international waterway, 2,342 mi (3,769 km) long, consisting of a system of canals, dams, and locks in the St. Lawrence River and connecting channels between the Great Lakes; opened 1959. It provides passage for oceangoing vessels into central North America. The seaway includes a 27-ft (8-m) deep waterway, a canal, and seven locks between the port of Montreal and Lake Ontario; a 27-ft (8-m) channel and eight locks through the Welland Ship Canal; and the Sault Sainte Marie Canals and locks.
The seaway has created a fourth seacoast accessible to the industrial and agricultural heartland of North America and has brought oceangoing vessels to lake ports such as Buffalo, Cleveland, Toledo, Detroit, Chicago, Milwaukee, Duluth, and Toronto. The maximum vessel size is 730 ft (223 m) in length with a cargo capacity of 28,000 tons. The shipping season has been extended to 250 days (mid-April to mid-December) by increased use of icebreakers and air pumps to control ice formation in the locks. Iron ore, wheat, and coal are the principal cargoes carried on the seaway.
Construction of the project was authorized by Canada in 1951 and by the United States in 1954. The St. Lawrence Seaway Authority was charged with construction and maintenance of required facilities in Canada; the St. Lawrence Seaway Development Corporation was responsible for facilities in the United States. Principal new locks on the St. Lawrence River section of the seaway are, from east to west, St. Lambert (18 ft/5.5 m lift); Côte Ste Catherine (30 ft/9.1 m), which enables vessels to bypass the Lachine Rapids; Lower and Upper Beauharnois (82 ft/25 m, including the Beauharnois Canal built in 1932); Bertrand H. Snell (45 ft/13.7 m); Dwight D. Eisenhower (38 ft/11.6 m); and Iroquois Guard Lock (3 ft/91 cm). Hydroelectric facilities were integrated with the project and developed and operated by the Power Authority of the State of New York and the Hydro-Electric Power Commission of Ontario. One serious unexpected side effect of the seaway has been the introduction of invasive nonnative aquatic species, such as the zebra and quagga mussels, into the Great Lakes and from there into other inland waterways.