St Lawrence

River in eastern North America. With the Great Lakes and linking canals such as the Welland Ship Canal, it forms the St Lawrence Seaway, an inland route for small ocean-going ships from the Gulf of St Lawrence, an arm of the Atlantic Ocean, to Thunder Bay at the head of Lake Superior; larger vessels stop at Montréal. The river is 1,200 km/745 mi long and icebound for four months each year. Enormous quantities of hydroelectric power are generated along its course.

The river issues from Lake Ontario, and is fed by the Great Lakes. In its upper course the St Lawrence includes the scenic Thousand Islands and forms the boundary between Ontario and New York State, USA. It then marks the Ontario–Québec border, before entering Québec province, where it flows past Montréal and Québec City. The river widens from about 3 km/2 mi below Québec to a maximum width of 145 km/90 mi as it empties into the Gulf of St Lawrence. The river had become polluted by the 1970s, and despite clean-up programmes there are continuing problems with chemical pollutants and invasive species.