ABSTRACT
Most of the meltwater from the Laurentide Ice Sheet was not discharged into proglacial outwash streams, but rather was ponded along the ice margin. Basins containing these proglacial lakes were the result of regional slope toward the ice sheet, crustal depression, and dams formed by accumulations of stagnant ice or previously formed ice-marginal glacial landforms. When lake levels rose and overtopped low points of their basins, or ice or debris dams failed, glacial-lake outburst floods occurred. Where lake outlets developed in glacial drift or poorly consolidated sedimentary bedrock, outburst floods rapidly downcut through the non-resistant substrate to completely drain the lake. Alternatively, where hard bedrock was encountered during downcutting, the lake level stabilized near that elevation.
Outburst floods were highly erosive and left their mark upon the landscape in the form of huge trench-shape channels with a suite of characteristic geomorphic components. These immense, distinctive channels lead away from all major glacial-lake basins at the southern margin of the Laurentide Ice Sheet. In addition, the midcontinent plains of the U. S. and Canada are crossed by hundreds of these deeply incised channels, which record the meltwater history of the retreating ice.
The volume of sediment eroded by the outburst floods was enormous. Floods from the southern outlets of Lake Agassiz and the lakes in the Great Lakes basins funneled into the Mississippi Valley and transported their sediment load to the Gulf of Mexico. These events probably dominate the Pleistocene glacial intervals recorded in the deposits of the Mississippi fan and Louisiana slope. In the western plains of the U. S. and Canada, outburst floods progressed from basin to basin along the ice margin like a line of falling dominoes; the arrival of a flood at a basin triggered the downcutting of its outlet and the draining of the lake. The coarse-sediment fraction, mostly sand, was dumped as huge fans which spread over the floors of the basins inundated by the floods. The concurrent enlargement of the96 outlets was so rapid that the fine fraction never had sufficient time to settle out and was washed along through downstream basins until a lake was reached that was large enough to contain the discharge without completely draining.
Meltwater flow across broad outwash plains, as traditionally envisioned, was, in fact, not typical along many segments of the Laurentide margin. The impoundment of meltwater to form glacial lakes, followed by sudden drainage and formation of glacial-lake spillways was a more significant meltwater process in most of the mid-continent area. The lasting effects of these outburst floods include the establishment of the courses of most major rivers in the area glaciated by the Laurentide Ice Sheet.
