The lake formed during the period, when the Michigan and Saginaw lobes of the Laurentian glacier had receded back to the Valparaiso and Kalamazoo moraines.

While the glacial advance became stagnant, the summer runoff formed a large lake covered parts of 13 counties in two states.

[1] The glaciers were static, only in that the glacial fronts melted at a rate matching the southward push of the ice mass.

[4] The Valparaiso Moraine formed the southern flank of the Michigan Lobe as well as the northern ridge of the Kankakee Valley.

South of the Sturgis Moraine, the future St. Joseph River was a marshy plain, extending southeastward towards the toe of the Huron-Erie Lobe.

First the Union City Moraine formed, then as the ice receded and again stabilized the Mississinawa Moraine formed along the front of this ice [4] The Saginaw Lobe's meltwater flowed across the St. Joseph valley, while the Huron-Erie Lobe's meltwater and sediments first flowed into the upper Tippecanoe River.

[4] When the glacier moved back to the Mississinawa Moraine, the off flowing water created the Eel River valley.

[2] The lake had two open bodies of water, split by the Iroquois Moraine, which formed a peninsula from the west.

The northern waters stretched from just west of Momence, Illinois, along the main stem of the Kankakee River to the marshes southwest of South Bend, Indiana.

These two basins were linked across 35 miles (56 km) of the nearly level Tipton Till Plain, west of the Tippecanoe River.

Meanwhile, the Huron-Saginaw lobe had melted back to the north and east along a line in Michigan extending from Holland eastward through Kalamazoo to Jackson.

At this point, the forward movement of ice coming from the north equaled the rate of melting over the summer season.

Not being able to cut a channel through the limestone ridge in Momence, the Kankakee Lake became 500,000 acres (202,346 ha) of marshland.

Here a sand ridge forms the east border of the till plain sloping westward to the Tippecanoe River.

[2] It topped the western divide near Morris, Illinois, where a lake had formed as the glacier retreated from the Marseilles moraine.