The ice front has almost completely left the region, and the ice dam has been removed.  This allowed an arm of the Atlantic Ocean to advance inland over the isostatically depressed lowlands, and thus creating the Champlain Sea.  Water levels in the lake Ontario basin are very low, however freshwater conditions still occupy the area.  Isostatic rebound now controls the destiny of the Champlain Sea.

How is it possible to have marine waters in the St. Lawrence lowlands, and not in the Lake Ontario basin?