200px|thumb|right|Canso Causeway from [[Cape Breton Island]]
200px|thumb|right|The [[Canso Canal Bridge crosses the Canso Canal at the eastern end of the causeway. The navigable canal is considered the official dividing line between Cape Breton Island and mainland Nova Scotia.]]
The Canso Causeway () is a rock-fill causeway crossing the Strait of Canso, which provides access by road between Cape Breton Island and the Nova Scotia peninsula.
Its crest thickness is , carrying the two vehicle traffic lanes of the Trans-Canada Highway, Nova Scotia Highway 104 on the mainland side, and Nova Scotia Highway 105 on the Cape Breton side, as well as the single track mainline of the Cape Breton and Central Nova Scotia Railway.
The causeway opened in 1955 after three years of construction. Constructed in an "S" shape, it has a base width of in waters having a maximum depth of .
Cape Breton Island remains circumnavigable as a result of the wide and long Canso Canal, which is located at the eastern end of the causeway to allow ship traffic to transit the Strait of Canso. The Canso Canal Bridge is a swing bridge which carries the road and railway line across the canal.
The word "Canso" is believed to be derived from the Mi'kmaq word kamsok, which means "opposite the lofty cliffs."
On July 2, 2014, it was announced that the Government of Canada would transfer ownership of the causeway to the Nova Scotia provincial government. The federal government will still maintain the Canso Canal and the navigational locks.
Ferries
Prior to the construction of the causeway, Cape Breton Island was connected to the mainland by separate railway car and motor vehicle ferries.
thumb|left|ICR rail ferry Scotia provides service across the Strait of Canso ca. 1903In 1880, the Intercolonial Railway (ICR) was opened between Mulgrave on the Nova Scotia side, west to New Glasgow. In 1890, the ICR built a line from Point Tupper on the Cape Breton side, east to Sydney. As a result, a rail ferry service was necessary so that rail cars from Cape Breton Island could be interchanged with the mainland North America rail network. Initially a small 2-railcar barge was used, however the growth of traffic from Industrial Cape Breton soon mandated that a dedicated rail ferry service be established. This service was operated by the ICR until 1918 when the ICR was merged into Canadian National Railways (CNR). CNR operated the ferries from 1918 until the causeway opened in 1955. Vessels used included: Mulgrave (1893–1901), Scotia (1903–1955), and Scotia II (1915–1955). The span, measuring , would have been longer than the Firth of Forth Bridge and roughly similar in size to the Quebec Bridge.
Construction
From 1947 to 1951, articles and cartoons in provincial newspapers indicated that the strait's crossing was to be a bridge. Finally, project engineers agreed that because of the size, the amount of ice, the strength of the currents that filled the Strait of Canso every winter, and the depth of the water, it would be difficult to construct a bridge, and instead, a causeway option was chosen.
The Canso Causeway was built at a narrow location on the Strait of Canso, about northwest of Port Hawkesbury and Mulgrave, crossing from Cape Porcupine near Auld's Cove on the mainland side to Port Hastings on the Cape Breton side. About of rock for building the causeway was quarried from a mountain on Cape Porcupine.
Contracts were awarded from May 1952 to build approach roads and rail lines for the causeway construction, and the project was officially started at a ceremony on September 16, 1952 that was attended by federal Minister of Transport Lionel Chevrier and Premier of Nova Scotia Angus L. MacDonald.
Legacy
thumb|The Canso Causeway viewed from the air, looking northwest toward the [[Aulds Cove, Nova Scotia|Aulds Cove (mainland) side from the Port Hastings (Cape Breton Island) side.]]
When the causeway was completed, the eastern end of the Strait of Canso became ice free during the winter. Several industries were attracted to the Point Tupper area, including the Stora pulp and paper mill, a Gulf Oil refinery and a heavy water plant.
Aside from blocking sea ice, the blocking of the Strait of Canso caused significant environmental damage from the enormous changes in the tidal regime of the southern Gulf of St. Lawrence all the way to the mouth of the St. Lawrence River. The causeway also had the effect of damming the waterway for migrating ground fish stocks for decades until some of these species were able to determine how to get around Cape Breton Island into the gulf. In addition, the causeway allowed several non-native species of land animals access onto Cape Breton Island. One particularly troublesome interloper has been the bobcat, which has slowly forced the more gentle lynx out of its traditional hunting grounds and up into the Cape Breton Highlands.
From 1955 to the early 1990s, the Canso Causeway charged a toll to motorists. The toll was discontinued after the construction costs (approximately $23 million) were paid off.
In 1993, CN Rail sold its Truro-Sydney railway line which crossed the Canso Causeway to the Cape Breton and Central Nova Scotia Railway. Today, CB&CNS employees operate the swing bridge across the Canso Canal.