CHICAGOLAND – A Metropolis Site Determined by Glacial and Coastal Process

The glacial processes that shaped the Great Lakes Region had similar impacts across most of the region, removing pre-existing unconsolidated materials down to bedrock, scouring the bedrock surface, depositing till in ground and end moraines, and establishing a new topography and new drainage patterns. However, at the southern end of the Lake Michigan basin, in the area of present-day Chicago, a unique glacial shaping of the landscape occurred. End moraines were formed that could act as a dam to hold glacial lake water at an elevation about 60 feet (18 m) higher than the historical mean, and glacial-fluvial erosion breaching this morainal dam allowed glacial and post-glacial lake water to drain to the Mississippi River system. As lake level declined to the modern level and as the Chicagoland river system developed, a natural waterway passage formed in the Chicago area between the Great Lakes and Mississippi River watersheds. A critical factor in this waterway connection was coastal erosion that established the location of the mouth of the Chicago River. The site of Chicago was thus destined by glacial and coastal processes to be the most significant waterway transportation center in the mid-continent, but major geo-engineering would be needed for urban growth. The Chicago River mouth needed straightening and protection from shoaling, the land of the central business district needed to be raised for improved drainage, and ultimately the flow direction of the Chicago River needed to be reversed to redirect sewage and protect the water quality of Lake Michigan. All of this geologic and human-induced landscape shaping is now a backdrop to ongoing considerations for future geo-engineering to modify or terminate this Mississippi River to Great Lakes watershed connection. That connection, interestingly, is the very reason for the founding and growth of Chicago.