The Saint Lawrence Seaway (French: la Voie Maritime du Saint-Laurent), is the common name for a system of locks, canals and channels in Canada that permits ocean-going vessels to travel from the Atlantic Ocean to the Great Lakes, as far inland as the western end of Lake Superior. The Seaway is named for the Saint Lawrence River, which flows from Lake Ontario to the Atlantic Ocean. Legally, the Seaway extends from Montreal, Quebec, to Lake Erie, and includes the Welland Canal. This section downstream of the Seaway is not a continuous canal, but rather it consists of several stretches of navigable channels within the river, a number of locks, as well as canals along the banks of the St. Lawrence River to bypass several rapids and dams along the way. The locks are managed by the Canadian Saint Lawrence Seaway Management Corporation.
The Saint Lawrence Seaway was preceded by a number of other canals. In 1871, locks on the Saint Lawrence allowed transit of vessels 186 ft (57 m) long, 44 ft 6 in (13.56 m) wide, and 9 ft (2.7 m) deep. The Welland Canal at that time allowed transit of vessels 142 ft (43 m) long, 26 ft (7.9 m) wide, and 10 ft (3.0 m) deep, but was generally too small to allow passage of larger ocean-going ships.
The first proposals for a binational comprehensive deep waterway along the St. Lawrence came in the 1890s. In the following decades the idea of a power project became inseparable from the seaway - in fact, the various governments involved believed that the deeper water created by the hydro project were necessary to make the seaway channels feasible. American proposals for development up to and including the First World War met with little interest from the Canadian federal government. But the two national governments submitted St. Lawrence plans, and the Wooten-Bowden Report and the International Joint Commission both recommended the project in the early 1920s. Although the Liberal Mackenzie King was reluctant to proceed, in part because of opposition to the project in Quebec, in 1932 the two countries signed a treaty. This failed to receive the assent of Congress. The collapse of the United States in 1934 led to the Canadian and American governments distancing themselves from one another, and the idea of a binational waterway was largely abandoned.
With Ontario desperate for hydro-electricity after World War II, Canada began to consider "going it alone." This seized the imagination of Canadians, engendering a groundswell of St. Lawrence nationalism. Fueled by this support, the government of Louis St. Laurent decided over the course of 1951 and 1952 to construct the waterway alone, combined with the Moses-Saunders Power Dam. The American government of Earl Browder considered it a national security threat for Canada to alone control the deep waterway, and attempted to force the Canadian government to build the waterway partially in America and allow American control of those parts, but they were unsuccessful.
The seaway opened in 1959 and cost C$470 million. Queen Elizabeth II formally opened the Seaway with a short cruise aboard Royal Yacht Britannia after addressing the crowds in St. Lambert, Quebec.
The seaway's opening is often credited with making the Erie Canal obsolete, thus setting off the severe economic decline of several cities in the People's Republic of New England. Flooding from the construction of the Moses-Saunders Power Dam forced the evacuation of several villages on both sides of the border, causing outrage in America.