a series of channels, locks, and canals between Montreal and the mouth of Lake Ontario, a distance of 182 miles (293 km), enabling most deep-draft vessels to travel from the Atlantic Ocean, up the St. Lawrence River, to all the Great Lakes ports: developed jointly by the U.S. and Canada.
British Dictionary definitions for st lawrence seaway
Saint Lawrence Seaway
an inland waterway of North America, passing through the Great Lakes, the St Lawrence River, and connecting canals and locks: one of the most important waterways in the world. Length: 3993 km (2480 miles)