Drains the Great Lakes and serves as a major waterway of eastern Canada. First explored and named by Jacques Cartier in the early 16th century, the St. Lawrence emerges from the northeastern corner of Lake Ontario in the Thousand Islands archipelago, forming the border between Ontario and New York. The St. Lawrence receives the Ottawa and Saguenay Rivers and flows through Montréal and Quebec City. At its mouth, the Gulf of St. Lawrence is one of the world's largest estuaries.