The St. Lawrence is part of a complex system, which includes the Great Lakes, the St. Lawrence Seaway, and a dredged channel between Quebec and Montreal. The main purpose of the Seaway is to provide a maritime link between Montreal, an ocean port, and the Great Lakes by using a series of locks and canals. The best known is the Welland Canal linking Lake Ontario and Lake Erie, a 99-meter climb over the Niagara escarpment. A major issue for the seaway is related to its limited capacity and being closed for about three months during the winter.
The St. Lawrence itself can be subdivided between the estuary, which is accessible to deep navigation, and the river, where vessels close to the Panamax standard can access year round the Port of Montreal. The channel between Quebec and Montreal has a depth of 11.3 meters (37 feet) and can handle containerships up to 4,200 TEU.