A lock or water lock is an enclosed, rectangular chamber with gates at each end, within which water is raised or lowered to allow boats or ships to overcome differences in water level. Locks have a history of over 2,000 years, and although they are most often used by boats on canals, they also are used to transport massive ships between seas.

All locks operate on the simple buoyancy principle that any vessel, no matter what size, will float atop a large enough volume of water. By raising or lowering the level of a body of water, the vessel itself goes up or down accordingly. Locks are used to connect two bodies of water that are at different ground levels as well as to "walk" a vessel up or down a river's more turbulent parts. This is done by a series of connecting or "stair-case" locks. Locks contributed significantly to the Industrial Revolution (period beginning about the middle of the eighteenth century during which humans began to use steam engines as a major source of power) by making possible the interconnection of canals and rivers, thus broadening commerce. They still play a major role in today's industrial society.


The ancestor of the modern lock is the flash lock. It originated in China and is believed to have been used as early as 50 B.C. The flash lock was a navigable gap in a masonry dam that could be opened or closed by a single wooden gate. Opening the gate very quickly would release a sudden surge of water that was supposed to assist a vessel downstream through shallow water. This was often very dangerous. Using the flash lock to go upstream was usually safe but extremely slow since the gap in the dam was used to winch or drag a vessel through.

At some future date, a second gate was added to the flash lock, thus giving birth to the pound lock. The first known example of a pound lock (whose dual gates "impound" or capture the water) was in China in A.D. 984. It consisted of two flash locks about 250 feet (76 meters) apart. By raising or lowering guillotine or up-and-down gates at each end, water was captured or released. The space between the two gates thus acted as an equalizing chamber that elevated or lowered a vessel to meet the next water level. This new method was entirely controllable and had none of the hazards and surges of the old flash lock.

Ships in the Miraflores locks on the Panama Canal. (Reproduced by permission of Photo Researchers, Inc.)
Ships in the Miraflores locks on the Panama Canal. (Reproduced by permission of
Photo Researchers, Inc.

The first pound lock in Europe was built at Vreeswijk, the Netherlands, in 1373. Like its Chinese ancestor, it also had guillotine gates. The pound lock system spread quickly throughout Europe during the next century, but was eventually replaced by an improved system that formed the basis of the modern lock system. During the fifteenth century, Italian artist and scientist Leonardo da Vinci (1452–1519) devised an improved form of pound lock whose gates formed a V-shape when closed. In 1487, Leonardo built six locks with gates of this type. These gates turned on hinges, like doors, and when closed they formed a V-shape pointing upstream—thus giving them their name of miter gates. One great advantage of miter gates was that they were self-sealing from the pressure of the upsteam water.

Construction and operation

The earliest locks were built entirely of wood, with stone and then brick becoming standard materials. The gates themselves were always wooden, with some lasting as long as 50 years. Filling or emptying these early locks was often accomplished by hand-operated sluices or floodgates built in the gates. On later and larger locks, it was found that conduits or culverts built into the lock wall itself were not only more efficient but let the water enter in a smoother, more controlled manner. Nearly all locks operate in the following manner: (1) A vessel going downstream to shallower water enters a lock with the front gate closed. (2) The rear gate is then closed and the water level in the lock is lowered by opening a valve. The vessel goes down as the water escapes. (3) When the water level inside the lock is as low as that downstream, the front gate is opened and the vessel continues on its way. To go upstream, the process is reversed, with the water level being raised inside the lock. What the operators always strive for is to fill or empty the lock in the fastest time possible with a minimum of turbulence.

In modern locks, concrete and steel have replaced wood and brick, and hydraulic power or electricity is used to open and close the gates and side sluices. Movable gates are the most important part of a lock, and they must be strong enough to withstand the water pressure arising from the often great difference in water levels. They are mostly a variation of Leonardo's miter gates, except now they usually are designed to be stored inside the lock's wall recesses.

Probably the best known locks in the world are those used in the Panama Canal—the most-used canal in the world. Completed in 1914, the Panama Canal is an interoceanic waterway 51 miles (82 kilometers) long that connects the Atlantic and Pacific Oceans through the Isthmus of Panama. It has three major sets of locks, each of which is built in tandem to allow vessels to move in either direction, like a separated, twoway street. Each lock gate has two leaves, 65 feet (20 meters) wide by 7 feet (2 meter) thick, set on hinges. The gates range in height from 47 to 82 feet (14 to 25 meters) and are powered by large motors built in the lock walls. The chambers are 1,000 feet (305 meters) long, 110 feet (34 meters) wide, and 41 feet (13 meters) deep. Most large vessels are towed through the locks. As with all locks today, they are operated from a control tower using visual signals and radio communications.

[ See also Canal ]

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