Jan M. Bednarski

Late glacial drainage systems along the northwestern margin of the Laurentide Ice Sheet

Abstract The evolution of drainage systems along the retreating northwestern Laurentide Ice Sheet was complex. The interaction of ice-margin configuration, topography and glacioisostasy resulted in a network of meltwater rivers that variably overflowed to the Arctic and Pacific Oceans and to the Gulf of Mexico. Glacial lakes also changed dramatically in size and location during the period of deglaciation. At the last (and all time) glacial maximum, the ice sheet extended into the eastern Cordillera, blocking northward and eastward drainage to the Arctic Ocean. Some meltwater and most non-glacial runoff were diverted through the mountains to the Yukon River basin, into Alaska and the Pacific Ocean. Retreat from the glacial maximum prior to 21 ka BP allowed proglacial drainage from the western margin of the ice sheet to flow into the Beaufort Sea/Arctic Ocean. Deglaciation was rapid after about 13 ka BP, with the present route of the lower Mackenzie River established between 13 and 11.5 ka BP. Continued ice retreat led to significant southward expansion of the Mackenzie/Beaufort drainage basin at about 11.5 ka BP through drainage capture of glacial Lake Peace, which previously had drained southeastward into the Missouri River and to the Gulf of Mexico. Very rapid ice retreat between 10.5 and 10 ka BP allowed glacial lake McConnell to expand down-slope in contact with the ice margin. Numerous glacial lakes occurred along the northwestern margin of the ice sheet during the maximum and retreat phases. These include ice-dammed glacial Lake Old Crow, which occupied unglaciated terrain of the northern Yukon, and glacial Lake Peace, which utilized a number of outlets as it migrated eastward with the ice front along the Peace Valley. The largest glacial lakes in the region were the result of glacioisostatic depression reversing the regional drainage. The Mackenzie Phase of glacial Lake McConnell was the second largest Pleistocene lake in North America (> 215,000 km 2 ). Late glacial and post-glacial changes in drainage systems were largely in response to isostatic rebound.