St Lawrence Seaway

St Lawrence Seaway, major development of the St Lawrence River, in south-eastern Canada and north-eastern United States that permits ocean-going vessels to navigate between the Great Lakes and the Atlantic Ocean. The seaway is sometimes taken to include the Welland Ship Canal, linking lakes Ontario and Erie, and the Sault Sainte Marie Canals, between lakes Huron and Superior. The name St Lawrence Seaway is also used to denote the entire Great Lakes-St Lawrence River waterway, which in effect forms a fourth coastline for Canada and the United States. This article is limited to a discussion of shipping facilities on the upper St Lawrence, plus a description of the adjacent St Lawrence Power Project.

From Montreal to Lake Ontario, a distance of about 295 km (183 mi), the St Lawrence rises 68 m (224 ft) in elevation and has several regions of rapids. In the 19th century, Canada constructed canals and locks to allow commercial vessels to navigate this part of the river, and by 1900 all of the component waterways had a minimum depth of 4.3 m (14 ft). In 1954 Canada and the United States agreed to build a system able to handle more traffic and larger ships. Construction began that year, and the seaway was opened in 1959. It provides a continuous channel at least 8.2 m (27 ft) deep and has seven locks. The seaway is operated by the St Lawrence Seaway Authority (Canada) and the St Lawrence Seaway Development Corporation (United States), and tolls are charged.

The seaway is divided into five sections (see map). The Lachine section connects Montreal and western Lake St Louis (a widened part of the river). It includes a 29-km (18-mi) canal bypassing the Lachine Rapids; the canal has two locks, named St-Lambert and Côte-Sainte-Catherine, which raise ships a total of 13.7 m (45 ft). The Soulanges section is made up of the 26-km (16-mi) Beauharnois Canal, between western Lake St Louis and eastern Lake St Francis (another broadened part of the river). This canal also has two locks, which together raise ships by 25 m (82 ft). The 47-km (29-mi) Lake St Francis section, from the eastern part of the lake to Cornwall Island, rises by only 30 cm (12 in); this section has a dredged channel but no locks. The 80-km (50-mi) International Rapids section includes the Wiley-Dondero Ship Channel, with the Bertrand H. Snell and Dwight D. Eisenhower locks (total lift, 27.4 m/90 ft), and the Iroquois Lock (lift, 1.8 m/6 ft). The 109-km (68-mi) Thousand Islands section, the fifth part of the seaway, has dredged channels and no locks. The first three sections are wholly in Canada (Quebec and Ontario provinces), and the last two contain segments (notably the Wiley-Dondero Ship Channel) in northern New York state.

The seaway plays a major role in overseas and United States-Canadian trade, handling large quantities of cargo during the open months of May through November (it is closed by ice the rest of the year). In 1983, more than 45 million tonnes of cargo were transported through the St Lawrence section of the seaway. Upstream shipments consisted mainly of iron ore from Quebec and Labrador to ports on the Great Lakes; downstream cargo was made up principally of the crops wheat, corn, barley, and soya beans from the plains region.

The St Lawrence River, with its great volume of flow and steep descent, is well suited to the production of hydroelectricity. During 1954-1960, roughly concurrent with the construction of the seaway, the large St Lawrence Power Project was built at the International Rapids section of the river. The project includes the Iroquois Dam (at Iroquois, Ontario) and the Long Sault Dam (near Massena, New York), both used to control the river's flow, and the Robert Moses-Robert H. Saunders Power Dam (generating capacity, 1,600,000 kw). The power dam was built between Barnhart Island, New York, and Cornwall, Ontario; its generating capacity is shared equally by the United States and Canada.