The great majority of natural lakes on our planet are found in glaciated regions. And, since lakes are among the more transient features of our landscape, most of these formed during the last glacial period of the Pleistocene (the Wisconsin Glaciation), which stretched from 70,000 to 10,000 years ago.
Glaciers produce lakes in three ways: scooping out lake basins, damming streams or calving ice chunks as they retreat. The Great Lakes of North America and the Finger Lakes of New York State are examples of the first mechanism; they occupy basins or deep gorges that were carved from the bedrock by advancing ice sheets. In alpine areas, mountain glaciers have produced smaller lakes in a similar fashion; these now occupy cirques and basins within glacial valleys.
Glacial deposits and the ice sheets themselves often blocked major streams and rivers, producing broad, shallow lakes; the classic example is Lake Agassiz, which formed when the Red River was dammed by a lobe of the Glacier. At its peak, this lake was four times the area of Lake Superior, covering much of south-central Canada and the adjacent parts of North Dakota and Minnesota; the dam melted away 8000 years ago and much of Lake Agassiz drained into Hudson Bay, leaving Lake of the Woods and Lake Winnipeg as remnants.
Finally, numerous kettle lakes dot the vast glaciated plain of Canada, New England and the Upper Midwest. Described more thoroughly in an earlier blog, these lakes formed as chunks of ice broke from beneath retreating ice sheets; embedded in the glacial till, they melted as the climate warmed, leaving depressions that refilled as drainage patterns developed. When viewed from the air, these kettles are aligned in sweeping curves, documenting the path of glacial retreat.
