Canal

I

Introduction

Canal, artificial waterway constructed for purposes of irrigation, drainage, or navigation, or in connection with a hydroelectric dam. This article deals only with navigational waterways, which are generally of two kinds: ship canals, which are deep enough to accommodate ocean-going vessels, and shallower canals used mainly by barges.

II

Construction

Canal construction consists chiefly of opencut excavation with ordinary power tools and construction machinery. The sides of the cut are often faced with masonry to prevent erosion of the banks by the wash of passing vessels and the subsequent blocking of the channel by a build-up of silt. Unlike roads and railways, canals cannot be made to conform to irregularities in terrain, but must consist of one or more level stretches, or reaches. Where reaches of different levels meet, vessels are transferred from one reach to the next usually by means of locks. A lock is a walled section of the channel, closed by water gates at both ends, in which the water level can be raised or lowered by means of valves or sluiceways to match the level in the upper or lower reach, as desired; when the levels are the same, the corresponding water gate is opened to permit a vessel to enter or leave the lock.

Other devices sometimes used to raise and lower small vessels are inclines and lifts. Inclines are paved or railed ramps over which vessels are hauled from one reach to the other by means of cables. In a lift the vessel is floated into a movable tank from one reach, water gates are closed, and the tank with the floating vessel is raised or lowered to the level of the next reach.

Locks, which are used in most multi-level canals, have certain disadvantages; frequently they are uneconomic because of the expense of construction and operation. Also, when traffic is heavy, the supply of water for the highest reach is difficult to maintain; in addition to the natural current flow, a lockful of water is lost from the upper reach in each locking operation. Consequently, to avoid construction of locks, canals are sometimes carried across depressions on embankments, over rivers on aqueducts, and through mountains in tunnels.

III

History

Canals date from a period long before the Christian era and served as means of navigation and communication for the Assyrians, Egyptians, Hindus, and Chinese. The remains of a canal near Mandalī, Iraq, date from 4000 bc; the 1,600-km-long (1,000-mi-long) Grand Canal of China, connecting Tianjin City (Tientsin) and Hangzhou (Hangchow), was begun in the 6th century bc (completed ad 1327) and is still in use. The lock was invented in Europe in the late 15th century. Several important French canals were built in the 17th century, including the Brière, Orléans, and Languedoc canals. During the 18th century in Russia a great system of canals connecting St Petersburg with the Caspian Sea was built. The Göta Canal, a 158-km-long (98-mi-long) canal from Sjötorp on Lake Vänern to Mem on the Baltic Sea, built in two sections on either side of Lake Vättern between 1810 and 1832, is a key element in the 558-km-long (347-mi-long) system of coastal waterways, lakes, rivers, and canals connecting Stockholm and Göteborg, also known collectively as the Göta Canal; about 87 km (54 mi) of the Göta Canal as a whole can accommodate ocean-going vessels. The Rhine-Main-Danube Canal (formerly Ludwig’s Canal), joining the Danube with the Main and Rhine rivers and totalling about 177 km (110 mi), was built in 1832. The Suez Canal, opened in 1869, links the Mediterranean and Red seas. The Panama Canal, first used in 1914, joins the Atlantic and Pacific oceans. In Germany the opening (1938) of the Mittelland Canal system (467 km/290 mi long) completed the east-west link in a system of about 11,265 km (7,000 mi) of inland waterways, extending from the Dortmund-Ems Canal east of the Rhine to the Elbe north of Magdeburg.

The first canal in England was completed in 1134 during the reign of Henry I, joining the rivers Trent and Witham. Canal building in Great Britain and Ireland flourished in the late 18th and early 19th centuries. The most notable canals of that period are the Leeds-Liverpool Canal (constructed 1816), which is 204 km (127 mi) long; the Grand Canal in Ireland (begun 1756), which extends 134 km (83 mi) east-west between Dublin and Shannon Harbour on the River Shannon; the Caledonian Canal (completed 1847), a 97.3-km-long (60.5-mi-long) waterway across Scotland that includes 37 km (23 mi) of canals; and the Manchester Ship Canal (1894), which opened up Manchester as a port to ocean-going vessels.

The Canadian canal system includes the St Lawrence River canals, the Ottawa River canals, the Chambly Canal, the Rideau Canal, and the Trent Canal. Of these the St Lawrence system has long been the most important, because it provides a waterway 4.3 m (14 ft) deep from the head of Lake Superior to the Gulf of St Lawrence. As part of the St Lawrence Seaway project, completed in 1959, this waterway was deepened to 8.2 m (27 ft) to permit ocean-going vessels with drafts up to 7.8 m (25.5 ft) to sail from the Atlantic Ocean to such Great Lakes ports as Chicago and Duluth.

In the United States, the construction of the Erie Canal, started in 1817, marked the beginning of an era of canal building, which produced an aggregate of more than 7,242 km (4,500 mi) of canals (mostly in the Middle Atlantic and Central states) and was largely responsible for opening the American Midwest to settlement. Many of the early canals, such as the Mississippi River system, which is navigable for 2,956 km (1,837 mi) and has 30 locks and dams, are no longer in active service, having been superseded by railway lines and by modern, enlarged waterways. The intracoastal waterways along the Atlantic and Gulf coasts are an important part of the inland-waterway system of the United States, which total 40,845 km (25,380 mi).

IV

Barge Canals

On most large canals barges are pushed or pulled by tugboats and towboats; one towboat may pull as many as 40 barges lashed together. Modern barges are designed to carry specific types of cargo. Open-hopper barges carry coal, gravel, and large equipment; covered dry-cargo barges are used for grain, dry chemicals, and other commodities that must be kept dry; tank barges carry petroleum and liquid chemicals. On some European canals, barges are towed in trains of two or more by petrol- or diesel-powered tractors running on a towpath beside the canal. In certain areas men and draught animals are still used for haulage.