It flows eastward from the source Lake Ontario to the Atlantic Ocean. The mouth St. Lawrence River crosses the border between the U.S and Canada. Some of the major cities include Brockville, Morristown, Ogdensburg, Massena, Cornwall, Montreal, Trois-Rivières, and Quebec City before entering the Gulf of Saint Lawrence the mouth of St. Lawrence River and it is one of the Biggest estuaries in the world.