Contained within the 3rd Edition (1957) of the Atlas of Canada is a map that shows three condensed maps of waterways known to be navigable as of the late 1950s, or those which had been used for navigation in the past. However, the map does not proport to show all the waterways upon which navigation was possible. The map entitled Existing Canals shows all the canals in operation as of the late 1950s. Their evolution is shown on the inset diagram entitled Development of Canadian Canals which gives the dates of construction and operation depths of canals. Several small maps featuring the Trent Canal System, St. Peter's Canal and the Welland Canal are also included on this plate. The St. Lawrence Seaway map shows the state of the St. Lawrence River, including the old canal system, dykes, locks and dams, prior to the construction of the Seaway, which began in 1954. Areas of land that were to be flooded are indicated along with railways and highways which were to be relocated because of the flooding. Construction of the St. Lawrence Seaway concluded in 1959.