 The Saint Lawrence River has some of the world's largest sustaining inland Atlantic ports at Montreal (the province's largest city), Trois-Rivières, and Quebec City (the capital).Its access to the Atlantic Ocean and the interior of North America made it the base of early French exploration and settlement in the 17th and 18th centuries.Since 1959, the Saint Lawrence Seaway has provided a navigable link between the Atlantic Ocean and the Great Lakes.