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How did the St. Lawrence Seaway connect the Great Lakes to the Atlantic Ocean?

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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 — St. Lawrence Seaway The St. Lawrence hydrographic system, with the five main sectors of the St. Lawrence Seaway project.

The St. Lawrence Seaway connects the Great Lakes to the Atlantic Ocean by creating a deep-draft navigable waterway through the St. Lawrence River and the Great Lakes system. Completed in 1959, this joint project between Canada and the United States involved several critical engineering operations:123

River Navigation: To bypass rapids and overcome a 69-metre drop between Montreal and Lake Ontario, engineers constructed a series of canals, dams, and locks to create a 27-foot-deep channel.3
Welland Canal: This canal connects Lake Erie and Lake Ontario, utilizing lift locks to bypass the 99-metre plunge of the Niagara River and Niagara Falls.3
System Improvements: The project included deepening connecting channels such as the St. Marys River, the Straits of Mackinac, and the Detroit River to ensure consistent depth for ocean-going vessels throughout the entire system.3

These modifications allow ships to travel approximately 3,766 km from Duluth, Minnesota, to the Atlantic Ocean.12