What did engineers build to make a smooth passageway from the great lakes to the atlantic ocean?

Engineers built a system of locks that enabled the smooth passage of ocean-bound vessels from the Great Lakes to the Atlantic Ocean.  Combined with existing rivers and canals, this system is known as The St. Lawrence Seaway.  Opened to navigation in 1959, this seaway is considered one of the most impressive engineering accomplishments in history. The 7 locks lift boats a total of 246 feet above sea level.

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