Paul Karrow, Quaternary Sciences Institute
Figure 1: Drainage divide and glacial lakes.
Glaciers make great dams. Ice is impermeable. Yes, it may crack, or dissolve in water, but if it's thick enough (more than about 50m) it flows under its own weight and tends to fill cavities (we might say "self-healing" or "self-sealing).
The last ice sheet, like its predecessors (there were many) in melting away as climate warmed about 20,000 years ago, released huge quantities of meltwater. As the southern limits reached by the ice sheet were along the Ohio River valley in the U.S.A., it was free draining, and remained so as long as the margin was south of the drainage divide between the Ohio River and the Great Lakes basin. However, once the ice edge melted back north of the divide, glacial lakes began to form in the low ground between the divide and the ice sheet (Fig. 1). In reality, such depressional areas were filled by meltwater to overflowing at the lowest available point around the edge. In the Great Lakes area, each of the present lake basins was occupied by a succession of glacial lakes. In general, the oldest lakes were the highest and successively lower lakes formed as the ice melted back to expose lower outlets. If the ice thickened and readvanced, lower outlets were sometimes blocked, raising water levels higher again (only temporarily).
Figure 2: Spillways of the Great Lakes area.
A network of meltwater escape routes has been recognized in the Great Lakes area (Fig. 2), occupied at different times as the ice melted back far enough to expose them. These routes, graded by flowing water, have been used for human transportation routes - e.g. Trent canal between Georgian Bay and Lake Ontario; 11,500 years ago the Trent Valley linked glacial Lake Algonquin in the Huron basin to glacial Lake Iroquois in the Ontario basin, and the route of the never-built Georgian Bay Ship Canal near North Bay (not to mention that of the voyageurs of the fur trade); ten thousand years ago it drained glacial Lake Algonquin of the Huron basin down the Ottawa Valley to the Champlain Sea.
Remember that the ice sheet caused the Earth's crust to sink and as the ice melted the crust rose again (see What on Earth Spring 2002), a process so slow it continues today. However, 10,000 to 12,000 years ago, as the ice sheet was melting away from southern Ontario, much of that rebounding had not yet taken place so the land to the northeast was much lower than it is today. With that in mind, picture the ice unblocking the east end of the Erie basin so the contained glacial lakes could flow freely to the northeast, and later the same thing in the Ontario and Huron basins. When these three basins were "uncorked" by the retreating ice front, water levels dropped to way below their present levels (by 50 to 100m or more). That left a lot of former glacial lake bottom dry land, which was soon occupied by plants and animals. Then as the crust tilted up to the northeast, water in the basins was raised to about their present levels over thousands of years. From time to time drowned forest stumps are found in the bottoms of lakes Erie and Huron, giving evidence of the former land areas drowned by uplift of the sills (Fig. 3).
Thus, glacier fluctuations have also caused great effects on Great Lakes water levels.
Figure 3: Drowned forests under lakes
