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Dike
Dike
A long embankment, often looking like a natural levee, that is used to prevent river water from flooding low-lying land.
A dike is usually made with a core of soil and faced with stone
or rush mats to prevent erosion. The top is broad and often planted with grass to help stabilize it. In
the United States artificial dikes or artificially heightened dikes are also called levees. (See also: Groin.)
 Dike—The Embankment in London is a stone dike. The front is a wall that has recently been raised to prevent flooding. Notice how the seats have had to be placed on raised platforms so that people can see over the wall while seated.
Diseases
Waterborne diseases are an important worldwide problem. Almost 250 million people each year contract a waterborne disease, and about 10 million of them die from such diseases.
One of the most serious of these diseases is cholera. Today, cholera affects
tropical countries
in particular, but
is certainly not
confined to them.
The connection
between cholera and
the contamination
of drinking water
by raw sewage was
first recognized in
London in 1854. This
led directly to rapid attempts by industrialized countries to purify their water. There may be more than a million people suffering from cholera today. It is a disease that particularly affects the slum areas of developing world cities where sanitation is poor.
Bilharzia, or schistosomiasis,
is another waterborne disease
that affects 200 million people. It relies on host snails in the water. Unfortunately, the still waters that the snails live in are being extended with the construction of more and more reservoirs and irrigation channels. All bodies of water in Africa are now contaminated with the snail, including all of the Great Rift Valley lakes.
Legionnaire’s disease is another disease connected with water. It grows in the water used in air- conditioning systems that are not sufficiently clean. It is resistant to the chemical chlorine—
normally regarded as a sure way of purifying water. However, Legionnaire’s disease does not affect drinking water.
Malaria is one of the world’s biggest killer diseases. It is carried by the anopheles mosquito. Two
 Diseases—This little girl is washing the family cooking pot in a disease-carrying sewer. Although many slum dwellers are aware of the risks of using polluted water sources such as this, they often provide the only water supply.
billion people are at permanent risk from malaria, with about
130 million cases reported each year, mainly in Africa and Asia. Malaria is responsible for one-third of all infant deaths. The larval stages of the mosquito develop in stagnant water.
Onchocerciasis, also called river blindness, is a disease of tropical countries produced by the bite of the black fly, which breeds on rivers.
(See also: Dehydration.)
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Dissolve
To cause one substance to disperse into another so that it seems to disappear. Water is the world’s most common solvent, and a wide range of substances dissolve in it.
Salt, carbon dioxide, and oxygen gases dissolve in water. All three are found in seawater. Carbon dioxide is also dissolved in water under pressure to make fizzy (carbonated) drinks.
Gases in the air readily dissolve in water. Often this is a good thing, such as when oxygen dissolves in water to provide
the oxygen necessary to marine life. But it can also be a danger, such as when pollution gases like sulfur and nitrogen oxide dissolve in raindrops and then fall as
acid rain.
When acid gases are dissolved in water, they can react with rocks and rot them away. Limestone rocks dissolve in water containing carbon dioxide. Over time
such solutions widen cracks in limestone, allowing them to form tunnels and caves.