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What agreement allowed Canada and the United States to begin St. Lawrence Seaway construction in 1954?

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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 construction of the St. Lawrence Seaway in 1954 was made possible by a final agreement reached between Canada and the United States in the early 1950s, following years of unsuccessful attempts to ratify earlier treaties.12

While previous efforts—such as the Seaway Treaty of 1932 and a 1941 agreement—failed to gain the necessary support in the U.S. Senate or remained unratified, the two nations eventually reached a consensus on the project's scope. Under this agreement, Canada committed to deepening the waterway to an 8.2-metre (27-foot) navigation depth between Montreal and Lake Erie. The United States agreed to perform complementary works, including the construction of a canal and locks to bypass the Barnhart Island–Cornwall generating dam at the foot of the Long Sault Rapids. Following the U.S. Congress's approval of participation in May 1954, construction officially began that summer.123