Color postcard from 1920 of a passenger steamer in the Weitzel Lock of the Soo Locks, Sault Sainte Marie, Michigan.The postcard is part of the "Souvenir of the Tourist Playground of Cloverland, Soo, Mich." souvenir postcard booklet. 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 Weitzel Lock, built in 1881, was replaced by the MacArthur Lock in 1942.