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When a river enters into a lake or a sea, its velocity and load transporting capacity are reduced. The river, therefore, deposits part of its load. All of the bed load and most of the suspended load is dropped near the point where the river enters a lake or the sea.

The bulk of the coarsest material of the load is deposited first, but the suspended load consisting of finer materials is carried farther out by the distributaries. Fine clays, carried in suspension, form clots in the salt water of the sea and sink to the sea floor causing the stream channel to become shallow and broad.

Thus, a fan-shaped alluvial deposit at a river mouth formed by the deposition of successive layers of sediment appears in the form of a delta, which has been named after the Greek letter ‘delta’ A. As the stream deposition blocks the channel, the stream moves to another position.

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After this new channel is also clogged, so the stream moves to another path. The multiple channels used by the stream to enter the sea are called distributaries.

Even within the large body of water such as a lake or the sea, natural levees build up along the sides of these distributaries. By continued deposition the steam builds up the land far out into the water.

In fact, Herodotus (fifth century B.C.) was the first person who observed the growth of the Nile Delta by the annual floods. The name refers to the Greek letter delta which resembles a triangle, which is the shape of the Nile Delta. But all deltas do not conform to this shape.

There is a lot of variance in the shape of different deltas formed by different rivers at their mouths. The shape of the delta, as a matter of fact, is governed by so many variables such as the river discharge and load, grain size, flood frequency and intensity, climate, vegetation, wave energy, tidal range, salinity, water temperature and tectonics etc.

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The sequential deposition is charac­terized by different types of bedding known as bottom-set beds, fore-set beds and top-set beds.

The bottom-set beds are formed of fine material and settle slowly on the sea floor. The fore-set beds form the bulk of the delta and grow towards the sea. They extend over the bottom-set beds, and as their upper surface grows seawards, they are overlain by the thin top-set beds.

It is a well-known fact that all the rivers do not form deltas, they grow only where the river-deposited sediment exceeds that removed by coas­tal processes such as waves and currents. In other words, deltas can attain great size only where wave and tidal action are not strong, and where the sediment supply is high.

Most of the deltas have a more or less arcuate shape resembling a bow. However, there are wide variations in the shape depending upon the pattern and permanence of the distributaries. One thing is common among all the deltas.

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Because of rich alluvial deposits and the abundance of moisture, there is dense growth of natural vegetation on them. As regards the growth of deltas, generally it is rapid. The Nile Delta’s growth rate has been recorded as 3 meters per year, whereas the growth rate of the Po River and the Mississippi River deltas is about 60 meters per year.

Many great rivers of the world have formed large deltas. For example, the Orinoco in Venezuela, the old delta of the yellow river in China, the Nile delta of Egypt, the Ganga- Brahmaputra delta in India, the Rhone delta in France, the Mississippi delta in the United States are all large deltas.

These large deltas have proved to be of great environmental importance because of their vast fertile lands which support dense agricultural populations. Besides, many important coastal cities are situated on or near the deltas.

Sunderbans in India on the delta of the Ganga River are famous for the supply of valuable timbers, and supports a large agricultural population. Big metropolitan cities like Kolkata, Alexandria, Amsterdam and Rotterdam, Shanghai and Marseilles etc. are situated on or near the deltas.

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