Origin and Evolution of the Great Lakes

 

Grahame Larson and Randall Schaetzl

 

           This paper presents a synthesis of traditional and recently published work regarding the origin and evolution of the Great Lakes.  It differs from previously published reviews by focusing on three topics critical to the development of the Great Lakes:  the glaciation of the Great Lakes watershed during the late Cenozoic, the evolution of the Great Lakes since the last glacial maximum, and the record of lake levels and coastal erosion in modern times. 

           The Great Lakes are a product of glacial scour and were partially or totally covered by glacier ice at least six times since 0.78 Ma.  During retreat of the last ice sheet large proglacial lakes developed in the Great Lakes watershed.  Their levels and areas varied considerably as the oscillating ice margin opened and closed outlets at differing elevations and locations; they were also significantly affected by channel downcutting, crustal rebound, and catastrophic inflows from other large glacial lakes. 

           Today, lake level changes of about a 1/3 m annually, and up to 2 m over 10 to 20 year time periods, are mainly climatically-driven.  Various engineering words provide small control on lake levels for some but not all the Great Lakes.  Although not as pronounced as former changes, these subtle variations in lake level have had a significant effect on shoreline erosion, which is often a major concern of coastal residents.