[1] A spatiotemporal computation of permafrost decay covering the period from 1957 to 2009 and validated by field investigations was made over a 76 km 2 river catchment straddling the tree line, in the discontinuous permafrost zone, east of Hudson Bay, in order to estimate the amounts of sediments and organic carbon released by thermokarst. Lithalsas and palsas are the dominant permafrost landforms, whereas thermokarst ponds, landslides, active layer failures, and gullies are the main features of permafrost degradation. Results show that 21% of the existing permafrost in 1957 had disappeared in 2009, resulting in a 96% growth of the thermokarst pond cover and a 46 to 217% increase of the number of active erosion landforms. An increase of stream connectivity related with the degradation of permafrost potentially allowed for an increase of sediments and carbon delivery to the main stream by a factor of 1.6. Volume of active landslides and gullies also increased by 12 to 38%, enhancing sediment and organic matter yields. Significant differences in permafrost degradation and in sediment and carbon inputs were observed along an east-west transect, from sites located at the head of the watershed near the tree line to sites located downstream close to the Hudson Bay coast. Thermokarst ponds in the forest tundra area released 2.3 times more sediments and dissolved organic carbon per unit of area in the fluvial system than in the shrub tundra area. Despite these yields by thermokarst, the Sheldrake River catchment currently does not seem to be yielding proportionally more sediments and carbon than a permafrost-free river catchment.Citation: Jolivel, M., and M. Allard (2013), Thermokarst and export of sediment and organic carbon in the Sheldrake

Permafrost degradation can be monitored through changes in the surface area and depth of thermokarst ponds. Radar remote sensing allows for discrimination of thermokarst ponds of different depths across large areas because different water depths produce different ice regimes in winter. In this study, patterns in the spatial distribution of ice‐cover regimes of thermokarst ponds in a typical discontinuous permafrost region are first revealed. Correlations of these ice‐cover regimes with the permafrost degradation states and thermokarst pond development in two historical phases were analyzed and compared. The results indicate that the ice‐cover regimes of thermokarst ponds are affected by soil texture, permafrost degradation stage and permafrost depth. Permafrost degradation is difficult to assess directly from the coverage area of floating‐ice ponds and the percentage of all thermokarst ponds consisting of such floating‐ice ponds in a single year. Therefore, continuous monitoring of ice‐cover regimes and surface areas can help to elucidate the hydrological trajectory of the thermokarst process and permafrost state.

In order to assess the impact of seasonal active layer thaw and thermokarst on river flow and turbidity, a gauging station was installed near the mouth of the Sheldrake River in the discontinuous permafrost zone of northern Quebec. The station provided 5 years of water level data and 3 years of turbidity data. The hydrological data for the river showed the usual high water stage occurring at spring snowmelt, with smaller peaks related to rain events in summer. Larger and longer turbidity peaks also occurred in summer in response to warm air temperature spells, suggesting that a large part of the annual suspension load was carried during midsummer turbidity peaks. Supported by geomorphological observations across the catchment area, the most plausible interpretation is that the rapid thawing of the active layer during warm conditions in July led to the activation of frostboils and triggered landslides throughout the river catchment, thus increasing soil erosion and raising sediment delivery into the hydrological network. These results indicate that maximum sediment discharge in a thermokarstaffected region may be predominantly driven by the rate of summer thawing and associated activation of erosion features in the catchment.Key words: permafrost, northern Quebec, thermokarst, turbidity, subarctic river.Résumé : Afin d'évaluer l'impact du dégel saisonnier de la couche active et du thermokarst sur le débit fluvial et la turbidité, une station hydrométrique a été installée près de l'embouchure de la rivière Sheldrake dans la zone de pergélisol discontinu au nord du Québec. La station a fourni des données portant sur le niveau d'eau sur une période de 5 ans et des données de turbidité sur 3 ans. Les données hydrologiques quant à la rivière ont montré le cycle de crue habituel se produisant à la fonte des neiges au printemps, avec de moindres crues liées à des événements de pluie en été. Des pointes de turbidité plus importantes et plus longues sont aussi survenues en été en réponse à des périodes de température de l'air élevée suggérant qu'une grande partie de la charge en suspension annuelle ait été portée pendant des pointes de turbidité au milieu de l'été. Appuyé par les observations géomorphologiques sur l'ensemble du bassin versant, l'interprétation la plus plausible est que le dégel rapide de la couche active pendant des conditions chaudes en juillet a mené à l'activation de ventres de boeuf et a déclenché des glissements de terrain sur tout le bassin versant de la rivière, augmentant ainsi l'érosion et la production de sédiments dans le réseau hydrologique. Ces résultats indiquent que le débit solide maximal dans une région touchée par le thermokarst peut être principalement lié au taux de dégel d'été et à l'activation de caractéristiques d'éro-sion dans le bassin versant.