Why build a canal
Moving heavy goods by road was slow and costly for most of history. A horse on a rough track could haul only a modest load, but the same horse walking along a towpath could draw a barge carrying many times as much. That difference is the basic reason people dug canals: water carries weight cheaply, and an artificial channel lets that advantage reach places natural rivers do not.
Canals have several purposes. Some irrigate fields, some drain land, and some carry boats. This article concerns navigation canals, and especially the lock, which solved the hardest problem: getting boats up and down slopes.
Early waterways
Artificial channels are very old. Irrigation canals were central to Mesopotamian and Egyptian farming, and rulers in several regions dug channels linking rivers. Ancient accounts describe a canal connecting the Nile to the Red Sea, but how it was built, used and maintained at different times is uncertain, so those accounts are best treated with caution. The Romans were famous for aqueduct engineering, which carried water to cities, and they also used canals and improved rivers for transport, though water supply and navigation are different problems.
China is the standout example. Parts of what became the Grand Canal date back to early centuries, and the major unification of the north-south route is usually attributed to the Sui dynasty around the early seventh century CE. It linked the Yellow River and Yangtze regions and moved grain to the capital, and later dynasties rebuilt and extended it.
The problem of changing levels
A canal crossing flat ground is simple. A canal that must climb is not, because water runs downhill and will not stay in a sloping channel. Early solutions included the flash lock, a single gate or weir that was opened to let a rush of water carry boats through. It worked, but it wasted water and could be dangerous and slow. Another approach was the slipway or double inclined plane, in which boats were hauled over a ridge on a ramp.
The pound lock
The pound lock is a chamber with gates at each end. A boat enters, the gates close behind it, and water is let in or out through small openings called paddles until the water level in the chamber matches the level on the other side. Then the far gate opens and the boat continues. The section of canal between two locks, which stays at one level, is the pound.
The chief benefit is that the system uses only a modest amount of water for each passage and can lift a boat safely a few feet at a time. Several locks in a row, called a staircase, can raise boats up a hill.
Who invented it is not settled. Chinese engineers are often credited with early pound locks in the Song dynasty, and a figure named Qiao Weiyue is commonly mentioned in connection with a lock built in the late tenth century. In Europe, pound locks appear in the Low Countries and Italy in the later medieval period, with the mitre gate, in which two angled leaves meet in a V pointing upstream so water pressure pushes them tightly shut, often associated with Italian engineers of the fifteenth century, sometimes including Leonardo da Vinci. These attributions are debated, and independent invention is entirely plausible.
Canals in early modern Europe
In seventeenth-century France, the Canal du Midi connected the Atlantic side of the country to the Mediterranean, passing through a high summit that had to be fed with water collected from reservoirs. It was completed in 1681 and is usually credited to Pierre-Paul Riquet, who financed and supervised much of the project. It showed that long canals with many locks could be built and maintained.
Supplying water to the highest point of a canal, called the summit level, is a recurring challenge. Engineers built feeder channels and reservoirs, and later used steam engines to pump water back up when natural supplies were short.
The canal age in Britain and America
Britain’s canal boom began in the eighteenth century. The Bridgewater Canal, opened in 1761 to carry coal to Manchester, is commonly cited as a turning point, because it cut the price of coal sharply and encouraged many other projects. Over the following decades, canals formed a network linking mines, ports and factory towns, which supported early industry.
In the United States, the Erie Canal, which opened in 1825 between the Hudson River and Lake Erie, cut travel time and cost between the Atlantic seaboard and the interior. It helped New York City grow as a trading center. Its construction also relied on many workers, including many immigrants, and on practical learning during the work rather than on formal engineering schools.
Competition and later uses
Canals soon faced competition. Railways were faster and could be built over a wider range of terrain, and the arrival of steam locomotives and expanding railway networks drew freight away from many canals from the mid-nineteenth century onward. Some canals declined or closed, while others adapted.
Large ship canals are a separate chapter. The Suez Canal (1869) is a sea-level channel needing no locks, while the Panama Canal (opened 1914) uses large locks to lift ships over the isthmus. Today, many historic canals are used for leisure boating and heritage, while working canals and rivers still carry bulk freight in parts of the world. The pound lock remains the basic device for moving boats between water levels.