Canal locks are devices on canals that allow changes in water-level such that a water-way can negotiate a hill without recourse to a lengthy detour. In more extreme circumstances, a tunnel, aqueduct, or elevator may be employed.

A canal lock traditionally consists of two pairs of oak or elm gates placed one after the other along a navigable channel of water. The system operates in much the same way as an air lock but acts between two levels of water as opposed to two levels of air pressure.

The following diagram gives a plan and side view of an empty (generic) canal lock:

A boat wishing to follow the canal down-hill, approaches the lock on a higher level of water to that which it will leave on. The water level within the lock - that is to say between the two pairs of gates - will be at one of these two levels. Presuming the lock is empty (- at the same level as the lower stretch of canal), the lock will need to be filled before the boat can enter it. This is generally achieved by opening a pair of ground paddles: sluices built into the canal bank, which when opened allow water to pass through culverts and into the lock. Some locks will not have ground paddles and, in such cases, paddles on the top gates are used.

Once the lock is full, the top gates are opened, and the boat enters the lock. The gates and paddles are then closed again, and the paddles on the bottom gates are opened, letting the water drain from the lock to the lower water-level. The bottom gates are then opened, and the boat continues on its way.

The two halves of a lock gate meet at a chevron which points against the flow of the water. This prevents the lock gates from bursting open from the differences in pressure between an empty lock and the full force of canal above it. It also has the effect of sealing the lock whenever the water levels at each side of the gate are different.

Some locks are manned by lock keepers and many locks are now self-operating, but most have to be worked by hand, by pushing against an overhanging part of the gate called the balance beam. Paddles are operated using a detachable handle device called a windlass, though many paddles now have their handles ready-attached. On some very large canals such as ship canals the locks gates and machinery are too large to be hand operated, and are operated by large scale hydraulic or electrical equipment.

Operating locks can be tricky, and hard work. Gongoozlers are people who take entertainment from observing the fruitless endeavours of hapless narrow-boat crews struggling with locks.

Also when a very steep gradient has to be climbed, what is known as a lock staircase is often used. A lock staircase is a group of locks which connect directly into each other without any intermediate pound. Many bargees consider lock staircases the stuff of nightmares. One example of a lock staircase is at Foxton Locks in England.

The worlds largest canal lock is the "Berendrecht lock" and can be found in Antwerp, Belgium. The lock is 500 metres (1,640 feet) long, and 68 metres (223 feet) wide, and has four sliding lock gates.

History and Development

In ancient times river transport was common, but rivers were often too shallow to carry anything but the smallest boats. Ancient people discovered that rivers could be made to carry larger boats by making dams to raise the water level. The water behind the dam deepened until it spilled over the top creating a weir. The water was then deep enough to carry larger boats. This dam building was repeated along the river, until there were "steps" of deep water.

This however created the problem of how to get the boats between these "steps" of water. An early and crude way of doing this was by means of a Flash lock a flash lock consisted essentialy of a small gap in the dam holding up the water, which could be quickly opened and closed.

When the gap was opened, a torrent of water would spill out, and the boat would be hauled through the opening against the water current with ropes, and when the boat was through, the opening would be quickly closed again.

This system was used extensively in Ancient China and in many other parts of the world. But this method was however highly dangerous, and risky, and many boats were deluged by the torrent of water.

The type of lock seen today is known as a Pound lock as described earlier in this article, which work by raising or lowering the water level within a double gated pound. It is believed that this type of lock originated in Medieval Europe.