Lock (water navigation)

A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a chamber in a permanently fixed position in which the water level can be varied. (In a caisson lock, a boat lift, or on a canal inclined plane, it is the chamber itself (usually then called a caisson) that rises and falls.

Locks are used to make a river more easily navigable, or to allow a canal to cross land that is not level. Over time, more and larger locks have been used in canals to allow a more direct route to be taken.

In Ancient Egypt, the river-locks were probably part of the Canal of the Pharaohs. Ptolemy II is credited by some^[who?] for being the first to solve the problem of keeping the Nile free of salt water when his engineers invented the lock around 274/273 BC.

During 960–1279 CE, the natural extension of the flash lock, or staunch, was to provide an upper gate (or pair of gates) to form an intermediate "pound" which was all that need be emptied when a boat passed through. This type of lock, called a pound lock, was first used in medieval China during the Song dynasty (960–1279 CE). The Songshi or History of the Song Dynasty, volume 307, biography 66, records how Qiao Weiyue, a high-ranking tax administrator, was frustrated at the frequent losses incurred when his grain barges were wrecked on the West River near Huai'an in Jiangsu. The soldiers at one double slipway, he discovered, had plotted with bandits to wreck heavy imperial barges so that they could steal the spilled grain.

In 984 Qiao installed a pair of sluice-gates two hundred and fifty feet apart, the entire structure roofed over like a building. By siting two staunch gates so close to one another, Qiao had created a short stretch of canal, effectively a pound-lock, filled from the canal above by raising individual wooden baulks in the top gate and emptied into the canal below by lowering baulks in the top gate and raising ones in the lower.

In 1373, in medieval Europe a sort of pound lock was built at Vreeswijk, Netherlands. This pound lock serviced many ships at once in a large basin. Yet the first true European pound lock was built in 1396 at Damme near Bruges, Belgium. The Italian Bertola da Novate (c. 1410–1475) constructed 18 pound locks on the Naviglio di Bereguardo (part of the Milan canal system sponsored by Francesco Sforza) between 1452 and 1458.

All pound locks have three elements:

The principle of operating a lock is simple. For instance, if a boat travelling downstream finds the lock already full of water: