisting of different types of geological materials or formed in different environments.

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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 m 2 ) 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 m 3 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.Correspondence to: A. F. McClymont (alastairmcclymont@gmail.com) 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.

[1] Where they can be correlated with geological exposures and trenches, 3-D ground-penetrating radar (GPR) data can contribute critical subsurface information to paleoseismic investigations. Because active faults are typically characterized by complicated near-surface structures that vary with the styles of faulting and the types of rock that are ruptured, GPR data can be difficult to interpret. We have acquired 3-D GPR data sets across three active fault zones within New Zealand that have different deformation styles: the strike-slip Wellington fault zone, reverse faults of the Ostler fault zone, and normal faults of the Maleme fault zone. To improve our interpretation of the processed GPR volumes, we employed two suites of geometric attributes. The first suite was computed using a coherence-based algorithm. It provided estimates of the coherency, azimuth, and dip of reflections. The second suite quantified the volumetric textures of reflections, which allowed different reflection facies to be defined objectively. We have demonstrated how some attributes were more successful at visualizing certain structural or depositional characteristics than others. For example, the coherency attribute was an excellent tool for highlighting normal faults within volcanic deposits of the Maleme fault zone, whereas the texture-based attributes were most useful for discriminating between the gravel and metasediment units juxtaposed by the Wellington fault zone. Our GPR data sets and associated attribute volumes showed details of near-surface fault geometry that were not obvious from surface mapping, and they revealed evidence of off-fault deformation, gravitational collapse, and topple structures.

Unconsolidated sediments in alpine watersheds can store glacier melt and snowmelt as groundwater, which helps sustain flow in mountain rivers during dry periods. However, the amount and distribution of groundwater storage in rugged alpine terrain is not well understood, hindering our ability to predict the rate and timing of groundwater discharge into alpine streams. We show how non-invasive time-lapse microgravity surveys can be used to gauge the spatial distribution of groundwater storage changes within a large (ca 1500 Â 1000 m) moraine-talus field of the Lake O'Hara alpine watershed of the Canadian Rockies. Additional ground-penetrating radar (GPR) and seismic refraction surveys provide complementary information on subsurface bedrock topography and reveal the location of a major northwest-southeast trending depression that likely controls groundwater flow to an alpine lake contiguous with the moraine-talus field. Repeat relative gravity measurements made on a network of 80 gravity stations over and around the moraine-talus field during the summers of 2009 and 2010 reveal gravity changes of up to 25 mgal. Although the small gravity changes associated with groundwater flowing out of storage areas are noisy, significant changes are evident on the eastern side of the moraine-talus field.