Canals

Canals are chanels or ditches filled with water. Sometimes they are natural channels, but most often they have been made by people. The earliest canal were probably dug to bring water from rivers or lakes to irrigate dry land. Other canals were used to drain water away from swampy cities and towns.
Early in history, people found another important use for canals tranportation. A canal can be used to join two cities. It can give in inland waterway in a country that has few rivers.
Natural bodies of water, such as rivers, lakes, or seas, can be connected by canals to provide shorter os safer water routes. Canals are also sometimes built in rivers that are too swift or too shallow for boats to use.
There are two main types of canals that are used for transportation: ship canals and barge canals. Ship canals are use by large seagoing vessels. Barge canals are used by cargo carrying barges and small ship.
Canals may be built all on one level, or can be opened and closed to let water in or out and let boat through.
A ship going a lower to a higher level in a canal first sails into a lock through its open lower gates. The gates are shut nehind it. Water is allowed to flow into the lock through small opening until the level of water in the lock is as high as the water in the upper level of the canal. Then the upper gates are opened, and the ship proceeds on its way. If ships must be lifted a great distance, more than one locks is used. Sometimes a whole series of locks is used, one after the other, like steps an a staircase.
If a ship is descending to a lower level, the process in simply reversed. The lock is filled with water and the upper gate is opened. The ship enters the lock, and the water is let out through small openings in the lower gate. The lower gate is opened, and the ship sails on.
Except is very small lock, the lock is always filled or emptied through small openings in the gates or through pipes in the bottom of the lock. If the gates themselves were opened to let water in or out, the water would rush in or out with terrific force. The ship might be dashed against the side of the lock and damaged. The banks of the canal might also be washed out by the rush of water.