The St. Marys Falls Canal consists of a 1.6 mile-long passage on the St. Marys River holding the so-called Soo (sometimes spelled Sault) Locks: a set of two parallel ship locks accommodating the 21-foot change of altitude between Lake Superior and Lake Huron in the Great Lakes of the United States and Canada. The river runs between twin cities, both named Sault Ste. Marie, in Ontario, Canada and Michigan, U.S.A. The Soo Locks are owned and operated by the United States Army Corps of Engineers, which provides free passage. They are closed seasonally between January and March, when icing prohibits shipping in the adjacent lakes.