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How did the new St. Lawrence Seaway replace older canals and locks?

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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 replaced older, shallower infrastructure by creating a continuous, deep-draft navigation channel that allowed large oceangoing vessels to reach the interior of North America.12

Before the completion of the modern seaway in 1959, navigation between Montreal and Lake Ontario was restricted by a series of six canals and 22 locks that were limited to a depth of only 4 metres (14 feet). To overcome these limitations and the navigational hazards posed by the St. Lawrence River's rapids, the Canadian and U.S. governments undertook a massive civil engineering project to raise the waterway's standard to a depth of 8.2 metres (27 feet).23

This modernization involved:
* New Construction: Building new canals and locks, such as those bypassing the Lachine and Long Sault rapids, to accommodate larger ships.2
* System Integration: Coordinating these new works with modernized locks in the Welland Canal and deepened channels throughout the Great Lakes system.2
* Infrastructure Upgrades: Raising bridges and relocating riverside communities to support the expanded, deeper waterway.2

These improvements successfully replaced the outdated, shallow-draft system with a unified, high-capacity route.24