A map of the Mackinac Channel, an incised river channel through which the waters of proglacial Lake Chippewa (Michigan) drained into proglacial Lake Stanely (Huron), passing over Mackinaw Falls en-route. At that time, both lakes drained to the sea through an isostatically-depressed valley in the North Bay (Ontario) area. As the land rebounded and closed off that drainage route, water levels in the upper Great Lakes rose, covering both the channel and the falls.