Canals and locks are types of artificial waterways constructed so that shipping can bypass difficult waters or reduce the length of a journey. Where ships need to be moved vertically from one water level to another, locks are built to make the transfer.
Transportation canals have an extremely long history - and irrigation canals, built to carry water to farms, are probably even older. Historians and archaeologists have identified a large number of canals in operation in ancient China, India and the Mediterranean. Today, however, there are essentially two forms of canals.
The first type of canal links freshwater rivers and lakes together in order to enable larger ships to travel along inland routes, or to help all ships bypass difficult stretches of water. Such canals crisscross the St. Lawrence and Great Lakes region of North America, for instance, although most were built during the 19th century and are now used only for recreational purposes, if at all. At 460 miles in length, the Wabash and Erie Canal - built to link the Ohio River with the Great Lakes in the 1830s - is North America's longest canal. The Rideau Canal, which links nearby waterways on the Canadian side of the border, is billed as the oldest still-operating canal in North America, but its primary purposes are tourism and recreation, not commercial shipping. The longest freshwater canal of all, the Grand Canal in China, was built around 600 A.D. and stretches over 1,100 miles long.
The second type of canal, and the most important to commercial shipping today, links two oceans or seas so that ships can avoid long trips around continents or peninsulas. The two most strategically important canals of the 20th century were the Panama Canal, which allows ships to move between the Pacific and the Atlantic without having to circumnavigate South America, and the Suez Canal, which cuts across Egypt and allows ships to move from the Mediterranean to the Red Sea and the Indian Ocean without travelling around Africa. However, the same function is also served by works like the Kiel Canal in Germany and the White Sea Canal in Russia.
Regardless of the purpose of the canal, construction usually involves the creation of a level body of water. Since the ground along which they are built is not level, and natural rivers all run gradually downhill, canal engineers must at some point build into the canal system something capable of moving ships - and water - vertically. These systems are known as locks.
All locks share the same purpose of raising or lowering ships between two separate bodies of water. Ships enter a specialized chamber, which can seal at both ends and then adjusted by the lock operator. What happens next depends upon the way the lock has been designed. Some locks are actually boat lifts like the innovative Falkirk Wheel or the more traditional Peterborough Lift Lock. In these cases, the entire chamber - containing both the boat and the surrounding waters - is physically moved up and down between the upper part of the canal and the lower part of the canal.
In traditional locks, however, ships enter a chamber, and then the water level in the chamber is raised or lowered by the lock operator. A ship in the high channel of the canal may enter the canal with the water level in the chamber set at that level, for instance. In this case, the operator then evacuates water from the chamber, allowing the water level to fall until it is equal to the lower channel of the canal. The chamber is then opened on the lower end and the ship leaves the lock.
Because some canal and lock systems are so important to world shipping, they have had immense influence on both politics and even the design of boats. Large freighters and tankers, for instance, may be designated as "Panamax" or "Suezmax" ships, indicating that they have been built to the maximum dimensions allowable within the Panama Canal and the Suez Canal, respectively.