Source: The St. Lawrence Seaway Authority and Saint Lawrence Seaway Development Corporation.
From its opening in 1959 up to the late 1970s, tonnage on the St. Lawrence Seaway increased steadily, particularly because of the growth of North American grain exports and the prevalence of the steel industry in the American Midwest. An enduring pattern has been an imbalance in its traffic flows with downbound tonnage being more dominant, which underlines the role of the Seaway as an outlet for bulk cargoes coming from the Midwest. Since the early 1990s, tonnage started to decline as grain got carried more by rail and as the steel industry in the Midwest experienced a decline. The Seaway has not been able to capture other types of cargoes, such as containers, than those it conventionally handled. It is therefore bound to the cycles of bulk and grain trades.