Canals—manmade waterways—divert water from natural sources to facilitate transportation, irrigate or drain land, and supply water. Irrigation canals were a feature of most ancient civilizations. The Nahrwan Canal, l85 mi. (300 km) long, was built between the Tigris and Euphrates rivers circa 2400 b.c., and Egypt's pharoahs linked the Mediterranean and Red seas with a canal that the Romans later restored and used for shipping. China built the initial stretch of its Grand Canal in 6l0 a.d., a waterway 600 mi. (l000 km) in length at its completion. Canal systems for transportation were not widespread in Europe until the 1600s and 1700s. The first network came into use in the Netherlands, which had adapted drainage canals to handle boats by the thirteenth and fourteenth centuries. France's Canal du Midi linked the Atlantic and Mediterranean by l68l. Cargo boats used in the canal systems were designed with flat bottoms to accommodate the shallow depths.
Making allowances for changes in land levels is a primary concern in canal construction. Lake Erie, for example, is roughly 325 ft. (99.125 m) higher than Lake Ontario. An early means of addressing such variations was to construct slipways between various levels, up or down which boats could be pulled. China relied on these as early as the fourth century a.d. A more sophisticated means of compensating for level variations is the lock--a chamber linking different water levels. Boats enter and leave the lock through a pair of gates. When both gates are closed, the water level in the chamber is raised or lowered, taking the boats to the desired level.
The forerunner of the contemporary lock was the navigation weir or flash lock--an opening in a masonry dam that was closed with a wooden gate by a sluice or other moving device. These were in use in China in 50 b.c. The first pound lock probably grew from a discovery by China's Chiao Wei-Yo, who constructed two flash-locks within 250 ft. (76.25 m) and found that the stretch of river between them functioned as an equalizer. The first such lock in the West was constructed in Belgium in l396. Leonardo da Vinci (l452-l519) is believed to have invented a pound lock featuring mitre gates that swung vertically while he was an engineer for the Duke of Milan.
Many of Western Europe's major rivers are today linked by an extensive canal system used for shipping, and the opening of the Erie Canal in l825 signalled the onset of a national water transportation network in America. Its success spurred construction of canals in cities like Philadelphia and Washington, D.C., although the proliferation of railroads dramatically curtailed America's reliance on canals for shipping and transportation. Canal construction reached its technological peak when canals began accommodating oceangoing vessels. These require depths of 30 to 40 ft. (9.l5-12.2 m). To justify the cost and effort involved, deep-water canals either must offer an alternative to lengthy shipping routes, as do the Suez (1869) and Panama (1914) canals, or permit access to inland ports. The Panama Canal, currently administered by the United States, will be turned over to Panama in December 1999, although a separate treaty guarantees international access in perpetuity. Completion of the St. Lawrence Seaway, for example, allowed ocean access to the Great Lakes ports. Modern canal systems are often intricately engineered affairs. Canada's Trent-Severn Waterway linking Lake Huron's Georgian Bay with Lake Ontario is 242 mi. (387 km) long; its main course alone features 33 mi. (53 km) of manmade channels, marine railways, and two hydraulic lift locks, one of which is the world's highest--65 ft. (20 m).
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