The Northumberland Strait is a tidal water body between Prince Edward Island and the coast of eastern New Brunswick and northern Nova Scotia. The strait extends 225 km west-northwest to east-southeast from Richibucto Cape, NB, to Cape George, NS, with a width of 13-43 km. It is 68 m deep at its eastern end but less than 20 m over a large central area. Preglacial and glacial valleys eroded into red sandstone and siltstone lead from both ends into the floor of the Gulf of ST LAWRENCE. The retreat of glacial ice from the strait and surrounding area about 13 000 years ago was followed by flooding by the sea. Soon after, isostatic uplift excluded the sea from the central area, which became an isthmus joining opposite coasts. By 5000 years ago, the rising sea level had flooded this link, establishing the strait, which has been deepening slowly.

A generally shallow depth causes strong tidal currents, water turbulence and a high concentration of suspended red silt and clay, which led early French colonists to name the strait "la mer rouge." Shallowness is also largely responsible for the warmest summer water temperature in eastern Canada (July, 20°C or higher) and a consequent concentration of summer tourist activity, as well as a prolific shellfish and lobster fishery. Equable climate and extensive tillable soils form the basis for mixed agriculture and vegetable growing (particularly POTATOES) on both coasts.

The strait is crossed by a ferry from Caribou, NS, to Wood Islands, PEI. Canada's longest bridge (12.9 km), Confederation Bridge, links Prince Edward Island to New Brunswick. It opened in 1997 replacing a ferry that ran between Cape Tormentine, NB and BORDEN-CARLETON, PEI. The strait's coastal areas were settled by ACADIAN French from the early 16th century, and by English, LOYALISTS and SCOTS in the 18th century. The principal coastal cities and towns are CHARLOTTETOWN and SUMMERSIDE, PEI; PICTOU, NS; and SHEDIAC, BOUCTOUCHE and Richibucto, NB. The strait was named for HMS Northumberland, flagship of Admiral Colville.