Why Locks Are Essential on the St. Lawrence Seaway

Overcoming a Major Elevation Change

The St. Lawrence Seaway relies on a system of locks because of the significant difference in water levels between Lake Ontario and the Montreal area, which connects to the Atlantic Ocean. This elevation gap measures approximately 22.9 meters (75 feet), making it impossible for vessels to pass without assistance.

The Seven Locks of the Seaway

To manage this change, the Seaway incorporates seven locks in total:

• St. Lambert Lock and Upper Beauharnois Lock on the Canadian side, just upstream of Montreal.
• Eisenhower Lock, Bertrand H. Snell Lock, Dwight D. Eisenhower II Lock, Robert H. Saunders Lock, and Iroquois Lock on the U.S. side near Massena, New York.

These locks operate in pairs to gradually raise or lower vessels by the full 75-foot difference.

How the Locks Work

When ships travel from Lake Ontario toward Montreal, the locks raise them to the higher water level. Conversely, vessels heading upstream are lowered safely. Each lock chamber is large enough to accommodate ships of varying sizes and weights. Water levels inside the chamber are adjusted to match the target elevation, and massive gates open and close to allow vessels to enter and exit. The entire process is highly controlled to ensure safe, efficient passage.

Economic Significance

The St. Lawrence Seaway locks are a vital commercial artery, enabling large ships to move between the Great Lakes heartland and the Atlantic Ocean. They provide access to major ports and industrial centers, driving economic growth and prosperity across the region.