The St. Lawrence Seaway (French: la Voie Maritime du Saint-Laurent) is a system of locks, canals, and channels in Canada and the United States that permits oceangoing vessels to travel from the Atlantic Ocean to the Great Lakes of North America, as far inland as Duluth, Minnesota, at the western end of Lake Superior.

Although Canada’s Liberal Prime Minister William Lyon Mackenzie King was reluctant to proceed, in part because of opposition to the project in Quebec, in 1932 he and the U.S. representative signed a treaty of intent.

Beukema and St. Lawrence Seaway proponents were convinced a nautical link would lead to the development of the communities and economies of the Great Lakes region by permitting the passage of oceangoing ships.

By 1941, President Roosevelt and Prime Minister Mackenzie King made an executive agreement to build the joint hydro and navigation works, but this failed to receive the assent of the U.S. Congress.

Proposals for the seaway were met with resistance; the primary opposition came from interests representing harbors on the Atlantic and Gulf Coasts and internal waterways and from the railroad associations.

The need for cheap haulage of Quebec-Labrador iron ore was one of the arguments that finally swung the balance in favor of the seaway.

By 1959, Beukema was on board the U.S. Coast Guard cutter Maple for the first trip through the U.S. locks, which opened up the Great Lakes to oceangoing ships.

On April 25, 1959,[4] large, deep-draft ocean vessels began streaming to the heart of the North American continent through the seaway, a project supported by every administration from Woodrow Wilson through Eisenhower.

[6] Elizabeth II, Queen of Canada and President Eisenhower formally opened the seaway on June 26, 1959 with a short cruise aboard the royal yacht HMY Britannia after addressing crowds in Saint-Lambert, Quebec.

Lester Olsen, president of the Milwaukee Association of Commerce, said, "The magnitude and potential of the St. Lawrence Seaway and the power project stir the imagination of the world.

But by the turn of the 20th century, the Erie Canal had already been largely supplanted by the railroads, which had been constructed across New York and could carry freight more quickly and cheaply.

Under the Canada Marine Act (1998), the Canadian portions of the seaway were set up with a non-profit corporate structure; this legislation also introduced changes to federal ports.

Europe is no longer a major grain importer; large U.S. export shipments are now going to South America, Asia, and Africa.

[5] The U.S. Army Corps of Engineers conducted a study to expand the St. Lawrence Seaway, but the plan was scrapped in 2011 because of budgetary issues.

After the opening of the seaway, many vessels designed for use on the Great Lakes were built to the maximum size permissible by the locks, known informally as Seawaymax or Seaway-Max.

Channel depths and limited lock sizes mean that only 10% of current oceangoing ships, which have been built much larger than in the 1950s, can traverse the entire seaway.

[19] A notable adverse environmental effect of the operation of the seaway has been the introduction of numerous invasive species of aquatic animals into the Great Lakes Basin.

The zebra mussel has been most damaging in the Great Lakes and through its invasion of related rivers, waterways, and city water facilities.

Invasive species and artificial water level controls imposed by the seaway have had a negative impact on recreational fishing.

[20] The seaway, along with the St. Lawrence River it passes through, also provides opportunities for outdoor recreation, such as boating, camping, fishing, and scuba diving.

[21] The seaway passes through the St. Lawrence River, which provides a number of diveable shipwrecks within recreational scuba limits (shallower than 130 ft (40 m)).

The first 10 ft (3 m) of Lake Ontario is warmed and enters the St. Lawrence River, as the fast-moving water body has no thermocline circulation.

On July 12, 2010, Richelieu (owned by Canada Steamship Lines) ran aground after losing power near the Côte-Sainte-Catherine lock.

[23] Among international shippers are found: The St. Lawrence Seaway (along with ports in Quebec) is the main route for Ontario grain exports to overseas markets.

[37] Commercial vessel transit information is hosted on the Great Lakes St. Lawrence Seaway Development Corporation website.

[41] A list of organisations that serve the seaway in some fashion, such as chambers of commerce and municipal or port authorities, is available at the SLSDC website.