The Great Lakes St. Lawrence Seaway Development Corporation is a government office founded in 1954 as part of the Department of Transportation to oversee the maintenance, operations, and construction of the St. Lawrence Seaway within the territorial waters of the United States. 

The St. Lawrence Seaway as a whole is a two-section navigational aid, in which the Corporation oversees the transit of commercial and pleasure crafts between the United States and Canada. 

The first system is the Welland Canal introduced in 1833 with a forty-lock system providing passage through the Niagara River and the falls between Lake Erie and Lake Ontario. However, the modern Welland Canal was finished in 1932, twenty-two years before the St. Lawrence Seaway Development Corporation was established. 

The second section of the seaway came in 1959, providing transit between the Atlantic Ocean, through Montreal, Canada, and Lake Ontario, which in turn connects to Lake Erie. The second section is made up of seven additional locks nearly matching the same dimensions as the Welland Canal Locks. The seaway is over 2,300 miles long and is a vital waterway to assist in the delivery of iron ore, coal, limestone, grain, cement, and other various essential and general cargo. 

This three-foot-long rectangular flag has a distinctive design with the United States ensign of the eagle and the Canadian maple leaf separated by a trident signifying the service between both countries.