ICSE Class 6 Geography Chapter 10 Denudation

Chapter Ten

Denudation

Syllabus

Denudation: Meaning and agents of denudation; work of river and wind.

Stages of a river course and associated landforms - V-shaped valley, waterfall, meander and delta.

Wind - deflation hollows and Sand dunes.

Processes Involved in Denudation

Weathering refers to the disintegration of rocks by atmospheric agents at or near the surface of the earth. It happens due to change in weather conditions like temperature, altitude and precipitation.

Erosion refers to the displacement of rocks by agents like wind, water or ice. Erosion is caused by the movement of these eroding agents and the resulting smaller particles are transported to another place, whereas in weathering no such movement occurs.

Mass Movement or Mass Wasting refers to the large-scale movement of loose materials (rock-waste) down the slope on account of gravity. The steeper the slope, the more rapid is the movement.

Denudation is the process of breaking and lowering the rocks from the surface of the earth. It is wearing away of landmass by various processes like weathering, erosion mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levelling the peaks.

Teacher's Note

When you notice weathered rocks on a hillside or see how a river valley deepens over time, you are witnessing denudation processes that shape landscapes around us daily.

Work of a River

Rivers are the greatest agents of erosion, transportation and deposition. The force of erosion of a river depends on the degree of inclination gradient of its course, the volume of water it carries, the load of the sediments, velocity of water, and water discharge.

Factors Affecting Work of a River

Various factors affect erosion, transportation and deposition work of a river. Some important factors include the following:

(i) Velocity of water: Erosion and transportation are both maximum when velocity is high. In other words, erosion and transportation are proportionate to the velocity of water. Deposition, on the other hand takes place when velocity of water is low.

(ii) Volume of water: The larger the volume of water, the greater is the power of erosion and transportation. Deposition, on the other hand, takes place better when there is reduced volume of water as well as when the load is greater.

(iii) Load: Load is the material transported by a river. The load of a river, does all the work of erosion and deposition. Mass wasting, deepening of river valleys and formation of landforms depend on the load a river carries.

Course of a River

Rivers normally originate in mountains from glaciers or from lakes. Some rivers like those in the Deccan Plateau region are formed due to meeting of various streams during rainfall. The place of origin of a river is called its source. Where a river enters the sea or disappears before joining the sea is called its mouth. The streams or small rivers which join the main river are called its tributaries. Where a river divides into channels or smaller rivers, they are called its distributaries. The path along which the river flows is known as the course of a river.

From its source to its mouth, a river is divided into three main sections - Upper Course, Middle Course and Lower Course. These three sections are also called the Profile of a River.

Upper Course

In the upper course of its flow, the river is young (i.e., it is in the initial stage). At this stage, the river flows swiftly, for the gradients are steep. The upper course of a river generally lies in a hilly area and the river flows a considerable distance over a steep slope. Along its course, it erodes its bed and banks; and carries large boulders of rock with it. Under the impact of rush of water and the load the river carries erosion action is swift and effective.

Therefore, in the upper course the dominant activity of a river is erosion. Many landforms are shaped by this activity of the river.

Landforms

The significant landforms resulting from erosion in the upper course of a river are the following:

(i) V-shaped Valleys: A V-shaped valley is a narrow valley that has a profile suggesting the form of the English alphabet 'V'. It is formed from a stream eroding downward, through a process, called down cutting. These valleys are process in the initial stages of rivers and have formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

In regions of hard rocks it develops almost vertically in an I-shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

(ii) Waterfall: A waterfall is defined as a vertical fall of water of enormous volume from a great height in the long profile of a river. A waterfall is generally formed due to large differences in the rate of erosion. The like features known as rapids. The water that falls down the edge of a hard resistant rock falls may have at the bottom a soft rock. It is this soft rock that gets eroded fast and creates a hollow basin called plunge pool. Some of the world's highest and best known falls are Angel Falls (986 m) in Venezuela and Yosemite Falls (778 m) in California, USA. Jog Falls or Gersoppa falls, on a tributary of Kaveri river, have a plunge of 60 metres.

Teacher's Note

When visiting hilly regions or waterfall locations, you can observe how rivers carve V-shaped valleys and create dramatic falls where they encounter resistant rock layers.

Middle Course

The second stage of a river is known as the maturity stage. It corresponds to the middle course. At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountains area and enters the plains. In the middle course the river flows more slowly, and the energy required to transport the materials is just enough to drag large particles.

Two other important characteristics of the middle course are:

(i) the lateral cutting is more active than down cutting; and

(ii) the volume of water in the river increases due to many tributaries joining it.

As a result of these characteristics, the river carries its load in different ways. Fine particles remain suspended in water, large particles are just dragged along the bed of the river and smaller ones are rolled down in the centre. The river also gets widened as a result of great volume of water and the impact of the load.

Landforms of The Middle Course

Meander: River meanders are bends of longitudinal courses. S-shaped meanders which are common to most rivers result when a channel forms a curved path. These are the result of both erosion and deposition work of rivers. Meanders generally form under conditions of a gentle slope and sufficient water in rivers. The river flow is diverted by an obstruction allowing the river to do lateral erosion work. When the curvature of meander loops are made more and more circular, the curvature then becomes more sharp and the river breaks through the meander to form an oxbow lake. The Ganga in India and Mississippi in the USA are famous for their meanders.

Teacher's Note

You can observe meandering patterns in rivers near your locality, where the water curves back and forth as it flows across flatter terrain.

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