River is an important external agent of denudation on the ever changing face of the Earth.
The work of river is more or less common in all the drainage systems of the world.
The work of river consists of three closely interrelated activities.
1. Erosional work: The process of wearing and taking away the part of rock is known as ‘Erosion’. It depends upon the volume and velocity of water, nature of slope and the nature of rocks. The erosional work of the river is performed in two ways.
a. The Mechanical and
b. The Chemical erosion.
There are various landforms associated to erosional work of river.
a. ’V’ Shaped valley: In the mountain course the speed of the river is greater and volume is less. As the water rushes down the steep slopes there is maximum vertical or later erosion. The rapid down cutting or vertical erosion results in the formation of ‘V’ shaped valley.
b. Gorge: A deep and narrow valley with steep rocky sides in the river course is known as ‘Gorge’. They are formed by the regular vertical cutting by the rivers in the valleys eg. Narmada gorge.
c. I shape valley: A vey steep, deep river valley formed by the river, looking like ‘I’, is called T Shaped valley. These are very deep compared to gorges.
d. Canyon: It is a wide, deep and steep valley almost with vertical walls like feature found in the arid or semi arid regions is called ‘Canyon’ eg. Grand Canyon of River Colorado in USA.
e. Potholes: These are the small depressions in the rocky beds of other river valley. They are formed by corrosion. Pebbles, sand and small rocks carried by the river swirled around on the river bed. This action erodes the rock on the river bed forming potholes.
f. Waterfalls: Huge volume of water falling from a great height along the course of a river is called “Waterfalls’. They are formed when the hard and soft rocks come in the way of flowing river. The soft rock gets eroded faster and hard rock does not erode easily. Therefore huge amount of water falls from great height and creates waterfalls. Eg. The Jog falls, The Angel falls, The Victoria falls.
g. River Capture: It is formed mainly due to head-ward erosion by the river near its source. When the source of a river is captured by another major and strong river it is called‘River Capture’.
2. Transportational work: The process of carrying away the eroded materials is known as ‘Transportation’. The rock materials and eroded particles carried by a river is called its Load. The transportation capacity of a river is based on velocity of water, volume of water, load, slope, smooth valley floor etc.
The major landforms associated with the transportational work of the river are,
a. Alluvial fans: The term alluvium refers to the debris transported and deposited by rivers. When the fast flowing river enters the plateau or plain region, it experience sudden decline in gradient and obstruction in its path. Due to obstruction of the river spreads and deposits many of its light materials in fan shape known as ‘alluvial fans’.
b. Alluvial cones: In the plateau and foot hill region when the river spreads out, the eroded materials carried by the river is deposited in conical shape called ‘Alluvial cones’.
3. Depositional work: The process of carrying and accumulating the eroded materials by the river at the lower course is called ‘deposition’. In the lower course due to gentle slope the river slows down and deposits most of its materials on the banks, course and the mouth.
The important landforms resulting from depositional work of the river are,
a. Meanders: In the lower course, river flows slowly in zig zag or curved manner due to smaller obstruction in its path. A curve or loop formed by the river in its path is called ‘Meander’. When the river course formed by such crescent shaped loops due to continuous lateral deposition it is called meandering course.
b. Ox-bow Lakes: The ox-bow lakes are formed by depositional and erosional actions taking place simultaneously and they are a result of excessive meandering. The River which flows through the shorter route leaving the curve of the meander cut off and crescent shaped lake is formed known as ‘Ox-bow lakes’.
c. Flood Plains: When the river is in floods the water over flows on its bank and spreads in the surrounding regions. The silt carried by the water gets deposited in these areas and creates flat plains on both the banks of the rive known as ‘Flood Plains’.
d. Delta: A triangular shaped alluvial deposition forced at the mouth of the river is called ‘Delta’. Important types of deltas are a. Arcuate or Common delta b. Bird-foot delta
e. Distributaries: As the river approaches the sea or Ocean, due to reduction in gradient, joining of tributaries, its volume increases, speed decreases hence, the river begins to break up into a number of branches from the main river called ‘Distributaries’.
f. Estuary: Estuaries are the tidal mouth of a riverhaving a narrow, gradually widening lay at the mouth. In Estuary River water is mixed with sea water. Eg. The Narmada estuary, The Kali estuary.
