A dam is a barrier (wall) of earth, concrete or rock. A dam is built across a river to restrict the flow of water. Dams are usually built in gorges or across narrow parts of rivers. An artificial (man-made) lake or reservoir is formed behind the dam. The first known dam was built in about 4000 BC across the Nile. This changed the course of the Nile so that the city of Memphis could be built.

Dam on the Zaire River

Reasons for Construction

A dam may be built for many reasons. Most dams are built for several purposes.

• To control the flow of water in drought. In a drought, the holes in the dam can be closed, allowing water to build up in the lake behind the dam.

• To control the flow of water in a flood. When there is a lot of rainfall, the river may overflow at its banks. More holes in the dam can be opened to let more of the water through.

• For better irrigation of crops. The water can be sent into irrigation channels and pumped directly onto the farmer's land.

• For navigation. Building a dam often makes the river deeper. Bigger ships can then travel on the river.

• For electricity. Hydro-electric dams are built which have turbines in them. The power of falling water turns the turbines.

• For recreation. The lake created behind the dam can be used for water sports.

Types of dam

There are 4 basic types of dam. These are:

• Gravity dams.

• Arch dams.

• Buttress dams.

• Embankment dams.

Most dams are made of concrete, except for embankment dams which are made of stones, earth and clay. Many dams have spillways for overflow purposes.

Gravity dams

Gravity dams are solid concrete dams where the base of the dam is thicker / wider than the top. They are vertical with very little curve on the wall part. One example is the Grande Dixence Dam in Switzerland (1962), which is 284 metres high and 700 metres long. Another example is the Grande Coulee Dam (1942) which is 168 metres high and 1272 metres long. Gravity dams are usually built on solid rock foundations.

Arch dams

This type of dam is like an arch bridge. The arch curves towards the flow of the water. Arch dams are normally built in a narrow canyon or gorge. They are rare because there are few sites where it is possible to build an arch dam. One example of an arch dam is the Hoover Dam across the Colorado River in the USA.

The Hoover Dam

Buttress dams

Buttress dams are sometimes called hollow gravity dams. They have an upstream wall to hold the water in the reservoir and a downstream wall. Between the two walls are triangular vertical walls which allow the water to pass between the two outside walls. These triangular walls also make the dam stronger. An example of a buttress dam is the Daniel Johnson Dam on the Manicouagan River in Canada (1968). It is 1306 metres long and is 214 metres high.

Embankment dams

Embankment dams are very low in height compared to their length. The wall of the dam has a gently sloping curve. They are normally made of clay, stones and rock although concrete is used as well. An example of an embankment dam is the Tarbela Dam on the Indus river in Pakistan (1977) . The Tarbela Dam is 148 metres high and 2743 metres long. The dam at Bewl Water in Kent is also an embankment dam.

The dam at Bewl Water in Kent