Connecting Lakes Superior, Huron, and Michigan, the St. Marys River is a key hydrological, ecological, and transportation link in the Great Lakes system. Formed less than 10,000 years ago at the end of the most recent Ice Age, the 125 km river has long been a vital artery to the heart of the continent. The St. Marys River valley is the cultural heartland of the Ojibwe, and for almost 5,000 years Aboriginal peoples gathered there to fish and trade. It later served explorers, missionaries, and fur traders as a gateway to the west. In the late 18th century, the British military recognized its strategic value for the defence of Canada and built Fort St. Joseph. In the 19th century, locks bypassing the rapids linked the commerce and industry of eastern North America to western natural resources. Still an important commercial route, the river is now also a major destination for sport fishers and boaters.