Beavers build them from sticks. Landslides create
them from trees, mud, and debris. Humans make them from earth and concrete.
These structures are dams. Dams hamper the flow of water in a river or stream.
Landslides don’t mean to create dams. They do so by
dumping a lot of earth and other stuff in a river. Scientists think beavers
build dams for protection. Beaver dams capture water in front of the lodges in
which beavers live. Beavers can hide from their enemies in this deeper water. A
dam also protects the beaver lodge by slowing the river’s speed.
WHY DO WE BUILD DAMS?
We build dams to control water. A dam built
across a river or stream stops the water’s flow. Water collects in a lake behind
the dam. The lake stores water for people to use later. The lake, or water
storage area, is called a reservoir.
The water in reservoirs travels in pipes to
people’s homes for drinking water. It can flow through canals for farmers to
use in watering their crops. People also sail boats and swim in reservoirs.
Many dams use reservoir water to produce
electricity. Water flows into large machines called turbines inside the dams.
The turbines power other machines that generate electricity. Electricity produced
in this way is called hydroelectric power.
Some dams are built to prevent flooding.
During the rainy season, the reservoir stores the river’s extra water. During
the dry season, the dam sends the reservoir water back into the river.
HOW BIG ARE DAMS?
If you’ve ever visited a large dam, you know it is
an amazing sight. Dams are some of the biggest structures ever built.
The Hoover Dam on the border of Nevada and
Arizona is as tall as a 72-story building. The Grand Coulee Dam in the state of
Washington contains enough concrete to build a sidewalk all the way around the
Earth.
Few dams are this big, however. Most dams are
small structures less than 10 feet (3 meters) tall.
WHAT ARE DAMS MADE OF?
Many large dams are made of concrete. Some are
made of packed earth or rocks. Because these materials are not as strong as
concrete, dams made of earth or rocks must be very thick. The Tarbela Dam in
Pakistan is made of earth and rock. It contains more than 15 times as much
material as the Grand Coulee Dam.
Dams must be strong enough to withstand the
pressure of water against them. Dams also must be cared for and repaired. A dam
that breaks can cause disaster. In 1889, a dam in Pennsylvania broke and let
loose a wall of water. The water submerged the town of Johnstown, knocking down
houses and killing more than 2,000 people.
