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The St. Lawrence river is among the world's greatest river systems and has the highest annual discharge of any Canadian river at 447 cubic km per year. No other river in the world discharges more water into the Atlantic Ocean and only the Mississippi River has a greater discharge in North America. The river is the chief outlet of the Great Lakes and travels more than 1200 km. It flows from the northeast end of Lake Ontario past Montreal to Quebec City where it forms the huge Gulf of St. Lawrence estuary. The entire St. Lawrence River System, including the Great Lakes, spans a distance of around 3200 km and drains an area of approximately one million square kilometres.