The U.S. and Canadian networks of inland waterways are based on the great navigable rivers of the continent linked by several major canals. Additionally, to reduce the hazards of navigating the Atlantic Seaboard and to shorten distances, intracoastal waterways (protected routes paralleling the coast) have been developed. The total inland U.S. system, including protected coastal routes, approximates 40,230 km (25,000 miles), of which well over half has a minimum depth of 2.7 metres (9 feet). The largest system is based on the Mississippi, which is navigable for about 2,897 km (1,800 miles) from New Orleans to Minneapolis, and its vast system of tributaries. This system connects with the St. Lawrence Seaway via Lake Michigan, the Chicago Sanitary and Ship Canal, and the Illinois River and with the Atlantic coast via the New York State Canal System (Erie Canal) and the Hudson River.

The two intracoastal waterways are the Atlantic and the Gulf, the former extending from Boston, Massachusetts, to Key West, Florida, with many sections in tidal water or open sea. The Gulf Intracoastal Waterway comprises large sheltered channels running along the coast and intersected by many rivers giving access to ports a short distance inland. New Orleans is reached by the Tidewater Ship Canal, a more direct and safer waterway than the Mississippi delta. The Pacific coast’s canals are not linked with the national network, but two major projects of importance are the Sacramento Deepwater Ship Canal and the Columbia River developments, the latter providing more than 805 km (500 miles) of navigable river from the Pacific to Lewiston, Idaho.

The opening of the St. Lawrence Seaway in 1959 saw the fulfillment of a project that had been envisaged from the times of the earliest settlements in Canada. A continuous navigable deep waterway from the Atlantic to the Great Lakes was the obvious route for opening up the interior of North America, but natural obstacles, such as the Lachine Rapids north of Montreal, had prevented its realization. The completion of such a waterway required agreement between the United States and Canada, which was difficult to achieve. In 1912 the Canadian government decided to improve the Welland Canal to provide an 8.2-metre (27-foot) depth with locks 244 metres (800 feet) long and 24 metres (80 feet) wide, but because of World War I it was not completed until 1932. Although a joint project to include hydroelectric power development on the International Rapids section had been provisionally agreed upon, final agreement between Canada and the United States was not reached until the early 1950s. The Canadian government undertook to raise the standard of the waterway to an 8.2-metre (27-foot) navigation depth between Montreal and Lake Erie, and the United States agreed to carry out other works, including the bypassing by canal and locks of the Barnhart Island–Cornwall generating dam at the foot of the Long Sault Rapids. This agreement enabled work on the seaway to begin in 1954. The resultant deep waterway, navigable by oceangoing ships, extends about 3,701 km (2,300 miles) from the Atlantic Ocean to the head of the Great Lakes in the heart of North America.

After Montreal Harbour the first lock is the St. Lambert, which rises 4.6 metres (15 feet) to the Laprairie Basin and proceeds 14 km (8.5 miles) to the second, Côte Sainte Catherine Lock, which rises 9 metres (30 feet) to Lake St. Louis and bypasses the Lachine Rapids. Thereafter the channel runs to the lower Beauharnois Lock, which rises 12.5 metres (41 feet) to the level of Lake St. Francis via a 21-km (13-mile) canal. The seaway then crosses the international boundary to the Bertrand H. Snell Lock, with its lift of 13.7 metres (45 feet) to the Wiley-Dondero Canal; it then lifts another 11.6 metres (38 feet) by the Dwight D. Eisenhower Lock into Lake St. Lawrence. Leaving the western end of the lake, the seaway bypasses the Iroquois Control Dam and proceeds through the Thousand Islands to Lake Ontario.

Eight locks raise the water 99 metres (326 feet) over 45 km (28 miles) from Lake Ontario to Lake Erie. The St. Marys Falls Canal, with a lift of about 6 metres (20 feet), carries the waterway to Lake Superior, where the seaway terminates.