 The Island of Montreal is the largest island in the Hochelaga Archipelago, which is formed by the confluence of the Ottawa and St. Lawrence rivers.The St. Lawrence widens into Lake Saint-Louis southwest of the island, narrows into the Lachine Rapids, then widens again into the Bassin de La Prairie before narrowing again and flowing toward Quebec City and the Atlantic Ocean.Saint Helen's Island and Notre Dame Island are in the Saint Lawrence southeast of downtown Montreal, and Nuns' Island is located southwest of downtown.