Childrens Illustrated Encyclopedia

151

EVERY DAY, FACTORIES and homes use

up huge amounts of water. For example,

an oil refinery uses 10 times as much

water as the gasoline it makes. Dams

help provide us with much of the water we

need by trapping water from flowing rivers.

Building a dam across a river creates a

huge lake, called a reservoir, behind the

dam. Reservoirs also provide water to

irrigate large areas of farmland. A

reservoir can store the water that

falls in rainy seasons so that there is

water during dry periods. By storing

water in this way, dams also prevent floods.

Flood barriers are dams that can stop the

sea from surging up a river and bursting

its banks. Some dams provide electricity as

well as water. They contain hydroelectric

power stations powered by water

from their reservoirs.

CONCRETE DAMS

There are two main types of concrete dam: arch dams
and gravity dams. Arch dams (either single-arch or

multiple-arch) are tall, curved shells of concrete as
little as 10 ft (3 m) thick. Because their arched shape

makes them very strong, they do not burst. Large
gravity dams are also made of concrete. Their vast

weight keeps them from giving way.

HOOVER DAM

The Hoover Dam in the United States, one of the world’s

highest concrete dams, is 726 ft (221 m) high. It is an arch dam

that spans the Colorado River, supplying water for irrigation

and electricity to California, Arizona, and Nevada. Lake Mead,

the reservoir formed by the dam, is 115 miles (185 km) long.

EMBANKMENT DAMS

Some of the biggest dams are embankment dams, made by

piling up a huge barrier of earth and rock. A core of clay or
concrete in the center keeps water from seeping through

the dam. The side is covered
with stones to protect it

from the water. The
world’s highest

embankment
dam is the Nurek

Dam in Tajikistan,
which is 984 ft

(300 m) high.

Lake Mead

Lift shaft inside dam goes

down to hydroelectric

power station.

Water from the

reservoir enters

the intake towers.

Roadway along

top of dam

Hydroelectric

power station

Overflow

water

Tunnel that

was excavated

to divert river

while the dam

was built.

Arched, concrete

dam wall

Pipes carry excess

water to the

Colorado River so

that the dam does

not break or

overflow.

Water flows down

pipes to hydroelectric

power station.

Water flows

down to

Colorado

River.

FLOOD BARRIERS

Movable dams called flood

barriers are built on rivers to

control flooding. Built in 1982,

this barrier across the Thames

River in England protects

London from flooding by North

Sea gales. Large, curved gates

rise if the river gets too high.

Dam shown with water

removed from one side.

A dam prevents fish, such as

salmon, from swimming up and

down a river. Some dams have

a fish ladder, a pipe, or pools

through which fish can swim

past the dam.

DAMS

THE EFFECTS OF DAMS

The reservoir that forms in

the valley behind a dam floods

the land, often damaging the

environment. For example,

the Aswan High Dam in Egypt

was built to control the

flooding of the Nile River,

but changing the river’s flow

has destroyed the fertility of

the surrounding land.

Hoover Dam

Find out more

Electricity

Farming

Lakes

Rivers

Water

Waterproof core