Great Lakes Geography: Correcting the Manitoba/Ontario Confusion

The premise contains two errors:

1. There are five Great Lakes (not five in Ontario + four in Manitoba): Superior, Huron, Michigan, Erie, Ontario.
2. None are in Manitoba. All five border Ontario (Canada) and/or U.S. states. Manitoba's large lakes (Winnipeg, Winnipegosis, Manitoba) are separate post-glacial lakes.

Why They're Smaller Than Glacial Maximums

The Great Lakes are remnants of glacial Lake Algonquin, Lake Chippewa, Lake Nipissing—proglacial lakes that filled the basin as the Laurentide Ice Sheet retreated (~14,000–8,000 years ago). Those ancestral lakes were larger because:

• Ice dams blocked outlets, raising levels
• Isostatic depression: crust weighed down by ice, lowering outlets
• Meltwater input >> evaporation

As ice retreated, outlets lowered (Niagara, St. Lawrence), crust rebounded (isostatic uplift, still ongoing ~30 cm/century at Hudson Bay), and climate warmed. Modern lakes stabilized ~3,000 years ago at current levels.

Lake Agassiz (Manitoba/NW Ontario/MN/ND) was the true giant—440,000 km², larger than the Caspian. It drained catastrophically (~8,200 ya), possibly triggering the 8.2 ka cooling event.