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| Sunday, 5 June 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Flowing bodies of water : Rivers
Rivers are flowing bodies of water. There are rivers in every continent except Antarctica. Rivers are an important part of the Earth's water cycle and the sculpting of the Earth's topography as they carry huge quantities of water from the land to the sea. What is a river? In simple words, a river is fresh water flowing across the surface of the land, usually to the sea. It flows in a channel. The bottom of the channel is called the bed and the sides of the channel are called the banks. All rivers start at the highest point in an area. As the river flows downstream, it gains more water from other streams, rivers, springs, added rainfall, and other water sources. Rivers begin in mountains or hills, where rain water (or melting snow) collects and forms tiny streams called gullies. Gullies either grow larger when they collect more water and become streams themselves or meet streams and add to the water already in the stream. When one stream meets another and merge, the smaller stream is known as a tributary. It takes many tributary streams to form a river. Most rivers end when they flow into a large body of water. The end of the river is called the mouth. At the mouth, there is usually a river delta, a large, silty area where the river splits into many different slow-flowing channels that have muddy banks. New land is created at deltas. Deltas are often triangular-shaped, hence the name (the Greek letter delta is shaped like a triangle). At the source of a river, the water is relatively pure. As the water flows downstream, it picks up silt and minerals (including mineral salts) from the soil and rock in the river bed. Many other chemicals enter river water as it flows downstream, including animal waste, human sewage, agricultural (farm) runoff, urban runoff, and mining or factory effluents. In basic terms, the existence of a river depends on three things: the availability of surface water, a channel in the ground, and an inclined surface. In this sense, the term river includes all kinds of water courses, from the tiniest of brooks to the largest of rivers. Essentially, a river represents the excess of precipitation over evaporation for a certain land area. Rivers are very dynamic and show tremendous variation through time and space. The shape, size and content of a river are constantly changing, forming a close and mutual inter-dependence between the river and the land it traverses.
Rivers are central to many of the environmental issues that concern society, and they are studied by a wide range of specialists including hydrologists, engineers, ecologists and geomorphologists. Rivers are very dependent on climate and their characteristics are closely related to the precipitation and evaporation regimes in their drainage areas. The three main types of rivers are Perennial, Periodic and Episodic. Perennial or permanent rivers have a constant flow of water (although there may be considerable seasonal variation in amount of flow) and occur in regions where precipitation generally exceeds evaporation. Periodic rivers may run dry occasionally but have stream flow during
regular periods of variable duration. These occur in regions where
evaporation exceeds precipitation on an annual average but periodically
precipitation is greater.Episodic rivers only rarely and fleetingly have
water in their channels. These occur in very arid climates (such as desert
regions). |
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| News | Business | Features
| Editorial | Security
| Politics | Produced by Lake House |
Sri Lanka is blessed with hundreds of beautiful rivers. Apart from
their beauty, they play a major role in nature. Rivers are vital carriers
of water and nutrients to areas all around the earth. They are of great
importance geologically, biologically, historically and culturally. 
Rivers are critical components of the hydrological cycle, acting as
drainage channels for surface water. The world's rivers drain nearly 75
per cent of the earth's land surface. They provide habitat, nourishment
and means of transport to countless organisms; their powerful forces
create majestic scenery; they provide travel routes for exploration,
commerce and recreation; they leave valuable deposits of sediments, such
as sand and gravel; they form vast floodplains where many of our cities
are built; and their power provides much of the electrical energy we use
in our everyday lives. 
