LATE-GLACIAL DRAINAGE SYSTEMS ALONG THE NORTHWESTERN MARGIN OF THE LAURENTIDE ICE-SHEET

The evolution of drainage systems along the retreating northwestern La urentide Ice Sheet was complex. The interaction of ice-margin configur ation, topography and glacioisostasy resulted in a network of meltwate r rivers that variably overflowed to the Arctic and Pacific Oceans and to the Gulf of Mexico. Glacial lakes also changed dramatically in siz e and location during the period of deglaciation. At the last (and all time) glacial maximum, the ice sheet extended into the eastern Cordil lera, blocking northward and eastward drainage to the Arctic Ocean. So me meltwater and most non-glacial runoff were diverted through the mou ntains to the Yukon River basin, into Alaska and the Pacific Ocean. Re treat from the glacial maximum prior to 21 ka BP allowed proglacial dr ainage from the western margin of the ice sheet to flow into the Beauf ort Sea/Arctic Ocean. Deglaciation was rapid after about 13 ka BP, wit h the present route of the lower Mackenzie River established between 1 3 and 11.5 ka BP. Continued ice retreat led to significant southward e xpansion of the Mackenzie/Beaufort drainage basin at about 11.5 ka BP through drainage capture of glacial Lake Peace, which previously had d rained southeastward into the Missouri River and to the Gulf of Mexico . Very rapid ice retreat between 10.5 and 10 ka BP allowed glacial lak e McConnell to expand down-slope in contact with the ice margin. Numer ous glacial lakes occurred along the northwestern margin of the ice sh eet during the maximum and retreat phases. These include ice-dammed gl acial Lake Old Crow, which occupied unglaciated terrain of the norther n Yukon, and glacial Lake Peace, which utilized a number of outlets as it migrated eastward with the ice front along the Peace Valley, The l argest glacial lakes in the region were the result of glacioisostatic depression reversing the regional drainage. The Mackenzie Phase of gla cial Lake McConnell was the second largest Pleistocene lake in North A merica (> 215,000 km(2)). Late glacial and post-glacial changes in dra inage systems were largely in response to isostatic rebound.