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The Northumberland Strait (French: détroit de Northumberland) is a strait in the southern part of the Gulf of Saint Lawrence in eastern Canada. The strait is formed by Prince Edward Island and the gulf's eastern, southern and western shores.
The western boundary of the strait is delineated by a line running between North Cape, Prince Edward Island and Point Escuminac, New Brunswick while the eastern boundary is delineated by a line running between East Point, Prince Edward Island and Inverness, Nova Scotia.
 Water temperature
The strait's shallow depths lend to warm water temperatures in summer months, with some areas reaching 25°C, or 77°F. Consequently the strait is reportedly home to the warmest ocean water temperatures in Canada, and some of the warmest ocean water temperatures on the Atlantic coast north of Virginia.
During the winter months between December and April, sea ice covers the entire strait and Gulf of St. Lawrence.
While the western shores of Cape Breton Island and northeastern shores of the Nova Scotia peninsula are dominated by granite, sedimentary rocks along the central and western parts of the strait, as well as the entire south shore of Prince Edward Island, consist of sandstone, lending to beautiful sandy beaches with minimal coastal development. The largest island in the strait is Pictou Island.
Major communities on the strait include the cities of Charlottetown and Summerside, Prince Edward Island as well as the towns of Souris, Prince Edward Island, Pictou, Nova Scotia, and Shediac, New Brunswick.
The Northumberland Strait is a minor shipping route, with ports such as Pugwash shipping salt, Summerside, Charlottetown, Georgetown and Souris shipping agricultural products and receiving petroleum and aggregate, and Pictou shipping forestry products and general cargo. Shipping has declined in recent decades with the decline of rail service to ports and the increased capacity of highways to larger ports outside of the Gulf of St. Lawrence, which freezes in the winter.
 Confederation Bridge
The narrowest part of the strait, the 13-kilometre (7–nautical mile) wide Abegweit Passage in the western part of the strait between Borden-Carleton, Prince Edward Island and Cape Jourimain, New Brunswick, is now spanned by the Confederation Bridge.
The strait hosts two seasonal ferry services: Northumberland Ferries Limited operates a passenger/vehicle service between Caribou, Nova Scotia and Wood Islands, Prince Edward Island, a separate passenger-only ferry service also operates from Caribou to Pictou Island. The other passenger/vehicle service is located between Souris, Prince Edward Island and Iles de la Madeleine, Quebec. Both services are located at the eastern end of the strait.
There was a ferry service between Borden-Carleton, Prince Edward Island and Cape Tormentine, New Brunswick until May 31, 1997. It was one of Canada's busiest ferry services and was operated by Marine Atlantic across Abegweit Passage. This service has been replaced by the Confederation Bridge.
 Undersea cables
Maritime Electric supplies electricity to customers in Prince Edward Island. Although the utility does have some electrical generation facilities, since the mid-1970s the majority of Prince Edward Island's electricity has been purchased from NB Power and supplied via an interprovincial electrical interconnection. This interconnection consists of two 23 km (14 mi) 138 kilovolt oil-filled submarine transmission cables installed 1975-78 under the Northumberland Strait at the western end of the Abegweit Passage between Murray Corner, New Brunswick and Fernwood, Prince Edward Island. Since the Northumberland Strait freezes in winter, the cables are designed to be protected from ice scouring and were copied from a Swedish design that was originally used to supply the island of Gotland in the Baltic Sea.
Bell Aliant has two fibre optic telecommunications cables spanning Northumberland Strait to serve Prince Edward Island. The first is a submarine cable running from Caribou, Nova Scotia to Wood Islands, Prince Edward Island. The other runs through the Confederation Bridge; this cable replaced a submarine cable that used to run from Borden-Carleton, Prince Edward Island to Cape Tormentine, New Brunswick.
 Swim crossings
Swim-related details about crossings, tides, currents, marine life, and navigation can be found at OpenWaterPedia's Northumberland Strait page.
There have been several documented unassisted swim crossings of the strait, although there is no official body to verify these claims. There are only two documented single-day double crossings (~28 km) to date:
- On July 26, 2007, Jennifer Alexander (who has Type 1 Diabetes) of Halifax, swam a double crossing in 19 hours, 17 minutes.
- On July 26, 2008, Kristin Roe, also of Halifax, swam a double crossing, recorded at 15 hours, 40 minutes for the Stephen Lewis Foundation and Farmers Helping Farmers.
 Cultural significance
The strait is famous in folklore circles for regular sightings of the Ghost Ship of Northumberland Strait, a flaming vessel which appears in the middle of the strait. The strait is also notable for the ice boats, small oared boats which made regular dangerous crossings of the strait in winter carrying mail and passengers before the era or ice breaking ferries.
- "The Canadian Press: Halifax woman completes two-way swim of Northumberland Strait". Retrieved 2008-08-13.
- "The Charlottetown Guardian: She's Doing It for Everyone But Herself". Retrieved 2008-08-13.