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How did the St. Lawrence Seaway deepen the route between Montreal and Lake Erie?

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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 deepened the route between Montreal and Lake Erie by replacing older, shallower infrastructure with a modernized system that achieved a uniform navigation depth of 8.2 metres (27 feet). Previously, the route relied on a series of canals and locks limited to a depth of only 4 metres (14 feet).12

To accomplish this, the project involved several major engineering operations:

St. Lawrence River: Between Montreal and Lake Ontario, the Canadian and U.S. governments constructed new canals and locks, built dams, and cleared shoals to overcome navigational hazards like the Lachine and International Rapids.1
Welland Canal: This canal, which connects Lake Ontario to Lake Erie, was modernized with lift locks of seaway dimensions to overcome the 99-metre plunge of the Niagara River and Falls.1
Connecting Channels: The U.S. Army Corps of Engineers deepened various connecting waterways, including the Detroit River and the St. Clair River, to ensure the entire system could accommodate deep-draft ocean vessels.1