St. Lawrence Estuary stretches from west to east for 655 km, from the outlet of Lake Saint Pierre to pointe des Monts, where it becomes the Gulf of St. Lawrence, in Quebec, Canada.

Lake Saint Pierre is a lake in Quebec, Canada, a widening of the Saint Lawrence River between Sorel-Tracy and Trois-Rivières. It is located downstream, and northeast, of Montreal; and upstream, and southwest, of Quebec City. The end of the lake delimits the beginning of the estuary of Saint Lawrence.