SUB-ARCTIC CLIMATE VARIABILITY INDICATED BY PATTERNS OF CHANNEL CHANGE IN THE UPPER YUKON BASIN, YUKON TERRITORY, CANADA

Climate changes have been correlated with increased river discharge in the arctic (Peterson et al., 2002), however gage records in Western North America are not long enough to present a clear picture of arctic and sub-arctic discharge changes. River and bar geomorphology, and vegetation patterns have the potential to enhance our understanding of stream response to climate variability on a longer time scale. This study uses vegetation age and bar mapping coupled with gage record data and remote sensing to investigate channel change patterns and their links to climate change in three tributaries of the Yukon River (Pelly, Ross, and Wheaton Rivers) in the upper Yukon basin. Vegetation communities were mapped on selected bars in the field and selected trees were cored or cross-sectioned to determine community age and thus minimum bar age. These data were used to ground truth regional near-stream vegetation mapping on ASTER and LANDSAT imagery. The Pelly (49000 km²) and Ross (7250 km²) Rivers in the eastern Yukon are snow-melt dominated whereas the Wheaton River (875 km²) in the western Yukon has a small glacial contribution. Preliminary results indicate that although avulsion and meander migration are common processes of channel change, the amount and rates of change are highly variable among the three sub-basins. Vegetation community patterns and ages indicate that Pelly River bars experience migration and avulsion which reworks substantial portions of bars in as little as 30 years, whereas bars on the Ross River have extensive areas over 100 years of age. The Wheaton River has experienced substantial avulsion during snow melt floods in the last decade (1999), however like the Ross River, some bar areas adjacent to the main channel are approximately 100 years of age. Potential explanations for the differences in channel change rates are interactions among climate, bedrock geology, glacial history, and slope that strongly influence discharge and bedload, thereby determining the effectiveness of flood events.