Describe any four depositional landforms formed by the river action.
Answers
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.
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
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.
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.