Canal (waterway)

I

Introduction

Canal (waterway), 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 oceangoing 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 buildup 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 multilevel 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.

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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 about 4000 bc; the 1,900 km (1,200 mi) Grand Canal of China, connecting Beijing and Hangzhou, was constructed over a long period of time, largely in the 7th and 13th centuries AD, 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 Saint Petersburg with the Caspian Sea was built. The Göta Canal, a 386-km-long (240-mi-long) system of lakes, rivers, and canals, about 87 km (about 54 mi) of which can accommodate oceangoing vessels, connects Stockholm and Göteborg and was completed in 1832; the Ludwigs Canal, joining the Danube with the Main and Rhine rivers and totaling about 177 km (about 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 (about 7000 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 King Henry I; it joined the Trent and Witham rivers. Canal building in Great Britain and Ireland flourished in the late 18th and early 19th centuries. Two of the most notable canals of that period are the Grand Canal in Ireland (begun 1756), which extends 134 km (83 mi) east-west between Dublin and the Irish town of Shannon Harbour on the Shannon River, and the Caledonian Canal (completed 1847), a 97.3-km-long (60.5-mi-long) waterway including 37 km (23 mi) of canals, across Scotland. The Manchester Ship Canal (opened 1894) opened Manchester Port to oceangoing 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, the waterway was deepened to 8.2 m (27 ft) to permit oceangoing 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. See St. Lawrence Seaway.

The first navigation canal in the U.S. was built around the rapids of the Connecticut River at South Hadley, Massachusetts, in 1793. It had two levels connected by an incline, over which boats were transported in tanks filled with water and dragged by cables operated by waterpower. 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 7242 km (4500 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 are no longer in active service, having been superseded by railroads and by modern, enlarged waterways. These include the Mississippi River system, which is navigable for 2956 km (1837 mi) and has 30 locks and dams; the Illinois Waterway, which links Lake Michigan with the Mississippi River; the 1579-km (981-mi) Ohio River waterway system, extending from Pittsburgh, Pennsylvania, to the Mississippi River; and the New York State Canal System, a principal section of which connects Lake Erie with the Hudson River. The intracoastal waterways along the Atlantic and Gulf coasts are an important part of the inland-waterway system of the United States, which in the late 1960s totaled 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 gasoline- or diesel-powered tractors running on a towpath beside the canal. In certain areas men and draft animals are still used for haulage.