Color cover of a 1930 Sault Sainte Marie, Michigan souvenir postcard booklet, showing a passenger boat passing through the Poe Lock of the Soo Locks. The Soo Locks, located on St. Marys River between the United States and Canada, provide ships safe passage around the rapids in the canal. The first locks in this canal were built in 1855, and have been improved upon ever since. The locks are the busiest, based on tonnage passing through, in the world. The Poe Lock, built originally in 1896, is the only one large enough to pass lake freighters.