The St. Marys River carries water from Lake Superior into Lake Huron.
The river forms part of the international boundary between Michigan in the United States and Ontario in Canada. It flows generally eastward for about 120 kilometres from the eastern end of Lake Superior to Lake Huron. Rapids near Sault Ste. Marie historically made navigation difficult, leading to the construction of the Soo Locks.
The St. Marys River is sometimes confused with the St. Clair River, which connects Lake Huron with Lake St. Clair farther south. The St. Marys is the first major downstream connection in the Great Lakes chain: Lake Superior drains through it before water continues through the St. Clair River, Lake St. Clair, and the Detroit River.
Hydroelectric stations, shipping channels, wetlands, and rapids all occur along the river. Its name is commonly written “St. Marys River” without an apostrophe in geographic references, although “St. Mary’s River” also appears in older usage.