When it is necessary for shipping and boating traffic to travel between two bodies of water that are at different elevations, they do it by means of a system of man-made "locks". Ships move into the locks, gates are closed (locked) and water floods in or out shifting the elevation of the water. Once the water has assumed the new level, the locks are opened into the next body of water and the ship moves on.
Sometimes spelled "Sault" locks but pronounced as Soo locks, the locks share their name with two cities; one in Michigan and one in Canada: Sault Ste Marie. The Soo locks are a set of four parallel locks, running east to west that enable ships to travel between the Lower Great Lakes and Lake Superior. Located on the St. Marys River between Lake Superior and Lake Huron and sitting between the Upper Peninsula of Michigan and the province of Ontario in Canada, they bypass the river rapids which fall 21 feet. Over 10,000 ships per year cross through the Soo Locks even though they are closed due to ice from January to March. The locks are maintained by the United States Army Corps of Engineers and passage through them is free. The Sault Ste. Marie International Bridge between the United States and Canada allows for roadway travel over the River.