St. Lawrence Seaway, a joint project of the United States and Canada that provides a deepwater ship route from the Atlantic Ocean to the Great Lakes. It consists of a series of dredged channels, canals, and locks on the St. Lawrence River between Montreal and Lake Ontario. In addition, the Welland Canal between Lakes Ontario and Erie is administered by Canada as part of the seaway. Combined with the Great Lakes, the seaway forms one of the world's greatest inland waterways, the Great Lakes-St. Lawrence Seaway system. A minimum channel depth of 27 feet (8.2 m) is maintained throughout the system. The seaway is closed to shipping from late December until early April.
Several major sections make up the seaway as it ascends from 20 feet (6 m) above sea level at Montreal to the 5 70-foot (174-m) level of Lake Erie. The first of these sections, at Montreal, is an 18-mile (29-km) channel whose St. Lambert and Cote Ste. Catherine locks bypass the Lachine Rapids, once the farthest upstream point ships could reach. After sailing across Lake St. Louis, a wide section of the river, ships enter the Beauharnois Canal through its two locks about 20 miles (32 km) upstream from Montreal.
Farther on, beyond Lake St. Francis, lies the international section of the seaway (shared by Canada and the United States). It consists of three dams, a canal, and three locks. The Moses-Saunders Power Dam—whose 32 generators have a total capacity of 1,800,000 kilowatts—and the Long Sault Dam have backed up the river here and flooded the once treacherous International Rapids. Ships bypass the dams through the Wiley-Dondero Canal and its Eisenhower and Snell locks. Twenty-five miles (40 km) upstream is the Iroquois Dam with its lock, the last step of the seaway within the river. This dam aids in the control of the river level above the Moses-Saunders Power Dam.
The final step in the seaway is the Welland Canal, whose eight locks bypass the Niagara River and Falls and overcome the 320-foot (98-m) difference in elevation between Lakes Ontario and Erie. Beyond here, ships have access to all parts of the Great Lakes.
Most vessels operating on the seaway are lake freighters, which are designed for operating within the Great Lakes-St. Lawrence Seaway system and are not oceangoing. Some oceangoing vessels, however, also use the seaway. More than 80 per cent of all the cargo transported on the system consists of bulk cargoes, such as grain, iron ore, and coal.
Projects to allow ships to skirt the rapids on the St. Lawrence were begun as early as the 18th century. By the early 20th century, small ships of up to 14-foot (4.3-m) draft could sail from the ocean into the Great Lakes. The canals and locks in use at that time, all Canadian-built, were even then considered by many to be inadequate.
The idea of a deepwater ship channel in the St. Lawrence soon became a controversial issue in business and government circles of both countries. The railroads and representatives of Atlantic coast ports were strongly opposed to the seaway. They argued that the severe winters would limit the shipping season to seven or eight months of the year, and that the project would be far too costly to build. Midwesterners, generally in favor of the seaway, contended that business generated by shipping would offset winter shutdowns, and that tolls could be charged so that the seaway could pay for itself.
For nearly 60 years the seaway issue was debated. After Congress failed to approve a 1932 treaty and a 1941 agreement between Canada and the United States, Canada announced in 1952 that it would build the seaway alone.
Finally, in 1954, Congress passed the Wiley-Dondero Act, which created the St. Lawrence Seaway Development Corporation, an agency of the federal government. The corporation was made responsible for the planning and construction of the United States' share of navigation works in the seaway, in the area from Lake Ontario to near Cornwall, Ontario. The corporation also cooperated with Canada's St. Lawrence Seaway Authority in the control and operation of the seaway after it was completed in 1959.
Canada assumed full responsibility for building navigation facilities between Cornwall and Montreal, and for improving the Welland Canal. Power development on the seaway was carried out by agencies of the Ontario and New York governments. Much of the approximately $500,000,000 cost of the entire project was spent for canals, locks, and hydroelectric plants in the international section.