Lake Saginaw was a prehistoric glacial lake in Michigan that occupied the basin now filled by Saginaw Bay during shifting late-glacial stages in the Huron basin. It existed twice as an independent lake, first draining west through the Grand River channel to Lake Chicago before a northern opening joined it to Lake Arkona, then reappearing when ice advanced again during the time of Lake Whittlesey. Its waters reached far into nearby river valleys, and changes in ice barriers and outlets eventually ended it as a separate lake. Its shorelines and outlet history remain important evidence for reconstructing how ice, water, and land rebound shaped the Great Lakes region.