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Why does winter shorten the St. Lawrence Seaway shipping season?

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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 shipping season is restricted to approximately eight and a half months primarily due to the region's severe winter climate. During the winter months, the formation of ice across the waterway creates significant navigational hazards that prevent standard operations.123

While icebreakers are sometimes used to maintain channels in specific areas, such as near Montreal, the overall environmental conditions—including the freezing of the river and the presence of ice—necessitate the closure of the seaway to ensure safe transit for vessels. Additionally, in northern passages like the Strait of Belle Isle, the cold Labrador Current further contributes to an extended period of ice cover, which limits the shipping season in that area to between June and late November.1234