Great Lakes–St. Lawrence Seaway Shipping

Updated 2026-11-10

The Great Lakes–St. Lawrence Seaway system is an inland shipping route that lets vessels travel between the Atlantic Ocean and the head of Lake Superior. It crosses the border between Canada and the United States, passes through a series of locks and canals, and carries mostly bulk goods. The sections below cover the length and structure of the route, the main locks, the navigation season, cargo and ships, history, governance and environmental questions.

Length and layout of the system

The waterway runs about 3,700 km from the Atlantic Ocean to Duluth, Minnesota, on Lake Superior, which is 2,038 nautical miles. Lake Superior lies 183 m above sea level, so the route is a long staircase of locks.

The name covers two connected parts. The St. Lawrence Seaway proper is the section between Montreal and Lake Erie. Its lock-and-canal works include the Montreal to Lake Ontario section, which is 306 km long and has seven locks, and the Welland Canal between Lake Ontario and Lake Erie, which has eight. Together they make 15 locks, 13 Canadian and two American. Transport Canada gives the lift on the Welland Canal as 99.4 m. The elevation difference between Montreal and Lake Erie is 168 m.

Beyond the Welland Canal the system is usually called the Great Lakes Waterway. It includes the Soo Locks between Lake Huron and Lake Superior, and dredged channels in the St. Marys River, the Detroit River, Lake St. Clair and the St. Clair River. Because the upper locks are larger than those downstream, the biggest freighters stay in the lakes above the Welland Canal.

The locks

Montreal to Lake Ontario

On the St. Lawrence River the locks, from downstream to upstream, are St. Lambert, Côte Sainte-Catherine, the two Beauharnois locks, Bertrand H. Snell, Eisenhower and Iroquois. The Snell and Eisenhower locks are at Massena, New York. The others are in Quebec and Ontario. Pilotage applies to vessels in foreign trade.

Each lock in the Seaway is 233.5 m long, 24.4 m wide and 9.1 m deep over the sill. A lock fills with about 91 million litres of water in 7 to 10 minutes, and a transit takes about 45 minutes.

Welland Canal

The Welland Canal is about 43 km long and lifts or lowers ships by 99.5 m between Lake Erie and Lake Ontario. Locks 1 to 7 are the lift locks, each 766 ft (233.5 m) long and 80 ft (24.4 m) wide, and each changes the water level by 43 to 49 ft (13 to 15 m). Locks 4, 5 and 6 are a twin-flight group at Thorold. Lock 8 at Port Colborne is longer than the others, at about 350 m, and adjusts for the level of Lake Erie. An average transit takes about 11 hours.

The current canal is the fourth on the route. The first, begun in 1824, had 40 wooden locks. The second and third were smaller and were closed in 1935. The current canal opened on August 6, 1932. The Welland By-Pass, about 13.4 km long, was built from 1967 to 1972 and opened for the 1973 season.

Soo Locks

The Soo Locks at Sault Ste. Marie lie between Lake Huron and Lake Superior. The US Army Corps of Engineers owns and maintains the American locks and provides free passage. The two active locks are the MacArthur Lock, opened in 1943, and the Poe Lock, rebuilt in 1968. The Poe Lock is 1,200 ft long, 110 ft wide and 32 ft deep, and the MacArthur Lock is 800 ft long and 80 ft wide. About 10,000 ships pass the locks each year. The Davis and Sabin locks were decommissioned in 2010, and a replacement lock is under construction, with completion slated for 2030. A smaller Canadian lock opened in 1895 and was rebuilt in 1987.

Navigation season

Ice closes the system for part of each year. The Seaway's operator reports that the season typically runs from the third week of March to late December. In 2026 a delayed winter closure ended with the Montreal to Lake Ontario section closing on January 12 and the Welland Canal on January 13, and Transport Canada scheduled the reopening for March 22. The Soo Locks close from January through March, which gives time for inspection and maintenance.

Ice cover across the Great Lakes varies widely from winter to winter. Icebreakers keep lanes open during icy periods in the fall and early winter.

Ships and size limits

Vessel size is set by the locks. The largest ship that can pass through the whole Seaway is called Seawaymax. It is 740 ft (225.6 m) long, 78 ft (23.8 m) wide and has a draft of 26.5 ft (8.1 m). The Seaway's operator gives ship limits of 225.5 m by 23.8 m, and such a vessel can carry roughly 30,000 tonnes of cargo. The Welland Canal also has a maximum air draft of 116 ft (35.5 m) under the Garden City Skyway.

Three ship types use the system. Lakers are built for the Seaway and the lakes, with vertical sides and snub-nosed bows. Salties are ocean-going vessels with V-shaped hulls. The third group is tug-propelled barges. Ships larger than Seawaymax operate only inside the upper lakes. Only a small share of modern ocean-going ships fit the locks, which limits container traffic.

Channel depths vary along the route. Downstream of Quebec City the channel is 41 ft (12.5 m) deep, and upstream of Montreal it is 27 ft (8.2 m).

Cargo and traffic

The Seaway's operator says more than 200 million tonnes of cargo move on the wider waterway in a year, while Transport Canada reports about 37 million tonnes through the Seaway locks. The main cargo types are grain, dry bulk such as stone, salt and potash, iron ore, general cargo such as steel and wind turbine components, and liquid bulk such as refined petroleum products and ethanol. Annual totals for Canadian marine traffic as a whole are covered in a separate page on Canadian shipping statistics.

Major ports on the waterway include Duluth-Superior, Chicago, Detroit, Toledo, Cleveland, Hamilton and Thunder Bay. Montreal lies at the lower end of the Seaway and Port Colborne on Lake Erie.

A 2023 study by Martin Associates, cited by the Seaway's operator, put the economic activity supported by cargo on the system at US$50.9 billion (CAD 66.1 billion), with 356,858 jobs. Transport Canada's briefing gives a lower figure of CAD 46.8 billion and more than 241,000 jobs.

History

Niagara Falls and the rapids of the St. Marys River blocked water travel between the lakes for centuries. The first Welland Canal opened in 1829. The Soo Locks opened in 1855 and were taken over by the US Army in 1881.

Plans for a deep waterway on the St. Lawrence date to the 1890s. A treaty signed in 1932 failed to win the required Senate vote in 1934, and a 1941 executive agreement between President Roosevelt and Prime Minister Mackenzie King was not approved by Congress. In September 1951 Canada told the United States it would build the Seaway alone if necessary, and Parliament authorized the St. Lawrence Seaway Authority in December 1951. The Wiley-Dondero Seaway Act, signed on May 13, 1954, authorized joint construction, which began that year.

About 22,000 workers took part. Construction moved 210 million cubic yards of earth and rock and used more than 6 million cubic yards of concrete. The cost was C$470 million, of which the Canadian government paid $336.2 million. Creating Lake St. Lawrence required the acquisition of properties in six Ontario villages and three hamlets, now known as the Lost Villages. The icebreaker D'Iberville made the first transit on April 25, 1959, and Queen Elizabeth II and President Eisenhower opened the Seaway officially on June 26, 1959. Expansion proposals have been studied since, and a US Army Corps of Engineers study of expansion ended in 2011 for budgetary reasons.

Governance

The Seaway operates under a binational arrangement based on international agreements that carry the weight of treaties. The Canadian side is run by the St. Lawrence Seaway Management Corporation, a not-for-profit corporation. Management was commercialized under the Canada Marine Act in 1998, and the federal government keeps ownership of the assets. Transport Canada and the corporation signed a long-term agreement in March 2024. The American side is run by the Great Lakes St. Lawrence Seaway Development Corporation, a federal agency within the US Department of Transportation. The Soo Locks are operated separately by the US Army Corps of Engineers.

Environmental aspects

The Seaway opened the lakes to ocean-going vessels, and aquatic species from other regions arrived with them. About 160 new species have entered the Great Lakes ecosystem since the 19th century, with ballast water and hull fouling named as major causes. Zebra mussels were first found in 1988 and quagga mussels in 1989. They have affected the lakes, connected waterways and city water facilities. The round goby, the ruffe, the sea lamprey and two waterfleas are other introduced species.

Water levels are a second concern. Low levels between 2010 and 2016 caused problems for vessels, communities, industry and agriculture. Seaway construction also changed the river: flooding that formed Lake St. Lawrence began on July 1, 1958.

Spills are a further risk. On July 12, 2010, the Richelieu ran aground near the Côte-Sainte-Catherine lock and spilled about 200 tonnes of diesel. The lock and the Seaway were closed during the cleanup.

Frequently asked questions

How long is the Great Lakes–St. Lawrence Seaway system?

The waterway runs about 3,700 km from the Atlantic Ocean to Duluth, Minnesota, on Lake Superior. The Montreal to Lake Ontario section is 306 km, and the Welland Canal is about 43 km between Lake Ontario and Lake Erie.

How many locks are there?

The Seaway has 15 locks: seven between Montreal and Lake Ontario and eight on the Welland Canal. Thirteen are Canadian and two are American. The Soo Locks, which connect Lake Huron and Lake Superior, are counted separately and are American.

When is the navigation season?

The season typically runs from the third week of March to late December. The Soo Locks close from January through March because of ice.

What is Seawaymax?

Seawaymax is the largest vessel size that can transit the whole Seaway. It is 740 ft (225.6 m) long, 78 ft (23.8 m) wide, with a draft of 26.5 ft (8.1 m). Larger lake freighters stay in the upper lakes.

What cargo moves on the system?

The main cargo types are grain, dry bulk, iron ore, general cargo and liquid bulk. The operator cites more than 200 million tonnes a year on the wider waterway, and Transport Canada reports about 37 million tonnes through the Seaway locks.

Who runs the Seaway?

The St. Lawrence Seaway Management Corporation, a not-for-profit corporation, runs the Canadian side under an agreement with Transport Canada. The Great Lakes St. Lawrence Seaway Development Corporation, part of the US Department of Transportation, runs the American side. The US Army Corps of Engineers operates the Soo Locks.

Why are invasive species linked to the Seaway?

Ocean-going vessels reached the lakes through the Seaway, and species such as the zebra mussel arrived in ballast water or on hulls. About 160 new species have entered the Great Lakes ecosystem since the 19th century.