Two problems: water and level
A canal for seagoing ships has to solve two problems that a simple ditch does not. The first is depth and width. Ships draw far more water than barges, so the channel must be deeper, wider and with gentler bends. The second is level. Land rises and falls, and water will not stay in a channel that slopes. The usual solution is to build the canal in separate levels and move vessels between them with locks. Where the land is flat enough, or where two seas are close and at much the same height, a canal can be dug at sea level and needs no locks at all.
From flash locks to pound locks
Early river navigation often used flash locks, also called staunches. A gap in a weir was closed with boards or a gate, which held back the water. When a boat needed to pass, the gate was opened, and the boat was carried through on the rush of water or hauled up against the flow. The method was wasteful of water and could be dangerous.
The pound lock improved on this by enclosing a chamber between two sets of gates. A vessel entered the chamber, the gates behind it closed, and water was let in or out through sluices until the level matched the next section. Then the far gates were opened. The idea is old: in China, canal engineers are credited with early versions of this arrangement in the tenth century, and the Grand Canal system used various devices to manage level changes. In Europe, pound locks with improved gates appeared in the later medieval and Renaissance periods, in Italy and the Low Countries among other places. Leonardo da Vinci is often associated with the mitre gate, in which two leaves meet at an angle pointing upstream so that water pressure holds them shut, though historians generally suggest that similar gates were in use in Italy before his time.
The Stecknitz Canal, completed in 1398, linked Lübeck with the Elbe and was an important inland route for the merchants discussed in the history of Hanseatic Baltic trade. It carried small vessels rather than seagoing ships, but it illustrates how commercial towns used canals to connect seas and rivers.
Early modern canals
The seventeenth century saw large-scale canal building in France, where the Canal du Midi, completed in 1681, linked the Atlantic and the Mediterranean through a chain of locks and a summit level supplied by reservoirs. It was built for modest vessels rather than ocean-going ships, but its scale showed what could be done.
In Britain and the Netherlands, canals were extended through the eighteenth century, mostly for barges and coasting craft. The Dutch had a long tradition of managing water, and their canals and sea locks protected land as well as moving boats.
Canals for seagoing ships
Canals capable of taking real seagoing vessels were a nineteenth-century development, driven by larger ships and by industrial needs. Three examples show the range.
The Caledonian Canal in Scotland, designed by Thomas Telford and opened in 1822, used a series of lochs linked by cut sections and locks to cross the Great Glen from coast to coast. It was meant to allow ships to avoid the long and stormy passage around the north of Scotland, although the locks proved smaller than ideal and steam vessels later grew too large for it.
The Manchester Ship Canal, opened in 1894, turned an inland city into a port. It runs roughly 36 miles from the Mersey estuary to Manchester and uses a series of locks to lift ships to the level of the city. Its backers wanted to avoid the charges paid at Liverpool, and the canal made Manchester one of the country’s busiest ports for several decades. Its decline followed the same pattern as other older ports, because modern ships grew too big for the locks and containerisation moved trade elsewhere.
The Kiel Canal, opened in 1895 as the Kaiser Wilhelm Canal, crosses the base of the Jutland peninsula and links the North Sea with the Baltic. It saves ships a long trip around Denmark and was valuable for naval movements as well as trade. Because it connects two tidal seas at similar levels, it needed only locks at the ends to manage differences in water level.
Sea-level canals and great ocean links
The Suez Canal, opened in 1869, was dug between the Mediterranean and the Red Sea without locks because the two seas are at much the same level. It transformed routes to Asia, and its effects on steamers are described in the account of Suez Canal shipping. The Panama Canal, opened in 1914, took a very different path: the terrain required a lake and sets of locks that lift ships from the ocean to an elevated level and lower them again. The working of those locks is part of the story of Panama Canal transit, and is mostly a matter of scale, not a different principle.
Why locks mattered
The size of the locks often decided which ships could use a canal. Designers built to the vessels of their day, and when ships grew, the canals could be enlarged only at great expense. Several nineteenth-century canals became obsolete for this reason, while others, such as Panama, have been widened in recent decades. Water supply was another constraint, since each passage of a lock uses a volume of water, and canals with summit levels depended on reservoirs and feeders.
The broader significance is economic. A canal could redirect trade, give a landlocked city a seaport, or save weeks of sailing. It could also be a weapon in competition between ports and between states. Behind the grand projects lay a long history of pragmatic engineering: gates, sluices, embankments and the careful management of water.