Lake Chicago

Lake Chicago
Map of middle stage of glacial Lake Chicago, USGS Report of 1915.
Lake Chicago
Location	North America
Group	Great Lakes
Coordinates	42°12′N 87°06′W / 42.2°N 87.1°W / 42.2; -87.1
Lake type	former lake
Primary inflows	Laurentide Ice Sheet
Primary outflows	Chicago River
Basin countries	United States
First flooded	18,000 years before present
Max. length	241 mi (388 km)
Max. width	57 mi (92 km)
Average depth	160 ft (49 m) [1]
Residence time	4000 years in existence
Surface elevation	160 ft (49 m)
References	The Pleistocene of Indiana and Michigan and the History of the Great Lakes; Frank Leverett and Frank B. Taylor; Monographs of the United States Geological Survey; Volume LIII; Washington; 1915

Lake Chicago was a prehistoric proglacial lake that is the ancestor of what is now known as Lake Michigan, one of North America's five Great Lakes. Formed about 13,000 years ago and fed by retreating glaciers, it drained southwest through the Chicago Outlet River.