Sault Ste. Marie is a city on the St. Marys River in Ontario, Canada, close to the U.S.-Canada border. It is the seat of the Algoma District and the third largest city in Northern Ontario, after Sudbury and Thunder Bay. To the south, across the river, is the United States and the city of Sault Ste. Marie, Michigan. These two communities were one city until a new treaty after the War of 1812 established the border between Canada and the United States in this area at the St. Mary's River. In the 21st century, the two cities are joined by the International Bridge, which connects Interstate 75 on the Michigan side, and Huron Street (and former Ontario Secondary Highway 550B) on the Ontario side. Shipping traffic in the Great Lakes system bypasses the Saint Mary's Rapids via the American Soo Locks, the world's busiest canal in terms of tonnage that passes through it, while smaller recreational and tour boats use the Canadian Sault Ste. Marie Canal.