1. Alluvial fans and Cones

When streams flow abruptly from steeper to gentler gradients, as at the base of a mountain or ridge, its velocity is checked and the huge quantities of material carried by the river are dropped their giving rise to a broad, low cone-shaped deposit called an alluvial fan.

Thus alluvial fans form where a stream leaves a confined valley and enters a flatter region. The material constituting a fan includes coarse boulders and pebbles at its head to finer material down its slope.

The term alluvial fan is commonly used when the slope of the deposit is below 10 degrees and alluvial cone when the slope is from 10 to 50 degrees.

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A series of adjacent fans may in time coalesce to form an extensive piedmont alluvial plain, also known as ‘Bajada’.

2. Flood-plain deposits

Flood plains are areas of low and relatively flat land bordering the channel on one or both the sides, at bank level.

These areas are readily submerged under water during flood time, when the river water overtops the banks of the channel and rises above the channel at low water. Deposits formed on flood plain by flood-water outside the actual channel are known as Over bank deposits.

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A number of features are associated with the flood plains, which are as follows:

(a) Meanders and oxbow lake

Dominating the flood-plain is the meandering river channel, i.e. the river flows across the flood- plain in broad sweeping curves, known as meanders. Mean­ders are common where the gradient of a river becomes extremely low.

In a flood plain a slight obstacle or accidental irregularity usually causes a deviation of the current with the initiation of a bend. Once started the bends tend to grow and gradually become more pronounced.

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The water flows faster around the outer side of the bend and is slow on the inner curve. Accordingly, erosion becomes more towards the outside of each bend and the channel deepens along the downstream part of the bend which is also termed as the under-cut side.

At the same time there starts deposition towards the inner side of each bend forming what is known as the slip-off slope. Thus the river shifts its channel towards the outer bank and leaves gently rounded slip-off slope on the inside of the growing curve.

A meander grows until it becomes bulb-shaped with a narrow neck, because of the constant broadening of a river’s bend during the erosion of the outer bank and deposition on the inner bank. During floods, the increased power of the flow may carry the stream across this neck.

As a result, the river straightens its channel. The former meander, therefore, re­mains as a back-water for some time and the entrances to it gradually get silted up since the river follows the shortest course and the water in the meander is still. Thus a meander loop gets abandoned.

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This phenomenon is called a cut off. The abandoned channel thus constitute a loop-shaped lake known as an oxbow lake or horse-shoe lake. The deposits formed at the slip off slope of a meandering river is known as Point-bar.

(b) Natural levees

These are broad, low-ridges formed along the bank of the river during floods. During floods, when the entire flood plain is inundated, water spreads from the main channel over adjacent flood plain deposits.

When the flood retreats, sand and silt etc. are deposited in a zone adjacent to the channel forming low ridges that parallel a river course. They are highest near the river bank and gradually slope away from it, because the deposition is more nearer to the channel and decreases away from it.

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3. Braided River

Braiding is a phenomenon of dividing and reuniting the river channels. In such cases, the river flows in a number of narrower channels separated by lenticular sand and gravel bars which may again meet the main channel somewhere downstream.

They are commonly developed where the amount of load is more and the river is incapable of transporting all of it.

This deposition starts near the centre of the channel. The coarser fractions of the load tend to form islands with a channel on each side and similarly other islands also develop. Accordingly the flow is divided into multiple branches (which may rejoin later on) to give rise to what is known as braided river.

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Apart from the above, there are also other causes for the develop­ment of a braided-river. Sometimes a considerable portion of water is lost due to evaporation and infiltration, thereby depositing the load of silt and clay carried by the river on the channel itself.

This makes the channel so shallow that the stream cannot be contained in it and therefore spills out over one side. This overflowing water is capable of cutting a new track gradually. With subsequent repetition of this process a braided river pattern is developed.

4. Delta

Deltas are basically features of river deposition. As we know, when a river enters a lake or sea its velocity is checked rapidly and the process of deposition is accelerated. Even the colloids carried in the river water get coagulated due to the electrolytes present in the sea water.

The coarser and heavier material is laid down first and the finer and lighter material is carried further out. Thus the load brought by the river gets deposited at its mouth, which gives rise to what is known as a delta, because these deposits are triangular in outline and resemble the Greek letter (delta). Deltas are considered to be the submerged equivalents of alluvial fans.

The essential condition for the growth of a delta is that the rate of deposition of sediments at the mouth of the river should exceed the rate of removal by waves and currents.

Sometimes the tides and currents may be sufficiently strong to prevent any considerable deposition and the mouth of the river remains open forming what is called an estuary; – whereas deltas are formed when the deposits of a river are not removal by tidal or other currents. Thus the factors favourable to the formation of a delta are:

(a) Abundant supply of sediments;

(b) Absence of powerful waves or shore currents;

(c) A stable body of water,

(d) A shallow water offshore;

Small deltas may exhibit a characteristic pattern of stratification not present in many large deltas-built into the ocean. Thicker layers of coarser-grained sediments known as foreset beds pile up on the sloping bottom close to shore, whereas finer sediments deposited in thinner layers further out are known as bottom-set beds.

The bottom-set beds are actually the continuations of the forest beds. On top of forest beds, thin layers of sediments lie, which have a gentle seaward slope. These are known as top-set beds.

Deltas show a variety of shapes, mostly because of the configura­tion of the coastline as well as the action of the sea-waves. On the basis of the shape the deltas are classified as-arcuate delta, bird-foot delta, cuspate delta etc.

Through the delta run a large number of channels which come out of the main channel. The smaller channels are termed as distributar­ies.

Deltas provide extensive flat fertile lands which support dense agricultural population, as for example, the Ganges delta, the Nile delta, the Mississipi delta etc.