Rivers: Landforms

Question 1

What are the erosional landforms that rivers can form?

Answer 1

1. Valleys
2. Waterfalls
3. Gorges

Question 2

Define a valley

Answer 2

A long deepening on land surface (that a river can run through)

Question 3

How does a V-shaped valley form?

Answer 3

• In the upper course of a river
• High vertical erosion due to steep gradient, hence the land erodes downwards
• The sides of the valley undergo weathering and collapse (slope transport)
• This forms a V-shaped valley

Refer to notes for drawing

Question 4

What are the two ways a waterfall can form?

Answer 4

1. By rocks of different resistance
2. By faulting

Question 4

How do wide valleys form?

Answer 4

• In the middle or lower course of river (lower course valleys are almost completely flat)
• Lateral erosion occurs predominantly as gradient is gentler
• The valley is wider (could be because the river meanders more often, covering greater lateral area)

Question 5

Define a waterfall

Answer 5

A steep plunge of water over a change of great height along a river

Question 6

Outline the process of a waterfall’s and its relevant features’ formation by rocks of different resistance

Answer 6

1. River flows over rocks of different resistance
2. Less resistant rocks will get eroded faster (via hydaulic action/abrasion), causing a change in gradient
3. Overtime, the river undercuts the less resistant rock, causing water to plunge from great height, forming a waterfall.
4. The repeated pounding of water leaves a depression at the base of the waterfall called a plunge pool
5. The overhang eventually collapses due to erosion/weathering and the rock pile swirls in the plunge pool, deepening it by abrasion

Refer to notes for drawing

Question 7

Outline the process of a waterfall’s and its relevant features’ formation by faulting

Answer 7

1. River flows over rocks before displacement
2. During faulting, rocks are displaced along a fault line, with one layer being higher than the other
3. When the river flows over, the water plunges from great height, forming a waterfall.
4. The repeated pounding of water leaves a depression at the base of the waterfall called a plunge pool
5. The overhang eventually collapses due to erosion/weathering and the rock pile swirls in the plunge pool, deepening it by abrasion

Refer to notes for drawing

Question 8

Define a gorge

Answer 8

A deep and narrow valley, with steep sided walls

Question 9

Where is a gorge commonly found and why?

Answer 9

In the upper course due to the steep gradient-> lateral erosion

Question 10

How does a gorge form?

Answer 10

When a river continues to erode through resistant rocks, the walls of the valley remain steep. forming a gorge

Check notes for drawing (reference only)

Question 11

What are the landforms formed by a combination of river processes?

Answer 11

1. Meanders
2. Oxbow lakes

Question 12

Define a meander

Answer 12

Bends in rivers

Question 13

Where are meanders commonly found?

Answer 13

Lower and middle course (gentler gradient)

Question 14

Where are the concave and convex banks of a meander found?

Answer 14

Concave banks are the outer side of the meander (it is concave from the river’s POV)

Convex banks are on the inner side of the meander (it is convex from the river’s POV)

Question 15

What are the landforms associated with a meander?

Answer 15

1. River cliff
2. Slip off slope

Question 16

How does a meander and its associated landforms formed?

Answer 16

1. Rivers erode laterally more than vertically
2. As erosion and deposition occur on the concave and convex banks, the meander is more pronounced and is separated by a narrow neck
3. The river starts to slow down to form bends; a meander
4. As rivers flow around a bend, the SOF is higher along the concave bend of the meander (lower friction)
5. Hence erosive energy is higher, undercutting occurs (erosion at bottom of wall, hence upper portion collapses) and a river cliff is formed
6. As rivers flow around a bend, the SOF is lower along the convex bend of the meander (higher friction)
7. Hence energy is lower and deposition occurs more on the convex bank, forming a slip off slope (gentle slope where water level is lower)

This causes the asymmetrical channel of a river; refer to notes for drawing

Question 17

Define an oxbow lake

Answer 17

Crescent shaped lake in the concave bend of a river channel

Question 18

Where are oxbow lakes commonly found?

Answer 18

Lower or middle course (gentler gradient)

Question 19

Outline the process of an oxbow lake’s formation

Answer 19

1. Constant erosion continues along the banks of two consecutive meanders
2. The two meanders move closer to each other forming a loop
3. Eventually the loop becomes distinct and separated by a narrow neck of land
4. The neck becomes narrower with continual erosion and deposition and the two meanders eventually meet
5. The river breaks through the neck and flows through a straight channel
6. Sediments deposited will dam up the water in the cut off, forming an oxbow lake

Check notes for drawing!!!

Question 20

What are the depositional landforms a river can form?

Answer 20

1. Floodplains and levees
2. Deltas

Question 21

Define a floodplain

Answer 21

Extensive low-lying flat land, built up with deposited materials or alluvium soil after repeated floods

Question 22

Define a levee

Answer 22

A raised bank along a river that is built up by deposition after repeated floods

Question 23

Where are floodplains and levees found?

Answer 23

Lower course (very gentle gradient)

Question 24

Outline the process of the formation of floodplains and levees

Answer 24

1. After a heavy downpour, volume of a river increases and overflows its banks, causing a flood
2. As the floodwater spreads, the speed of flow drops and materials are deposited on the riverbank and riverbed
3. Coarser and heavier materials are deposited first while fine and lighter materials are deposited further away
4. After a series of floods, layers of sediments thicken on plains next to the riverbanks, forming floodplains
5. Riverbanks become higher doe to deposition of materials; forms levees
6. Riverbeds may become elevated due to deposition, thus flooding continues

Check notes for drawings

Question 25

Define a delta

Answer 25

Flat land area at the river mouth that is heavily deposited with fine sediments

Question 25

Define alluvium soil

Answer 25

Fertile soil, eg clay

Question 26

Outline the process of the formation of a delta

Answer 26

1. As the river enters the water body at the river mouth, its speed drops
2. The energy of the river drops and it starts to deposit materials
3. The heavier materials are deposited first while lighter materials are deposited further out in the sea
4. Layers of sediments build up and rises above the sea level to develop into an extensive network called a delta

Question 27

Why do distributaries form?

Answer 27

Deposition in deltas forces a river to split up before entering the sea

Question 28

What are the conditions for a delta to form?

Answer 28

1. Enough load (Enough tributaries, less lakes along the way, active erosion along the river
2. Load can settle (Tidal currents are not too strong, sea is not too deep)