The St. Marys River is a vital industrial artery connecting Lake Superior to Lake Huron.

Countries & Cities · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Flowing 74.5 miles along the international border between Michigan and Ontario, the St. Marys River is far more than a waterway. It is a highly engineered system of locks, canals, and hydroelectric plants that has served as a critical hub for North American trade and defense for over a century.

The river serves as the natural outlet for Lake Superior, dropping 23 feet as it flows into Lake Huron. Because of the treacherous rapids at its head, the river has been a focal point for human intervention since the Ojibwe people first maintained a portage there. European exploration began in 1621 with Étienne Brûlé, and by 1641, Jesuit priests had named the site Sault Ste. Marie, referencing the way the water jumps over the rapids.

Industrial transformation accelerated in the 19th and 20th centuries. The American Soo Locks, completed in 1855, allowed massive freight ships to bypass the rapids, eventually becoming a cornerstone of the Great Lakes Waterway. During World War II, the waterway was so essential to the American war effort—carrying 90 percent of the nation's domestic iron ore in 1939—that it was heavily fortified against potential German air attacks, with thousands of troops and radar stations deployed to protect the locks.

Today, the river is a marvel of water management. The flow is strictly controlled by the International Joint Commission through compensating works consisting of 16 sluice gates. These gates divert over 93 percent of the water to power three hydroelectric plants, including the massive Edison Sault Electric plant, which stretches 1,340 feet. To balance this industrial demand with ecological needs, a concrete berm was constructed in 1985 to ensure water continues to flow over fish spawning habitats, even when other gates are closed to maximize power generation.

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