What is the St. Lawrence Seaway?

The St. Lawrence Seaway is a navigable waterway system between Canada and the United States that stretches from the Atlantic Ocean to the Great Lakes. It opened North America’s industrial and agricultural heartlands to deep-draft ocean vessels. The St. Lawrence Seaway extends 3,766 km (2,340 miles) from Duluth, Minnesota, to the Atlantic Ocean. Its construction, one of the largest civil engineering projects ever, began in 1954 and took nearly five years. The seaway became operational in April 1959. The seaway has had a major economic impact on the United States and Canada.

Where is the St. Lawrence Seaway located?

The St. Lawrence Seaway is a navigable waterway system in North America jointly constructed by Canada and the United States. It facilitates access between the Atlantic Ocean and the Great Lakes. The seaway extends 3,766 km (2,340 miles) from Duluth, Minnesota, on Lake Superior's westernmost point, to the Atlantic Ocean. The official seaway includes a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario, as well as the Welland Canal, which connects Lake Ontario and Lake Erie. The entire Great Lakes-St. Lawrence Seaway System has 15,300 km (9,500 miles) of navigable waterways.

Why was the St. Lawrence Seaway built?

The St. Lawrence Seaway, a joint project between Canada and the United States completed in 1959, was built to create a continuous navigable waterway for deep-draft ocean vessels, linking the Atlantic Ocean to the Great Lakes. This 3,766 km (2,340 miles) waterway opened North America’s industrial and agricultural heartlands to the world by clearing a throughway in a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario. The seaway project also aimed to tap into the hydroelectric power potential of the river. Vast deposits of iron ore discovered in Quebec and Labrador also motivated the seaway's construction, as they were needed by steel mills in the United States.

How does the St. Lawrence Seaway help ships travel between the Great Lakes and the Atlantic Ocean?

The St. Lawrence Seaway is a navigable waterway system that facilitates ship travel from the Great Lakes to the Atlantic Ocean. It stretches 3,766 km (2,340 miles) from Duluth, Minnesota, on Lake Superior's westernmost point, to the Atlantic. To create a continuous deep waterway, the St. Lawrence Seaway included construction and modernization efforts: * The governments of Canada and the U.S. built canals and locks to bypass rapids. * The Welland Canal has seven lift locks to overcome the Niagara River and Falls. * The U.S. Army Corps of Engineers deepened connecting straits and rivers. These modifications allow ships to navigate a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario, overcoming a 69-metre (226-foot) drop in elevation. The Seaway has become one of the world’s most heavily used international trade routes, allowing transport of grain, iron ore, and coal.

What types of structures, like locks and canals, are part of the Seaway?

The St. Lawrence Seaway includes structures such as locks and canals that facilitate navigation. To overcome the natural obstacles, the Canadian government constructed two canals and five locks around the Cedar, Cascades, and Lachine rapids. The U.S. government built two locks and a 16-km canal around the International Rapids. The Seaway has a series of locks, including the St. Lambert, Côte Sainte Catherine, Beauharnois, Bertrand H. Snell, and Dwight D. Eisenhower locks. Additionally, the Seaway includes the Iroquois Control Dam. These structures help raise and lower large ships a total of 170 metres (557 feet).

How does the St. Lawrence Seaway impact trade and the economy in North America?

The St. Lawrence Seaway significantly impacts trade and the economy of both the United States and Canada. It opened North America’s industrial and agricultural heartlands to deep-draft ocean vessels. The seaway facilitates the export of grain from farms in the Canadian prairies and the American Midwest at considerable savings. Additionally, the discovery of vast iron ore deposits in Quebec and Labrador led to Canada exporting ore to the United States, reversing its previous status as an importer. Coal is transported from U.S. mines to Canadian steel mills and power plants. The Great Lakes–St. Lawrence River system has become a heavily used international trade route, moving approximately 140 million metric tons of cargo annually.

St. Lawrence Seaway

References & Edit History Related Topics

Images

Map showing the St. Lawrence hydrographic system and the five main sectors of the St. Lawrence Seaway project, stretching from Lake Ontario near Kingston through various locks, canals, and dams along the Canada-United States border to Montreal. The map highlights key locations such as the Thousand Islands, International Rapids, Lake St. Francis, Soulanges, and Lachine sections, with detailed labeling of major locks, canals, and power dams. Cross sections of the lock and canal system are depicted

Map showing the St. Lawrence hydrographic system in eastern Canada and the northeastern United States, highlighting the five main sectors of the St. Lawrence Seaway project from the Great Lakes to the Atlantic Ocean. The map labels major cities, waterways, and lock and canal locations along the seaway, illustrating its route from Lake Superior and Lake Michigan through the St. Lawrence River to the Gulf of St. Lawrence.

St. Lawrence Seaway

waterway, North America

Written by

Pierre CamuAll

Fact-checked by

Britannica Editors

Last updated

Nov. 12, 2026 •History

Top Questions

What is the St. Lawrence Seaway?
The St. Lawrence Seaway is a navigable waterway system between Canada and the United States that stretches from the Atlantic Ocean to the Great Lakes. It opened North America’s industrial and agricultural heartlands to deep-draft ocean vessels.

The St. Lawrence Seaway extends 3,766 km (2,340 miles) from Duluth, Minnesota, to the Atlantic Ocean. Its construction, one of the largest civil engineering projects ever, began in 1954 and took nearly five years. The seaway became operational in April 1959. The seaway has had a major economic impact on the United States and Canada.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where is the St. Lawrence Seaway located?
The St. Lawrence Seaway is a navigable waterway system in North America jointly constructed by Canada and the United States. It facilitates access between the Atlantic Ocean and the Great Lakes.

The seaway extends 3,766 km (2,340 miles) from Duluth, Minnesota, on Lake Superior's westernmost point, to the Atlantic Ocean. The official seaway includes a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario, as well as the Welland Canal, which connects Lake Ontario and Lake Erie. The entire Great Lakes-St. Lawrence Seaway System has 15,300 km (9,500 miles) of navigable waterways.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why was the St. Lawrence Seaway built?
The St. Lawrence Seaway, a joint project between Canada and the United States completed in 1959, was built to create a continuous navigable waterway for deep-draft ocean vessels, linking the Atlantic Ocean to the Great Lakes. This 3,766 km (2,340 miles) waterway opened North America’s industrial and agricultural heartlands to the world by clearing a throughway in a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario.

The seaway project also aimed to tap into the hydroelectric power potential of the river. Vast deposits of iron ore discovered in Quebec and Labrador also motivated the seaway's construction, as they were needed by steel mills in the United States.
2 Britannica Sources
1. St. Lawrence River: The economy
2. St. Lawrence Seaway
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does the St. Lawrence Seaway help ships travel between the Great Lakes and the Atlantic Ocean?
The St. Lawrence Seaway is a navigable waterway system that facilitates ship travel from the Great Lakes to the Atlantic Ocean. It stretches 3,766 km (2,340 miles) from Duluth, Minnesota, on Lake Superior's westernmost point, to the Atlantic.

To create a continuous deep waterway, the St. Lawrence Seaway included construction and modernization efforts:
* The governments of Canada and the U.S. built canals and locks to bypass rapids.
* The Welland Canal has seven lift locks to overcome the Niagara River and Falls.
* The U.S. Army Corps of Engineers deepened connecting straits and rivers.

These modifications allow ships to navigate a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario, overcoming a 69-metre (226-foot) drop in elevation. The Seaway has become one of the world’s most heavily used international trade routes, allowing transport of grain, iron ore, and coal.
2 Britannica Sources
1. St. Lawrence River
2. St. Lawrence Seaway
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What types of structures, like locks and canals, are part of the Seaway?
The St. Lawrence Seaway includes structures such as locks and canals that facilitate navigation. To overcome the natural obstacles, the Canadian government constructed two canals and five locks around the Cedar, Cascades, and Lachine rapids. The U.S. government built two locks and a 16-km canal around the International Rapids.

The Seaway has a series of locks, including the St. Lambert, Côte Sainte Catherine, Beauharnois, Bertrand H. Snell, and Dwight D. Eisenhower locks. Additionally, the Seaway includes the Iroquois Control Dam. These structures help raise and lower large ships a total of 170 metres (557 feet).
2 Britannica Sources
1. St. Lawrence Seaway: Construction of the seaway
2. canals and inland waterways: Major inland waterways of North America
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does the St. Lawrence Seaway impact trade and the economy in North America?
The St. Lawrence Seaway significantly impacts trade and the economy of both the United States and Canada. It opened North America’s industrial and agricultural heartlands to deep-draft ocean vessels.

The seaway facilitates the export of grain from farms in the Canadian prairies and the American Midwest at considerable savings. Additionally, the discovery of vast iron ore deposits in Quebec and Labrador led to Canada exporting ore to the United States, reversing its previous status as an importer. Coal is transported from U.S. mines to Canadian steel mills and power plants. The Great Lakes–St. Lawrence River system has become a heavily used international trade route, moving approximately 140 million metric tons of cargo annually.
2 Britannica Sources
1. St. Lawrence River: The economy
2. St. Lawrence Seaway
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

St. Lawrence Seaway, continuous navigable deep waterway project from the Atlantic Ocean to the Great Lakes, undertaken jointly by Canada and the United States and completed in 1959. The St. Lawrence Seaway opened North America’s industrial and agricultural heartlands to deep-draft ocean vessels. It forged the final link in a waterway some 3,766 km (2,340 miles) long from Duluth, Minnesota (at the westernmost point of Lake Superior), to the Atlantic by clearing a throughway in a 299-km (186-mile) stretch of the St. Lawrence River between Montreal and Lake Ontario. Although the official seaway consists of only this stretch and the Welland Canal (connecting Lakes Ontario and Erie), the entire Great Lakes–St. Lawrence Seaway System, with 15,300 km (9,500 miles) of navigable waterways, has come to be known as the St. Lawrence Seaway. See also St. Lawrence River.

The economy

The seaway has had a major economic impact on the United States and Canada. A principal reason for its construction was the discovery, in Quebec and Labrador, of vast deposits of iron ore needed by steel mills in the United States. Canada, an importer of American iron ore before the seaway opened, exported ore, the second largest commodity, to the United States thereafter. The largest commodity moved is grain, from farms on Canada’s prairies and in the American Midwest, shipped through the seaway at considerable savings. Major users of the seaway are vessels known as lakers, which are designed to the maximum limits of the seaway locks in order to facilitate two-way trade. A laker can pick up grain in the western Great Lakes, destined for world markets, and return with Canadian iron ore, loaded in the lower St. Lawrence. The third largest seaway commodity is coal, moved chiefly from U.S. mines via the Welland Canal to Canadian steel mills and power plants. Another commodity that is significant—because of its value rather than the amount moved—is imported iron and steel.

Bulk commodities make up about 90 percent of annual cargo tonnage, but vessels of many nations also use the seaway to deliver or pick up general cargoes.

The Great Lakes–St. Lawrence Seaway System has become one of the world’s most heavily used international trade routes. Although the system is often characterized as a vast inland sea comparable to the Mediterranean, its use is restricted by limited access and by a severe winter climate that shortens the shipping season to about eight and a half months. In 1959 the seaway allowed passage of about 80 percent of the world’s ships, a figure that has since decreased. The size of a vessel that uses it is limited to a draft of 8 metres (26 feet), a length of 223 metres (730 feet), and a beam of 23 metres (76 feet). These dimensions have become relatively small by the standards of world cargo-ship construction.

St. Lawrence Seaway

The economy

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