The Great Lakes were created when their north shore rebounded from the retreating ice, capturing glacial runoff. Initially, the island was submerged entirely up to 75 metres under Lake Algonquin, a proglacial lake that existed as a single body of water atop the basins of today's lakes Superior, Huron and Michigan. As the land surface to the lake's south rebounded 9,000 years ago, it began draining eastward through the North Bay outlet towards the ancestral Ottawa River, resulting in a low water Lake Stanley in the Huron basin, during which St. Joseph Island was part of the mainland.