McCormick Bridgehouse and Chicago River Museum
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How Do You Make a River Run Backwards?

All water flows downhill. To reverse a river, you need to change the shape of the hill. The Chicago River used to flow into Lake Michigan because the lake was the region’s lowest point.

But the Chicago Sanitary and Ship Canal changed that. The canal becomes progressively deeper so by the point where it meets the Des Plaines River it is lower than Lake Michigan. Gravity allows the water to flow without the aid of pumps toward the Mississippi River and eventually the Gulf of Mexico.

The locks separating the Chicago River from Lake Michigan did not reverse the river. Instead, they prevent too much lake water from entering the river.