The two Soo Canals--Sault Sainte Marie Canals--link Lake Huron and Lake Superior along the St. Mary's River. The river flows between Ontario and Michigan. The canal on the American side has four locks to accommodate the river's rapid drop. The canal on the Canadian side has one lock. Towns on either side of the canals are named Sault Sainte Marie. The canals are part of the St. Lawrence Seaway and are a major shipping route for copper and iron ore, lumber, and steel. The present canals were built between 1881 and 1919.