Valley morphology and sediment in the Fort McMurray region of Alberta indicate that a catastrophic flood discharged down the lower Clearwater and Athabasca river valleys 9900 yr B.P. Geomorphic and chronologic evidence suggests that glacial Lake Agassiz (Emerson phase) was the probable water source. As the flood incised a drainage divide located near the Alberta-Saskatchewan border, the level of glacial Lake Agassiz decreased by 46 m, discharged 2.4 x 106 m3/s for at least 78 days, and stabilized at 438 m above sea level in the Lake Wasekamio area. At that time water entered the Arctic Ocean via glacial Lake McConnell and the Mackenzie River, rather than the Gulf of Mexico via the Mississippi River, as previously thought. Such a large influx of fresh water (8.6 km3/h) into the Arctic at the close of the last glaciation may have had an abrupt, major influence on northern climate.