Chapter 10: Denudation
Somewhere in the northern Sahara, the desert floor stops being flat and begins to fall away. Walk on and the ground keeps dropping, past the level of the sea and then below it. The floor of the Qattara Depression lies almost 450 feet below sea level, and it is one of the great hollows of the world. Nothing dug it. No river runs across it, no machine has ever worked there, and no earthquake tore it open. The wind made it, one grain at a time. Loose sand and dust on a dry surface have nothing to hold them down — no water to stick them together and almost no plant roots to bind them. A strong wind lifts them, carries them off and drops them somewhere else. Repeat that for long enough and the land surface itself is lowered. This is denudation: the slow stripping of the earth’s surface, carried out by agents that seem far too gentle for the job. A river cutting a gorge and a wind scooping a hollow are doing the same work at different speeds. In this chapter you will watch both of them at it, and learn to read the landforms they leave behind.
Diagram
Fig. 10.A1 Caption: Four processes work together to strip a slope — rock breaks, moves, travels and settles. Brief: One hillside profile from ridge to plain; label weathering on the exposed rock face, mass movement as material sliding down the slope, erosion and transportation along the stream, deposition where the slope flattens; arrows point downslope throughout.
10.1 Processes Involved In Denudation
Denudation is the process of breaking and removing the rocks from the surface of the earth. It is the wearing away of landmass by several processes working together — weathering, erosion, mass movement and transportation. No single one of them could strip a landscape on its own. Rock has to be broken loose before it can be moved, and it has to be moved before the land beneath it is exposed to be broken in turn.
Denudation leaves three marks on the land that a student can see for themselves in any hilly area: (a) It lowers the general level of the land, because material is steadily carried off the high ground and dropped lower down. (b) It rounds exposed rock surfaces, since the corners and edges of a rock face are attacked from more sides than a flat surface is. (c) It levels the peaks, wearing sharp summits down into blunt ones over very long stretches of time.
Weathering And Erosion
Weathering refers to the disintegration of rocks by atmospheric agents at or near the surface of the earth. It happens because weather conditions change — temperature, moisture and precipitation. Rock heated by day and cooled by night expands and contracts, water works into every crack, and the rock slowly falls apart where it stands.
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 smaller particles that result are transported to another place.
The difference between the two is movement, and it is worth fixing now, because the rest of the chapter depends on it. Weathering breaks rock and leaves the pieces lying where they fell; in weathering no such movement occurs. Erosion breaks rock and takes the pieces away. Both attack the same rock with the same agents of weather, and both make the fragments smaller — but only erosion empties the site.
Mass Movement, Transport And Settling
Mass movement, also called mass wasting, refers to the large-scale movement of loose materials — rock-waste — down the slope on account of gravity. Nothing carries this material; it simply falls, slides or creeps because it is on a slope and gravity pulls it down. The steeper the slope, the more rapid is the movement.
Transportation refers to the movement of material loosened by erosion and transported to another place by the action of wind or water. Where mass movement needs only gravity and a slope, transportation needs a moving agent to carry the load.
Deposition refers to the laying down of material that has been weathered, eroded and transported by natural processes such as water, wind and ice. Deposition is where the journey ends, and it is not the opposite of denudation but the last step in it — the material removed from one place has to arrive somewhere.
Agents Of Denudation
Agents of denudation erode, transport and deposit sediments at the earth’s surface, and in doing so they produce two families of landforms — erosional landforms where material has been stripped away, and depositional landforms where it has been laid down. The chapter’s whole subject is these two families.
The four active agents of denudation are the following: (a) Water, working mainly as running water in rivers and streams. (b) Wind, working where the surface is dry and bare. (c) Waves, working along coastlines. (d) Glacial ice, working in cold high mountains and polar lands.
In this chapter we shall study the work of rivers and wind as agents of denudation.
Flowchart
Fig. 10.A2 Caption: Denudation is a chain — rock is broken, moved, carried and finally laid down. Nodes: rock exposed at the surface → weathering breaks it where it stands, no movement → loose rock-waste → mass movement drags waste down the slope under gravity → erosion displaces material by water, wind or ice → transportation carries it to another place → deposition lays it down → landforms; branch: where material is removed → erosional landforms; where material settles → depositional landforms
A river does all of this along a single course, and the balance between the steps changes as the river travels. The figure below sets out that change in advance, so that you can see the whole journey before we take it stage by stage.
Diagram
Fig. 10.1 Caption: Work of the river. Brief: Long profile of a river split into Young, Maturity and Old stages, with three labelled rows — effect on river (gorge, rapids, waterfalls; meanders, river cliffs; deltas, flood plains, ox-bow lakes), work of the river (predominantly erosion; transportation with some deposition; deposition), and effect on land with valley cross-sections beneath (steep V-shaped valley; wide gentle-sloping valley; very gently sloping smooth wide valley); river flows left to right.
10.2 Work Of A River
Rivers are the greatest agents of erosion, transportation and deposition. A river does all three at once, along different parts of the same course, and the landscape you see is the record of which of the three was winning at that spot.
How much erosion a river can do is not fixed. The force of erosion of a river depends on the following: (a) The degree of inclination, or gradient, of its course — the steepness of the slope it runs down, which decides how fast the water moves. (b) The volume of water it carries, since more water means more weight and more force pressing on the bed and banks. (c) The load of the sediments, which is the material the water is already carrying with it. (d) The velocity of water, that is, the speed at which the river flows. (e) The water discharge, meaning how much water passes a point in the channel.
Before looking at these one by one, it helps to see the whole river at once. The figure below runs from the source, where the river begins, to the mouth, where it meets the sea, and shows the features a river builds along the way.
Diagram
Fig. 10.2 Caption: Typical features of a river from source to mouth. Brief: Block diagram of one valley from mountain source to sea, labelled with source, waterfall, gorge, entrenched meander, lake, river terrace, beginning of floodplain, natural bridge, levee, interconnected channels, oxbow lake, lake, river mouth and sediment deposited on sea bed, with inset photographs of each feature; water flows from the high ground on the right to the sea on the left.
10.2.1 Factors Affecting Work Of A River
Various factors affect the erosion, transportation and deposition work of a river. Some important factors include the following: (a) Velocity of water: erosion and transportation are both maximum when velocity is high, and 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. (b) Volume of water: the larger the volume of water, the greater is the power of erosion and transportation. Deposition takes place better when there is a reduced volume of water, as well as when the load is greater. (c) Load: load is the material transported by a river, and the load of a river does all the work of erosion and deposition. Mass wasting, the deepening of river valleys and the formation of landforms all depend on the load a river carries.
Read the three together and one pattern runs through them. Fast water carrying plenty of volume tears material up; slow water, or water with more load than it can hold, sets material down. The same river therefore erodes in one stretch and deposits in another, without changing anything about itself except its speed and its slope.
Flowchart
Fig. 10.A3 Caption: Speed decides whether a river takes material away or leaves it behind. Nodes: river reaches a stretch of its course → is the gradient steep? yes → high velocity → erosion and transportation dominate → material carried downstream; no → low velocity → transporting power falls → deposition dominates → material laid down on the bed → back to river reaches the next stretch of its course
10.3 Course Of A River
Rivers normally originate in mountains, from glaciers or from lakes. Some rivers, like those in the Deccan Plateau region of peninsular India, are formed instead by the meeting of various streams during rainfall, so that no single spring or ice mass can be called the beginning.
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. Between the two, a river is rarely alone. The streams or small rivers which join the main river are called its tributaries, and where a river divides into channels or smaller rivers, they are called its distributaries. Tributaries bring water in; distributaries take it out.
The path along which the river flows is known as the course of a river. From the 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. The sections are not three different rivers. They are three stretches of the same water, doing different work because the slope beneath it has changed.
10.3.1 Upper Course
In the upper course of its flow, the river is young, that is, 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 its banks, and carries large boulders of rock with it. Under the impact of the rush of water and the load the river carries, erosion action is swift and effective. Water moving that fast can lift heavy fragments, and those fragments then grind against the rock beneath.
Therefore, in the upper course the dominant activity of a river is erosion, and many landforms are shaped by this activity of the river.
10.3.2 Landforms
The significant landforms resulting from erosion in the upper course of a river are the following.
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. The river cuts a deep, narrow slot; the sides above it are then loosened by weathering and slip down towards the channel, and the two together open the slot out into a ‘V’. These valleys are formed in the initial stages of rivers and have steep slopes.
In regions of hard rocks it develops almost vertically, in an I-shape, because the walls resist collapse and stay near-upright. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.
Diagram
Fig. 10.3 Caption: River valleys. Brief: Two cross-sections side by side — left, a narrow I-shaped valley with near-vertical walls in hard rock; right, a wider V-shaped valley with steep sloping sides; in both, the river at the base of the notch with a downward arrow showing down cutting.
Waterfalls And Rapids
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 — one band of rock wears away far faster than the band beside it, and a step is left in the river bed. The smaller differences lead to the formation of step-like features known as rapids, where the water is broken and turbulent but does not fall free.
The water that falls down the edge of a hard resistant rock may have at the bottom a soft rock. It is this soft rock that gets eroded fast and creates a hollow basin called a plunge pool. The soft rock is cut back further than the hard rock above it, so the hard rock is left standing out as an overhang, unsupported.
Diagram
Fig. 10.4 Caption: Waterfall. Brief: Cross-section with a band of hard rock lying over soft rock, numbered labels for (1) undercutting at the base, (2) overhang collapses, (3) plunge pool develops, (4) steep, gorge-like valleys downstream; water falls vertically over the hard band, with arrows showing the fall and the swirl in the plunge pool.
Some of the world’s highest and best known falls are Angel Falls (986 m) in Venezuela, in northern South America, and Yosemite falls (778 m) in California, USA. A drop of 986 m is close to a kilometre of falling water. In India, Jog Falls or Gersoppa falls, on a tributary of Kaveri river, has a plunge of 253 metres.
10.3.3 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 energy required to transport the materials is just enough to drag large particles — the river can still move them, but only along the bed, not up into the water.
Two other important characteristics of the middle course are: (a) The lateral cutting is more active than down cutting, so the river works sideways into its banks instead of downward into its bed. (b) 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 the great volume of water and the impact of the load.
10.3.4 Landforms Of The Middle Course
In the Middle Course, the river’s deposition work is more important than its erosion work. Therefore, the landforms of deposition are more prominent than the landforms of erosion.
Meanders And Oxbow Lakes
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. Once the water is thrown against one bank it cuts into it, while the slack water on the inside of the bend drops its load — so the bend deepens on one side and fills on the other, and the loop grows.
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 abandoned loop is sealed off by sand and silt and left as a curved sheet of standing water beside the new channel. The Ganga in India and Mississippi in the USA are famous for their meanders.
Diagram
Fig. 10.5 Caption: Meander. Brief: Two panels of the same stretch of river in green farmland — panel A labelled with a meander loop and the narrow neck between the loop’s ends; panel B with the neck broken through, labelled new channel, sand and silt plugging both ends, and the cut-off loop labelled oxbow lake.
Flowchart
Fig. 10.A4 Caption: An oxbow lake is a meander that grew until the river gave up on it. Nodes: obstruction diverts the flow → water strikes the outer bank → lateral erosion on the outer bank, deposition on the inner bank → loop becomes more circular and the curvature sharpens → neck between the two ends narrows → river breaks through the neck → shorter new channel taken; ends of the old loop sealed by sand and silt → cut-off loop left as an oxbow lake → back to the flow being diverted at the next bend
10.3.5 Lower Course Of The River
The third stage of a river is known as the old stage. This corresponds to its lower course. The river flows sluggishly and makes many landforms. In the lower course of the river all the ideal conditions for depositional landforms exist — a very gentle slope, a huge volume of water and a heavy load already being carried.
But the load-transporting capacity is drastically reduced, due to sluggish flow as well as the division of a river into many distributaries. Slow water cannot hold material up, and water split between several channels is slower still. So the river arrives at its mouth with far more material than it can keep moving.
10.3.6 Landforms Of The Lower Course
Delta
The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to deposition of sediments over a large area near the mouth of rivers. Such sedimentation occurs on the sides of the stream, at the mouth, in the front part as well as in the bed of the river — the channel silts up from every direction and the water is forced to find new ways around the blockage.
The deposition over a large area results in a triangular-shaped formation called a delta. It resembles the Greek letter Δ, which is where the landform gets its name.
Diagram
Fig. 10.6 Caption: Delta. Brief: Plan view of a river mouth with R. Ganga entering from the left and R. Brahmaputra from the top right, splitting into a fan of distributaries and mini-distributaries that spread across a triangular area of deposited sediment before reaching the sea.
Not all the rivers form deltas. There are certain conditions for the formation of deltas. These conditions include the following: (a) A nearly calm, sheltered sea, so that waves and currents do not sweep the deposited material away as fast as the river brings it. (b) A large amount of sediment supply, since a delta is built out of load and nothing else. (c) A reasonable size of the river, because the small rivers do not form deltas.
The deltas have also their own structure and undergo upliftment or subsidence, and they grow at a certain rate. For example, the Ganga-Brahmaputra delta is growing towards the sea. It is also one of the largest in the world.
10.4 Work Of Wind
The movement of air over the earth’s surface is known as wind. It is almost horizontal, although there is a small vertical component, and it is that small upward part of the movement that lets air lift material off the ground at all.
Wind picks up loose debris like sand and pebbles and transports as well as deposits them in another location. Wind can lift and carry sand grains away. When the speed of the wind slows down, the sediments brought by wind begin to drop on the ground, because slower air can no longer hold them up. If this process continues, it results in a huge expanse of sand known as deserts.
10.4.1 Action Of Winds
The wind is the most effective agent of erosion, transportation and deposition in arid regions, that is, in dry regions where rainfall is very low. Since there is little vegetation or moisture to bind the loose surface materials, the effects of wind erosion are more pronounced in the desert region. Damp sand sticks together and roots hold it down; dry bare sand has neither, and the wind takes it.
Wind erosion is operative in the following ways.
Deflation Hollows
Deflation involves the lifting as well as the blowing away of loose materials from the ground. The sands and pebbles are carried in the air or driven along the ground. The finer dust and sands may be deposited even outside the desert margins, because the lightest particles stay airborne longest and travel furthest.
Deflation results in the lowering of the land surface to create depressions called deflation hollows. The Qattara Depression of the Sahara Desert, in north Africa, which lies almost 450 feet below sea level, is a good example.
Diagram
Fig. 10.7 Caption: Deflation hollow. Brief: Cut-away block of desert ground with a hollow scooped from the surface, labelled depression produced by deflation, prevailing wind arrow across the top, a note that sand removed from the depression by the wind is deposited as dunes, layers of sand and aquifer shown in the block face, and water seeping out of the aquifer to form swamps or an oasis in the floor of the hollow.
Sand Dunes
Sand dunes are called hills of sand, and they are formed by the movement of winds. Such hillocks may be active dunes, constantly on the move, or they may also be inactive fixed dunes, rooted with vegetation. Dunes are found in deserts where the sand is being continuously moved, reshaped and redeposited — a dune is not a fixed object so much as a pile of sand that the wind keeps rebuilding.
Dunes have heights ranging from a few metres to about 150 metres, which at the upper end is the height of a tall building. In the desert regions of the Sahara, the Thar in north-western India, and West Australia, there are many sand dunes. They are in different shapes and sizes.
In fact, their shape and size depend upon factors such as: (a) The direction and force of wind. (b) The speed of wind, carrying the dust and sand particles. (c) The nature and amount of sand brought by the wind. (d) Water features, for example lakes, streams and the like, in the close-by area. (e) The nature of vegetation on the land. (f) The nature of the surface, where the formation of a sand dune takes place.
Shifting Or Migration
Sand dunes often move in the direction of the wind. The dunes of Trade Wind deserts, which lie in the belt of steady winds that blow from one quarter for most of the year, take definite forms, moving in a definite direction.
Migratory sand dunes are uncertain and dangerous. When they migrate with the wind, they damage towns and villages, because a moving hill of sand does not stop at a wall or a field boundary. The shifting of a sand dune can be checked by the growth of vegetation on the wind-ward slope, which is the side the wind strikes first — roots hold the sand and the wind can no longer lift it. Dunes are found in the tropical deserts of Asia and Africa.
Diagram
Fig. 10.8 Caption: Shifting of sand dune. Brief: Profile view of one dune drawn twice along a baseline, position A marked as the original position of the dune and position B further along as the later position, with a large arrow at top left showing the direction of the wind and dotted outlines between A and B showing the intermediate movement.
Two Types Of Dunes
The following two types of dunes are seen in the tropical deserts.
Barchan dunes are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction, and they are found in the deserts in the Sahara. Barchans are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks — the obstacle slows the wind, the wind drops its sand there, and the pile grows.
They occur transversely to the wind, that is, lying across the wind’s path, so that their horns thin out and become lower in the direction of the wind. The windward side is convex and gently-sloping, while the leeward side, being sheltered, is concave and steep. The crest of the sand dune moves forward as more sand is accumulated.
Diagram
Fig. 10.9 Caption: Barchan. Brief: Plan view of a crescent-shaped dune with a labelled wind direction arrow striking the convex outer curve, ripple lines across the gentle windward slope, the steep concave inner face in shadow, and both trailing tips labelled horns pointing downwind.
The migration of the barchans is a threat to desert life. They sometimes encroach on an oasis, a place in the desert where water reaches the surface, burying palm trees or houses. Long-rooted trees and sand-holding grasses are planted to halt the advance of the dunes, in order to prevent the fertile land from being devastated.
Longitudinal dunes or seifs are long and narrow sand-ridges which grow parallel to the direction of the prevailing wind. They are found in the interior parts of deserts. In the Libyan desert, in north Africa, they are known as seifs, which means ‘sword’ in Arabic. In the Thar Desert they are confined to the western margin, where the force of the south-west monsoon influences their formation.
An important feature of such a dune is that in its crest line there are rise and fall patterns. Seifs are found in the Sahara, Iran, Thar Desert (India) and West Australia.
Diagram
Fig. 10.10 Caption: Longitudinal sand dunes. Brief: Plan view of several long parallel sand-ridges running diagonally across the frame, with a labelled wind direction arrow entering from the left and further arrows running along the ridges to show that the ridges lie parallel to the prevailing wind, not across it.
Flowchart
Fig. 10.A5 Caption: What the wind removes in one place, it builds into a landform in another. Nodes: wind blows over dry, bare desert surface → erosion by deflation: loose sand and pebbles lifted and blown away → land surface lowered → deflation hollows; deposition: wind slows and drops its sand → sand dunes → branch: sand piles across the wind at an obstacle → barchan, crescent-shaped, horns pointing downwind; sand grows along a steady prevailing wind → seif, long parallel ridge → dunes migrate downwind → back to sand being lifted from the surface
10.5 Summary
- Denudation is the process of breaking and removing the rocks from the surface of the earth. It lowers the level of the land, rounds exposed rock surfaces and levels the peaks.
- Denudation is not one process but several working together.
- Weathering is the disintegration of rocks by atmospheric agents at or near the surface of the earth, brought on by changes in temperature, moisture and precipitation.
- Erosion is the displacement of rocks by agents like wind, water or ice, and the particles produced are carried to another place.
- Mass movement, or mass wasting, is the large-scale movement of loose rock-waste down a slope on account of gravity, and the steeper the slope, the more rapid the movement.
- Transportation is the movement of material loosened by erosion to another place by the action of wind or water.
- Deposition is the laying down of material that has been weathered, eroded and transported by natural processes such as water, wind and ice.
- Weathering and erosion differ in one thing only — movement. In weathering the broken material stays where it is; in erosion it is taken away.
- The four active agents of denudation are water, wind, waves and glacial ice. This chapter studies the work of rivers and of wind.
- Rivers are the greatest agents of erosion, transportation and deposition, and the force of a river’s erosion depends on the gradient of its course, the volume of water, the load of sediments, the velocity of water and the water discharge.
- Three factors decide what work a river actually does at a given place.
- Erosion and transportation are proportionate to the velocity of water, while deposition takes place when velocity is low.
- A larger volume of water gives greater power of erosion and transportation, while deposition is favoured by reduced volume and a greater load.
- The load of a river does all the work of erosion and deposition, and the deepening of valleys and the forming of landforms depend on it.
- The place of origin of a river is its source and the place where it enters the sea is its mouth. Streams that join the main river are its tributaries, and channels into which it divides are its distributaries.
- The path along which a river flows is its course, and from source to mouth it is divided into Upper, Middle and Lower Course, together called the Profile of a River.
- In the upper course the river is young, gradients are steep, the flow is swift, and the dominant activity is erosion.
- A V-shaped valley is a narrow valley formed by down cutting, with steep slopes, developing almost vertically into an I-shape in regions of hard rocks.
- A waterfall is a vertical fall of water of enormous volume from a great height, formed where rates of erosion differ greatly; smaller differences give rapids, and soft rock at the base is cut into a plunge pool.
- In the middle course the river is in the maturity stage, the gradient is reduced, lateral cutting is more active than down cutting, and the volume of water increases as tributaries join.
- Deposition matters more than erosion in the middle course, so its landforms are chiefly depositional.
- Meanders are bends of longitudinal courses, formed by erosion and deposition together under a gentle slope and sufficient water.
- When a meander loop grows too circular, the river breaks through it and leaves the cut-off loop as an oxbow lake.
- In the lower course the river is in the old stage, flows sluggishly, and its load-transporting capacity is drastically reduced by the slow flow and by division into many distributaries, so depositional landforms dominate.
- A delta is the triangular formation built where a river drops sediment over a large area near its mouth, resembling the Greek letter Δ. It needs a calm sheltered sea, a large sediment supply and a river of reasonable size, so small rivers do not form deltas.
- Wind is the movement of air over the earth’s surface, and it is the most effective agent of erosion, transportation and deposition in arid regions, where little vegetation or moisture binds the loose surface material.
- Deflation is the lifting and blowing away of loose material, and it lowers the land surface into depressions called deflation hollows.
- Sand dunes are hills of sand built by the movement of winds, either active and moving or fixed and rooted with vegetation, and their shape and size depend on the wind, the sand supply, nearby water features, the vegetation and the nature of the surface.
- Migratory dunes move downwind and damage towns and villages, and their shifting is checked by growing vegetation on the windward slope.
- Two types of dune are seen in the tropical deserts.
- Barchans are moon-shaped live dunes lying across the wind, with a gently sloping convex windward side, a steep concave leeward side, and horns thinning out downwind.
- Seifs, or longitudinal dunes, are long narrow sand-ridges growing parallel to the prevailing wind in the interior parts of deserts.
Flowchart
Fig. 10.A6 Caption: One process, two agents, and the landforms each of them leaves behind. Nodes: denudation → weathering, erosion, mass movement, transportation and deposition → agents: water, wind, waves, glacial ice → branch one, river: upper course, erosion dominant → V-shaped valley, waterfall, rapids, plunge pool → middle course, deposition growing → meander, oxbow lake → lower course, deposition dominant → delta; branch two, wind in arid regions: deflation removes material → deflation hollow; wind slows and drops sand → sand dunes → barchan, seif → material laid down becomes new rock-waste exposed at the surface → back to denudation
What This Chapter Cannot Tell You
The chapter names the landforms but not the clock. Nothing here says how many years a gorge takes to cut, how fast a waterfall retreats upstream, or how far a barchan travels in a season, and those rates differ enormously from one place to another.
The three stages of a river are a model, not a rule. A real river can be sluggish in a stretch of its upper course and cut hard into rock far downstream, and a diagram drawn from source to mouth smooths all of that away.
Two of the four agents of denudation are named and then set aside. The work of waves along coasts and of glacial ice in cold lands shapes a large part of the earth’s surface, and none of it is described here.
10.6 Check Your Concepts
Look at Fig. 10.9. From which side is the wind blowing? Give two features of the dune’s shape that you used to decide.
Using only the heights given in this chapter, work out how much higher Angel Falls is than Jog Falls. Then state one thing this difference in height does not tell you about the two waterfalls.
Write, in order, the steps by which a meander becomes an oxbow lake. Name the work the river is doing at each step.
Seifs in the Thar Desert are confined to its western margin. Explain why this type of dune forms where it does, and say what would have to change about the wind for barchans to form there instead.
State the difference between weathering and denudation.
Give a geographical reason for each of the following:
- Erosion is the dominant activity of a river in its upper course.
- Landforms of deposition are more prominent in the middle course of a river.
- All the rivers do not form deltas.
Describe the three stages of a river.
Give a geographical reason for each of the following:
- The effects of wind erosion are more pronounced in desert areas.
- Migratory sand dunes are uncertain and dangerous.
- Long-rooted trees and sand-holding grasses are grown bordering fields near deserts.
On the outline map of the world mark one of the following in each continent:
- Rivers which form deltas.
- Desert.
10.7 Case-Based Questions
Diagram
Fig. 10.A7 Caption: Purwa Waterfall on the River Tamsa in Madhya Pradesh. Brief: Photograph of a wide waterfall dropping in several sheets over a broken rock ledge into a pool below, with the flat wooded plateau surface visible along the top of the fall.
- Study the photograph of Purwa Waterfall on the River Tamsa in Madhya Pradesh (Fig. 10.A7) and answer the following questions:
- What stage is the River Tamsa in to cause formation of this waterfall? Give reasons to support your answer.
- What other landforms would a river make in this stage?
- What landforms would the River Tamsa make in the next stage of its flow?
- Besides being a tourist attraction, waterfalls are important sources of energy. Elucidate.