The Gulf of Saint Lawrence (French: golfe du Saint-Laurent), the world's largest estuary, is the outlet of North America's Great Lakes via the Saint Lawrence River into the Atlantic Ocean. It is a semi–enclosed sea, covering an area of about 236 000 km2 and containing 35000 km3 of water (including the St. Lawrence estuary). It opens to the Atlantic Ocean through the Cabot Strait (104 km wide and 480 m at its deepest) and the Strait of Belle Isle (17 km wide and 60 m at its deepest).

The gulf is bounded on the north by the Labrador Peninsula, to the east by Newfoundland, to the south by the Nova Scotia peninsula and Cape Breton Island, and to the west by the Gaspé and New Brunswick. It contains Anticosti Island, Prince Edward Island, and the Magdalen Islands.

Besides the Saint Lawrence River itself, semi-major tributaries of the Gulf of Saint Lawrence include the Miramichi River, the Natashquan River, the Restigouche River, the Margaree River, and the Humber River. Arms of the Gulf include the Chaleur Bay, Miramichi Bay, St. George's Bay, Bay of Islands, and Northumberland Strait.

Cultural importance

The gulf has provided a historically important marine fishery for various First Nations that have lived on its shores for millennia and used its waters for transportation.

The first documented voyage by a European in its waters was by French explorer Jacques Cartier in 1534; the Cartier expedition is reported to have been the first known encounter between Europeans and First Nations inhabiting the Gulf of St. Lawrence basin, which occurred in present-day New Brunswick on July 7, 1534.

Outlets

The gulf flows into the Atlantic through the Strait of Belle Isle between Labrador and Newfoundland, the Cabot Strait between Newfoundland and Cape Breton Island, and the Strait of Canso between Cape Breton Island and peninsular Nova Scotia. It should be noted that since construction of the Canso Causeway in 1955, the Strait of Canso does not permit free-flowing exchange of waters between the gulf and the Atlantic.

Protected areas

St. Paul Island, Nova Scotia, off the northeast tip of Cape Breton Island, is referred to as the "Graveyard of the Gulf" for its many shipwrecks; access to the island is controlled by the Canadian Coast Guard.
Bonaventure Island on the eastern tip of the Gaspé Peninsula, Île Brion and Rochers-aux-Oiseaux (Bird Rock) northeast of the Magdalen Islands are important migratory bird sanctuaries administered by the Canadian Wildlife Service.

The Government of Canada maintains national parks in the Gulf of St. Lawrence estuary at Forillon on the eastern tip of the Gaspé, Prince Edward Island on the north shore of the island, Kouchibouguac on the northeast coast of New Brunswick, Cape Breton Highlands on the northern tip of Cape Breton Island, Gros Morne on Newfoundland's west coast, and a national park reserve in the Mingan Archipelago on Quebec's Côte-Nord.

The five provinces bordering the gulf also maintain various provincial parks, some of which preserve coastal features.

Undersea features

The Laurentian Channel is a feature of the gulf floor that was formed during previous glaciations, where the continental shelf was eroded by the St. Lawrence River during periods of global sea level minimums. Over the last century, the bottom waters of the end of the channel (i.e. in the St. Lawrence estuary) have become hypoxic.

The large extension of the continental shelf southeast of Newfoundland is known as the Grand Banks, and is the focus of fishing and oil exploration. Portions of the Grand Banks lie outside Canada's Exclusive Economic Zone. The easternmost portion of the shelf is known as the Flemish Cap, and it lies completely in international fishing waters.
Submarine canyons and fans can be found off the Scotian shelf.