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Why is grain the largest commodity transported on the St. Lawrence Seaway?

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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.

Grain is the largest commodity transported on the St. Lawrence Seaway because it serves as a vital, cost-effective export route for agricultural products from Canada’s prairies and the American Midwest to world markets.1

The seaway facilitates this trade by allowing specialized vessels known as lakers—designed to the maximum dimensions of the system's locks—to transport grain from the western Great Lakes toward the Atlantic Ocean. This shipping method provides significant economic savings for producers. Furthermore, the seaway’s design enables efficient two-way trade; after delivering grain, these vessels can return inland carrying other essential goods, such as iron ore.1