Le rôle des formes périglaciaires dans l’hydrologie et l’évolution des pentes d’un désert polaire dans le Haut-Arctique canadien

Geomorphology exerts a fundamental control on ecosystems. In the case of periglacial landscapes, the mechanisms by which they evolve are not clearly defined, as research on the subject has often been excessively focused on the expressions of freeze-thaw processes as climatic control over landform development. It is however essential to identify and clarify what are the fundamental dynamics of sediment transport in periglacial regions to understand the evolution and the functions of these landscapes. At the scale of a unit slope, the mechanisms originating from cryo-conditioning are the main drivers of evolution. These include the effects of freeze-thaw processes on surface deposits and the production of large, periodic water supplies by snow redistribution and snowmelt. This thesis investigates the role played by periglacial landforms in the establishment of drainage pathways on a High Arctic polar desert landscape. It aims at establishing links between the azonal processes of runoff generation and sediment transport, and the climate-dependant processes of sorting and mass movement associated with the formation of patterned ground. It achieves this objective by focussing on surface deposit morphology (ch. 2), on the hydrological functions those patterned ground possess (ch. 3), on sediment transport on the slope (ch. 4) and on the impacts of those flowpaths on limnological conditions in a downstream headwater lake (ch. 5). The organisation of the drainage pathways is dependant upon the combination of sorting processes with erosion from snowmelt of large snowdrifts. The coarse nature of the reworked glacial drift and gelifracted slope deposits allowed water to move through the large voids between the particles, creating underground flow pathways of a relatively high hydraulic conductivity. These networks of preferential flow pathways similar to water tracks were efficient at transporting snowmelt, preventing the interaction of water with the rest of the soil outside of the flow paths. Some of the seeping water sources downslope from the snowdrifts yielded amounts of suspended sediments of similar amplitudes as other surface flow pathways in the Arctic. Solute transport was however the main driver of denudation, with concentrations exceeding suspended sediments by up to an order of magnitude. The presence of these flow paths influenced the timing of ice melting on the lake, contributing to the novel disappearance of perennial ice on the donwstream lake in 2011 and 2012. In addition, these flowpaths reacted to the degradation of the transient layer of permafrost in 2014, releasing solute rich waters in the soils and into the downstream lake. Further degradation of permafrost is expected to increase inputs of salt-rich waters to the littoral zone. The thesis effectively exemplifies how the evolution of a polar desert slope under severe periglacial conditions can be dependant upon the relation between sorting and water flow. This underlines the relevance of azonal processes for the geomorphology of a frost-dominated region, but maintains the importance of the periglacial control on soil morphology exerted by freeze-thaw processes.

Keyphrases: Ward Hunt, Geomorphology, Limnology, Patterned ground, Periglacial hydrology, Periglacial slope, Polar desert, Preferential flow, Sediment transport, Water tracks