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How do locks raise and lower ships in 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.

A lock is a basin with gates at each end that allows ships to move between different water levels. In the St. Lawrence Seaway, these structures function by varying the water level within the chamber to match the level of the waterway ahead of the vessel.12

The process generally follows these steps:

Entering the lock: When a lock is filled to the level of the upper pound, the upstream gates are opened to allow a ship to enter.1
Adjusting the level: Once the ship is inside, the upstream gates are closed. Water is then drawn out of or poured into the chamber—using mechanically or manually operated sluices—until the water level inside the lock matches the level of the lower pound.12
Exiting the lock: After the levels are equalized, the downstream gates are opened, allowing the ship to proceed.1

This operation is essential for navigating the seaway, which lifts or lowers ships a total of 170 metres (557 feet). On average, a single locking takes about 30 minutes to complete.2