Denudation - River
The word denudation is derived from the Latin word denudate means to lay bare. The wearing or tearing away of the land surface is referred to as denudation.
The surface of the earth is changing regularly due to the influence of various natural agents viz, endogenic (internal) and exogenic (external) agents.
- V-shaped valley
- I’ shaped Valley
- River capture
- Ox-Bow lake
- flood plain
Agents of denudation
The natural agents responsible for modelling and shaping the earth’s surface are known as Agents of Denudation. The important agents are :
River – Its activity is common in the drainage (river) regions.
Underground water – Its activity is familiar to the limestone regions.
Glaciers– Its activity is widespread in the polar and high mountain regions.
Wind –Its activity is regular in arid regions
Waves – Its activity is familiar in the coastal regions.
The denudation agents perform three important works. they are:
Erosion– the wearing away of the land surface.
Transportation– carrying the eroded materials.
Deposition– deposition of materials eroded and transported.
Work of river
The river is an important external agent of denudation on the ever-changing face of the earth.
The work of the river is more or less common in all the drainage systems of the world.
A river is a natural watercourse usually freshwater, flowing towards a sea or an ocean. The river gets water from the rainfall, underground water, and glacier.
The place where a river rises is called the source or the Birthplace of the river
The place where the river meets or drains into a sea or an ocean is called the river mouth.
The path of the river in which it flows from source to its mouth is called the course of the river.
A ridge of high land which separates two river systems is called a watershed or water divide.
Catchment area is the region from which a river or stream receives water from different sources.
The main river is joined by a number of streams or small rivers called
The place where a tributary river joins the main river or another river is called
The area drained by a river and its tributaries including distributaries form a river basin.
The river courses
The entire path of the river from source to mouth is divided into three courses
The upper course
The middle course
The lower course
1. The upper course
It is near the source of a river and is normally found in a mountainous area. Here the erosional work is dominant because the river flows on steep slopes with great velocity. This stage of the river course is known as the young stage.
2. The middle course
the middle course begins when the river enters the plateau and plains from the mountains. The dominant work of the river in this stage is transportation. this stage of the river course is called the mature stage.
3. The lower stage
is the plain course of a river near its mouth. in the lower course, the slope is very gentle, the valley is broad, and the speed of the river is very slow hence depositional work is dominant. This stage of the river is known as the old stage.
The work of the river consists of three closely interrelated activities:
1. Erosional work
The process of wearing and taking away the part of the rock is known as erosion. It depends upon the volume and velocity of water, the nature of the slope, and the nature of rocks.
The erosional work of the river is performed in two ways:
Mechanical erosion– It involves hydraulic action.
The chemical erosion – It involves corrosion or solution.
There are various landforms associated with the erosional work of the river. They are V-shaped valleys, gorges, I-shaped valleys, canyons, potholes, waterfalls, river capture, etc.
1. V-shaped valley
In the mountain course the speed of the river is greater and the volume is less. As the water rushes down the steep slopes there is maximum vertical or lateral erosion. The rapid downcutting or vertical erosion results in the formation of a V-shaped valley.
A deep narrow valley with steep rocky sides in the river course is known as gorges. They are formed by the regular cutting and the regular vertical cutting by the rivers in the valleys. Eg: Narmada gorge, Gangotri gorge, etc.
3. ‘I’ shaped Valley
A very steep, deep river valley formed by the river, looking like the ‘I’ alphabet, is called the ‘I’ shaped valley. These are very deep compared to gorges.
It is a wide, deep, and steep valley almost with a vertical walls-like feature found in the arid or semi-arid regions is called canyon eg . grand canyon of river Colorado in the USA.
These are the small depressions in the rocky beds of the river valley. They are formed by corrosion. Pebbles, sand, and small rocks carried by the river swirled around on the river bed. This action eroded the rock on the river bed forming potholes.
Huge amount of water falling from a great height along the course of a river come in the way of a flowing river. The soft rock gets eroded faster and hard rock does not erode easily. Therefore, the huge amount of waterfalls from a great height and creates water falls. Eg: The Jog falls
7. River capture
It is mainly due to headward 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. Eg: the river Testa was a tributary to ganga but, now it is a tributary to the river Brahmaputra.
2. Transportation work:
The process of carrying away the eroded materials is known as Transportation.
The rock materials are eroded particles carried by a river called its Load.
The transportation capacity of a river is based on the velocity of water, the volume of water, load, slope, smooth valley floor, etc.
The most important landforms associated with the transportation and depositional work of the river are alluvial fans, alluvial cones, etc.
the term alluvial refers to the debris transported and deposited by rivers. When the fast-flowing river enters the plateau or plain region, it experiences a sudden decline in gradient and obstruction in its path. Due to obstruction the river spreads and deposits many of its light materials in fan shapes known as alluvial fans.
In the plateau and foothill 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 the 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 in its lower course are meanders, ox-bow lakes, flood plains, natural levees, river distributaries Delta, Estuary, etc.
In the lower course, the river flows slowly in a zig-zag or curved manner due to smaller obstructions in its path. A curve or loop formed by the river in its path is called a meander. When the river course is formed by such crescent-shaped loops due to continuous lateral deposition it is called a meandering course.
2. 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 flows which flow through the shorter route leaving the curve of the meander cut off and a crescent-shaped lake is formed known as ox-bow lakes.
3. Flood plain
when the river is in flood the water overflows on its bank and spreads to the surrounding regions. the silt carried by the water gets deposited in these areas and creates flat plains on both the banks of the river known as flood plains.
4. Natural levee
During and the water of the river crosses its banks and deposits the sediments near the bank. Here the river banks are higher than flood plain. this high wall-like feature is called a natural levee.
As the river approaches the sea or ocean, due to a reduction in gradient, and the joining of tributaries, its volume increases, and speed decreases hence, the river begins to break up into a number of branches from the main river called distributaries. Eg the Hoogly.
A triangular-shaped alluvial deposition formed at the mouth of the river is called a delta.
Estuaries are the tidal mouth of a river having a narrow, gradually widening lay at the mouth. In the estuary, river water is mixed with seawater. eg: Narmada estuaries, Nethravathi estuary.
Potamology -The science of rivers
Limnology– The scientific study of lakes and ponds with respect to their physical, chemical and biological properties