VOLUME AND ROUTING OF LATE-GLACIAL RUNOFF FROM THE SOUTHERN LAURENTIDE ICE-SHEET

Melting of the Laurentide Ice Sheet during the last deglaciation added large volumes of water to many rivers and lakes of North America and to the world's oceans. The volume and routing of this meltwater not only helped shape the land's surface but also played a role in the evolution of late-glacial climate. A computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the Laurentide Ice Sheet at 500-yr time slices between 14,000 and 8000 yr B.P. Nearly all waters reaching the oceans flowed through the St. Lawrence, Hudson, or Mississippi River valleys. Discharge through the Mississippi River to the Gulf of Mexico during late-glacial time varied by more than a factor of 5, ranging between 17,400 m3 sec-1 (550 km3 yr-1) and 98,200 m3 sec-1 (3200 km3 yr-1). Discharge entering the North Atlantic Ocean through the St. Lawrence and Hudson valleys ranged between 20,300 m3 sec-1 (640 km3 yr-1) and 65,300 m3 sec-1 (2060 km3 yr-1), with very abrupt, twofold changes at about 11,000, 10,000, and 9500 yr B.P. as a result of the rerouting of water from the Lake Agassiz basin. As the areal extent and mass of the Laurentide Ice Sheet diminished, the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr-1 at about 14,000 yr B.P. to 2100 to 2600 km3 yr-1 between 11,500 and 8000 yr B.P. No meltwater entered the Gulf of Mexico after 9500 yr B.P. After the demise of the ice sheet over Hudson Bay about 8000 yr B.P., the modern continental drainage network was established and flows through the St. Lawrence declined to modern values of about 320 km3 yr-1. © 1990.