River, southern Quebec and southeastern Ontario, Canada.
It flows northeast out of Lake Ontario into the Gulf of Saint Lawrence and is about 760 mi (1,225 km) long. It passes through the Thousand Islands and for about 120 mi (195 km) forms the boundary between New York and Ontario. Entering Quebec, it widens into Lake St. Francis, then flows past Montreal island. Below Quebec city it widens to 90 mi (145 km) at its mouth in the Gulf of St. Lawrence. Major tributaries include the Ottawa, Saguenay , Richelieu , and Manicouagan rivers, all in Canada. It links the Atlantic Ocean with the Great Lakes through the Saint Lawrence Seaway .