Water tracks play a major role in the headwater basin hydrology of permafrost landscapes in Alaska and Antarctica, but less is known about these features in the High Arctic. We examined the physical and hydrological properties of water tracks on Ward Hunt Island, a polar desert site in the Canadian High Arctic, to evaluate their formation process and to compare with water tracks reported elsewhere. These High Arctic water tracks flowed through soils that possessed higher near-surface organic carbon concentrations, higher water content, and coarser material than the surrounding soils. The water track morphology suggested they were initiated by a combination of sorting, differential frost heaving, and eluviation. The resultant network of soil conduits, comparable to soil pipes, dominated the hydrology of the slope. The flow of cold water through these conduits slowed down the progression of the thawing front during summer, making the active layer consistently shallower relative to adjacent soils. Water tracks on Ward Hunt Island, and in polar desert catchments with these features elsewhere in the High Arctic, strongly influence slope hydrology and active-layer properties while also affecting vegetation distribution and the quality of runoff to the downstream lake.Key words: Patterned ground, permafrost hydrology, polar desert, sorted stripes, water tracks.Résumé : Les nappes d'eau (« water tracks ») jouent un rôle important au niveau de l'hydrologie des bassins d'amont des paysages de pergélisol en Alaska et en Antarctique, mais on en connait moins sur ces caractéristiques dans le Haut-Arctique. Nous avons examiné les propriétés physiques et hydrologiques de nappes d'eau sur l'île Ward Hunt, un site de désert polaire dans le Haut-Arctique canadien, afin d'évaluer leur processus de formation et de comparer avec les nappes d'eau documentées ailleurs. Ces nappes d'eau dans le HautArctique coulaient à travers des sols dont les concentrations en carbone organique proche de la surface et la teneur en eau étaient plus élevées, et dont le matériau était plus grossier que celui des sols environnants. La morphologie de nappes d'eau laisse entendre qu'elles ont été engendrées par une combinaison de triage, de soulèvement par le gel différentiel et d'éluviation. Le réseau de conduits de sol (comparables à des tuyaux de sol) qui en résulte dominait l'hydrologie de la pente. L'écoulement d'eau froide par ces conduits ralentissait la progression du front de dégel pendant l'été, entraînant la diminution systématique de l'épaisseur de la couche active par rapport aux sols adjacents. Les nappes d'eau sur l'île Ward Hunt, et dans les bassins hydrologiques de désert polaire ayant ces caractéristiques

Field records, aerial photographs, and satellite imagery show that the perennial ice cover on Ward Hunt Lake at Canada's northern coast experienced rapid contraction and thinning after at least 50 years of relative stability. On all dates of sampling from 1953 to 2007, 3.5 to 4.3 m of perennial ice covered 65-85% of the lake surface in summer. The ice cover thinned from 2008 onward, and the lake became ice free in 2011, an event followed by 26 days of open water conditions in 2012. This rapid ice loss corresponded to a significant increase in melting degree days (MDD), from a mean (±SD) of 80.4 (±36.5) MDD (1996MDD ( -2007 to 136.2 (±16.4) MDD (2008MDD ( -2012. The shallow bathymetry combined with heat advection by warm inflows caused feedback effects that accelerated the ice decay. These observations show how changes across a critical threshold can result in the rapid disappearance of thick perennial ice.

Mean annual air temperatures in the High Arctic are rising rapidly, with extreme warming events becoming increasingly common. Little is known, however, about the consequences of such events on the ice‐capped lakes that occur abundantly across this region. Here, we compared 2 years of high‐frequency monitoring data in Ward Hunt Lake in the Canadian High Arctic. One of the years included a period of anomalously warm conditions that allowed us to address the question of how loss of multi‐year ice cover affects the limnological properties of polar lakes. A mooring installed at the deepest point of the lake (9.7 m) recorded temperature, oxygen, chlorophyll a (Chl a) fluorescence, and underwater irradiance from July 2016 to July 2018, and an automated camera documented changes in ice cover. The complete loss of ice cover in summer 2016 resulted in full wind exposure and complete mixing of the water column. This mixing caused ventilation of lake water heat to the atmosphere and 4°C lower water temperatures than under ice‐covered conditions. There were also high values of Chl a fluorescence, elevated turbidity levels and large oxygen fluctuations throughout fall and winter. During the subsequent summer, the lake retained its ice cover and the water column remained stratified, with lower Chl a fluorescence and anoxic bottom waters. Extreme warming events are likely to shift polar lakes that were formerly capped by continuous thick ice to a regime of irregular ice loss and unstable limnological conditions that vary greatly from year to year.