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Crevasse
Crevasse
A deep crack in the surface of
a moving mass of ice, either an
ice sheet or a glacier. A crevasse forms because the ice near the surface is brittle, while the deeper ice is under much more pressure and acts more like a liquid. As the deeper ice flows, it begins to drag the surface ice along. If the ice flows down an even slope, then no crevasses occur. But if the ice flows over a rock step or around a bend, then the deep ice is able to flow over these obstructions, while the ice at the surface is too brittle to flow and breaks up into crevasses. As a result, the pattern of crevasses on the surface of ice gives good clues to how the deeper ice is moving. (See also: Ice crystals.)
D
Dam
An artificial wall or embankment designed to hold back water. The water held back makes a reservoir (sometimes called a “lake” if it is large).
Earth and rock dams are broad structures with gently sloping sides and a very wide base. They are used only for dams of modest height. An earth dam is usually faced with stone so that it is not eroded by running water or lake waves. Masonry and concrete are used to build the world’s tallest dams, or where a dam must be strong enough to block a narrow valley. The world’s biggest dam will be the Three Gorges dam on the Yangtze River in China. It is currently under construction and due to be completed in 2009.
Dams are important because they often do several useful things. For example, they hold back
river water to prevent floods;
 Dam—The Glen Canyon dam on the Colorado River, Arizona. The diagram below shows how water is led over the dam to the turbine house at its foot. This generates electricity.
they also store water in reservoirs
for use in irrigation, to produce Dam hydroelectric power, or to keep
rivers flowing in times of drought.
Reservoirs can also be used for
recreation.
The construction of dams
and reservoirs is of concern to environmentalists because it often destroys the natural environment. (See also: Barrage.)
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Reservoir
Turbine house