What happens when Lake Superior has too much water? It dumps it into an already overflowing Lake Michigan

For nearly a century, a dam at the head of the St. Marys River near Sault Ste. Marie, Mich., has been used like a faucet, controlling the amount of water flowing from Lake Superior into lakes Michigan and Huron.

In the past five years, following a swift rise in lake levels, the relatively obscure...