What is a dam?

   A dam is a wall across a stream. It holds back the water of the stream. It may make it spread out to form a big lake.
   It sounds much easier to build a wall across a stream than it is. Water is heavy. A tin can a foot wide, a foot long, and a foot deep would hold just a cubic foot of water. A cubic foot of water weighs about 62½ pounds. If the lid of such a can were lying at the bottom of a river 20 feet deep, and that is not at all deep for a river, the water on top of it would weigh more than half a ton. A dam to hold back a river has to be very strong. It has to be especially strong at the base, because there the pressure of the water it is holding back is greatest.
   People were not the first engineers to build dams. Beavers built them long before men did. But men have been building dams for several thousand years. The ancient Egyptians built dams along the Nile. The Babylonians built dams along the Tigris.
   Beavers build their dams of tree trunks and mud. The first dams men built were made of earth. Now most of them are made of concrete strengthened with steel.
   Beavers need ponds to build their homes in. Their dams make the water of streams spread out to form ponds. People build dams for several reasons. They may dam a tiny stream flowing in a gully to keep the stream from making the gully wider and deeper and spoiling much good land. They may dam a bigger stream to hold back the water after heavy rains and prevent floods. They may build a big dam to make an arti­ficial lake. The lake may furnish water for a city. It may furnish water for irrigation. At the same time it may provide miles of shore line, with chances for bathing and for boating.
   Another reason for building dams is to keep the water in a river deep enough for boats to travel on the river. Still another is to make an artificial waterfall. The falling water may be used to turn mill wheels or big generators for generating currents of electricity. Many dams serve more than one purpose.