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Rivers, famous rivers of the world

Most rivers begin, unspectacularly, as a trickling stream of water from a melting glacier or from a tiny spring high in the hills or mountains. This stream is joined by others as it flows through the mountains and along the plains towards the sea. These side branches of the river are called its tributaries.

The area which is drained by the river and its tributaries is known as the river basin. The one of the famous rivers of the world with largest basin is the Amazon in South America, which drains almost three million square miles. The main river itself is 3,900 miles long, only 250 miles shorter than the Nile, the world's longest river.

Famous rivers of the world
A waterfall is an irregularity in a 'youthful' river's course which eventually will be worn away (click image to increase)	Rivers provide a means of communication and transportation. Most great cities have grown from settlements along river banks. In this picture you can see the European river cruises ship

Naturally enough, rivers rise on each side of a mountain range and form separate river systems. The line of separation between the two systems is called a divide, or watershed. The Great Divide in North America, for example, runs along the Rocky Mountains. It separates the rivers flowing westward towards the Pacific Ocean from those flowing eastward towards the Atlantic Ocean.

Rivers, whether they are in Brazil or Britain, develop in much the same way. Three fairly distinct stages of development can be recognized which are often appropriately termed youth, maturity, and old age. In youth, up in the mountains, the river is unpredictable and full of vigor. In maturity, it runs at a more leisurely pace through well-formed valleys. In old age, it weaves rather aimlessly over a wide, level, featureless plain. In the mountains, the river is narrow, but swift-flowing down steep slopes. The running water carries along with it pieces of rock that cut into and deepen the stream bed. The swirling, stone-filled waters drill pot-holes that eventually merge to lower the bed. The river valley at this stage is deep, narrow, and steep-sided.

Famous rivers of the world
In 'old age' a river winds sluggishly in great loops across a plain and has far less power than a 'youthful' or 'mature' river	In the past many people lived in huts built on stilts in rivers for protection. Some still do in eastern lands

Where parts of the bed are harder than others, they will be worn away less rapidly, and rapids and waterfalls will form. Along the Nile River, major rapids are called cater acts. The river valley gradually widens out as tributaries join the main stream. The gradient, or slope, of the river becomes more gentle. Large amounts of transported material settle out as the river slows down. Gradually the river begins to meander, or swing from side to side of the valley. This serves to widen the valley. In times of flood, the river covers the whole of the valley floor. It washes down and deposits sand, gravel, and rocks to form a so-called flood plain. The steep sides of the valley which mark the limit of the flood plain are called bluffs.

A view of the Suwanee River. Made famous river by Stephen Foster's song (The Swanee River), it rises in Florida and winds its way to the Gulf of Mexico

As the land becomes flatter, the river slows down, and more and more material is deposited. The meanders become more pronounced because the current attacks the outside of the bends more than the inside. Material is deposited in the quiet waters on the inside of the bends.

In old age, then, the river has cut its way down practically to sea-level and has a vast flood plain extending back from the sea. Sometimes the sea-level drops or the land rises. This increases the speed of the river, and it begins cutting a new valley for itself through the old flood plain. This process is called rejuvenation, because the river once more has the vigor of its youth. When this process happens several times, a series of river terraces are formed. They are notable features of most river valleys.

When the river enters the sea, it slows down and deposits the fine sand and mud it is carrying. Usually the action of the sea tends to push the deposits back up river, and constantly shifting sand-bars are formed. Whenever more material is brought to the sea than is removed by tides and currents, deposition occurs. In a tideless sea, such as the Mediterranean, there is nothing to check the outflow and deposition of material by the river. The material gradually builds up and extends seawards in a roughly triangular shape to form a delta. As the delta builds up, the river gradually splits into a number of channels, or distributaries. Well known famous rivers deltas examples include those of the Nile, the Ganges and the Mississippi. Because deltas are made up largely of silt carried by the river, they are generally fertile.

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