Chicago's absolute location has not changed, but its relative location changed markedly in 1959 because the St. Lawrence Seaway was opened.
The St. Lawrence Seaway was a collaborative project between Canada and the United States that opened in 1959 and is a continuous navigable deep waterway that runs from the Atlantic Ocean to the Great Lakes. Deep-draft ocean vessels now have access to the industrial and agricultural heartland of North America, thanks to the St. Lawrence Seaway.
The Great Lakes, the St. Lawrence River, and the Atlantic Ocean are all connected by the St. Lawrence Seaway. The St. Lawrence Seaway is crucial for trade between the US and Canada.
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