A major aggradation took place from 8000 BP to 4000 BP in the Porcupine and Old Crow rivers, northern Yukon. The aggradation was a consequence of an increase in ow and the result of a perturbation of the permafrost active layer following the early-Holocene climatic warming of northwestern Canada. These ndings are useful for improving our understanding of how natural landscapes and river systems evolved in regions that experienced a permafrost history and, more particurlarly, an increase in climatic warming. This article also contributes to an improved understanding of natural landscape evolution along the Porcupine and Old Crow rivers in eastern Beringia, where there has been much interest in questions related to animal and human migration and adaptation.

Traditional methods of measuring surface net radiation involve point measurements that represent only a small area surrounding the instrumented sites. Remotely sensed spaceborne data offer the means by which to obtain estimates of the outgoing fluxes at the regional scale. The objective of this study was to estimate surface albedo, surface thermal exitance, and net radiation using Landsat Thematic Mapper (TM) data over wetland tundra at northern treeline near Churchill, Manitoba, Canada. Ground-based measurements of each component of the radiation balance were acquired at 5 locations coincident with 2 TM overpasses during summer 1991. Each location was representative of 1 of the major terrain types found in the Hudson Bay Lowlands (i.e. sedge-dominated wetland, upland lichenheath, tundra lakes and ponds, willow/birch wetland, and open spruce-tamarack forest). The mean absolute differences between remote sensing estimates and field measurements (all sites combined) are 0.01 for albedo, 25.7 W m -2 for thermal exitance, and 14.1 W m -2 for net radiation. The 2 summer 1991 TM images (June and August) were then used to examine within and between terrain type variations in surface net radiation during the growing season. TM imagery from August 1984 and August 1991 were also utilised to investigate differences in surface fluxes between a dry year (1984) and a wet year (1991). Results indicate that surface wetness and, to a lesser extent, phenology are the 2 main factors controlling the radiation balance during the summer period in this subarctic tundra-forest landscape.

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