4. Read the text to get the gist of it. Answer the following question.

What is a canal?

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Canal is a waterway dug across land. Canals have served as important means of transporting goods and water for thousands of years. There are two major types of canals: navigation canals and water conveyance canals. Navigation canals link bodies of water, enabling vessels to travel between them. Water conveyance canals, which include irrigation canals and drainage canals, carry water from one place to another and are often referred to as aqueducts or irrigation structures.

Navigation canals may connect two similar bodies of water, such as two lakes, or two different waterways, such as a lake and a river. Navigation canals also link oceans with seaports that lie near, but not directly on, the oceans. In addition, many navigation canals are parts of canalized rivers—that is, rivers whose navigable stretches are connected by a series of dams and locks. These structures enable vessels to travel an entire river by avoiding waterfalls, shallow areas, and other navigation hazards. Such major inland waterways as the Mississippi and Ohio rivers in the United States and the Rhine River in Europe are canalized rivers.

Early canals were ditches 3 to 5 feet (0.9 to 1.5 meters) deep and 15 to 40 feet (4.6 to 12 meters) wide. Through the years, boats and canals have become larger. The Panama Canal, opened in 1914, is about 40 feet (12.2 meters) deep and about 500 feet (152.4 meters) wide. Its locks are about 110 feet (33.5 meters) wide.

When planning a canal, builders first decide whether a canal is needed in an area. They consider such factors as the kinds of goods to be shipped in and out of an area and what other forms of transport are available. Canals are usually used for shipping large quantities of goods when the speed of movement is not important. In such cases, canals are cheaper than such alternatives as railroads or trucks.

Engineers study the terrain to determine the course of a canal and to decide whether it will need locks. The width and depth of a canal are based on the size of the vessels it will handle. Engineers also consider how a canal will affect surrounding plant and animal life. For example, a canal might disrupt the migration of certain animals and alter their food supply. Water conveyance canals sometimes have crossover bridges to allow for animal migration. However, such bridges may be impractical for large ship navigation canals.

The construction of a canal primarily involves the digging and moving of earth and rock. Such materials as clay or crushed rock may be added to a canal to reduce leakage and prevent erosion. Some canals are lined with asphalt or concrete.

Canal locks are rectangular chambers that enable ships to move from one water level to another by varying the amount of water in the lock. Most locks are made of concrete and have watertight gates at each end. Water flows in and out of locks by gravity, and so no pumps are needed. Locks do, however, require a supply of water at the upper level of the canal. As a result, some locks have special reservoirs and auxiliary canals to ensure an adequate water supply when the normal supply runs low.

Locks are used to overcome changes of elevation along the course of a canal and variations in tide level near a seacoast. To move a vessel upstream, where the water level is higher, the water level in the lock is lowered to that of the water just downstream. The downstream gates are opened and the ship moves slowly into the lock. After the ship is secured to posts, the gates close and valves open to fill the lock with water from upstream. The flow of water is carefully controlled to prevent the ship from colliding with another vessel or from ramming the gates. As the lock fills, the ship rises the level of the water upstream. The upstream gates are then opened and the ship passes through. To move a ship downstream, the process is reversed. After a ship enters the lock, the water is drained out and the ship is lowered to the downstream level.