Which Great Lake is connected to Lake Superior by the St. Marys River?

Lake Huron is connected to Lake Superior by the St. Marys River, which forms part of the boundary between Michigan and Ontario.

The river flows from Lake Superior at Sault Ste. Marie into the northern end of Lake Huron. Rapids on this route historically prevented large ships from passing directly, leading to construction of the Soo Locks. The locks opened in 1855 and allow vessels to move between the two lakes while overcoming the elevation difference.

Lake Huron and Lake Michigan are hydrologically connected through the Straits of Mackinac and have nearly the same water level. For that reason, some scientific descriptions treat them as one connected water body, even though maps and everyday usage identify two Great Lakes.

The St. Marys River is distinct from the Saint Lawrence River, which carries water east from Lake Ontario. It is also different from the St. Clair River, which continues the route from Huron toward Lake Erie.