Rivers: Landforms Flashcards
What are the erosional landforms that rivers can form?
- Valleys
- Waterfalls
- Gorges
Define a valley
A long deepening on land surface (that a river can run through)
How does a V-shaped valley form?
- 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
What are the two ways a waterfall can form?
- By rocks of different resistance
- By faulting
How do wide valleys form?
- 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)
Define a waterfall
A steep plunge of water over a change of great height along a river
Outline the process of a waterfall’s and its relevant features’ formation by rocks of different resistance
- River flows over rocks of different resistance
- Less resistant rocks will get eroded faster (via hydaulic action/abrasion), causing a change in gradient
- Overtime, the river undercuts the less resistant rock, causing water to plunge from great height, forming a waterfall.
- The repeated pounding of water leaves a depression at the base of the waterfall called a plunge pool
- 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
Outline the process of a waterfall’s and its relevant features’ formation by faulting
- River flows over rocks before displacement
- During faulting, rocks are displaced along a fault line, with one layer being higher than the other
- When the river flows over, the water plunges from great height, forming a waterfall.
- The repeated pounding of water leaves a depression at the base of the waterfall called a plunge pool
- 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
Define a gorge
A deep and narrow valley, with steep sided walls
Where is a gorge commonly found and why?
In the upper course due to the steep gradient-> lateral erosion
How does a gorge form?
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)
What are the landforms formed by a combination of river processes?
- Meanders
- Oxbow lakes
Define a meander
Bends in rivers
Where are meanders commonly found?
Lower and middle course (gentler gradient)
Where are the concave and convex banks of a meander found?
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)
What are the landforms associated with a meander?
- River cliff
- Slip off slope
How does a meander and its associated landforms formed?
- Rivers erode laterally more than vertically
- As erosion and deposition occur on the concave and convex banks, the meander is more pronounced and is separated by a narrow neck
- The river starts to slow down to form bends; a meander
- As rivers flow around a bend, the SOF is higher along the concave bend of the meander (lower friction)
- Hence erosive energy is higher, undercutting occurs (erosion at bottom of wall, hence upper portion collapses) and a river cliff is formed
- As rivers flow around a bend, the SOF is lower along the convex bend of the meander (higher friction)
- 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
Define an oxbow lake
Crescent shaped lake in the concave bend of a river channel
Where are oxbow lakes commonly found?
Lower or middle course (gentler gradient)
Outline the process of an oxbow lake’s formation
- Constant erosion continues along the banks of two consecutive meanders
- The two meanders move closer to each other forming a loop
- Eventually the loop becomes distinct and separated by a narrow neck of land
- The neck becomes narrower with continual erosion and deposition and the two meanders eventually meet
- The river breaks through the neck and flows through a straight channel
- Sediments deposited will dam up the water in the cut off, forming an oxbow lake
Check notes for drawing!!!
What are the depositional landforms a river can form?
- Floodplains and levees
- Deltas
Define a floodplain
Extensive low-lying flat land, built up with deposited materials or alluvium soil after repeated floods
Define a levee
A raised bank along a river that is built up by deposition after repeated floods
Where are floodplains and levees found?
Lower course (very gentle gradient)
Outline the process of the formation of floodplains and levees
- After a heavy downpour, volume of a river increases and overflows its banks, causing a flood
- As the floodwater spreads, the speed of flow drops and materials are deposited on the riverbank and riverbed
- Coarser and heavier materials are deposited first while fine and lighter materials are deposited further away
- After a series of floods, layers of sediments thicken on plains next to the riverbanks, forming floodplains
- Riverbanks become higher doe to deposition of materials; forms levees
- Riverbeds may become elevated due to deposition, thus flooding continues
Check notes for drawings
Define a delta
Flat land area at the river mouth that is heavily deposited with fine sediments
Define alluvium soil
Fertile soil, eg clay
Outline the process of the formation of a delta
- As the river enters the water body at the river mouth, its speed drops
- The energy of the river drops and it starts to deposit materials
- The heavier materials are deposited first while lighter materials are deposited further out in the sea
- Layers of sediments build up and rises above the sea level to develop into an extensive network called a delta
Why do distributaries form?
Deposition in deltas forces a river to split up before entering the sea
What are the conditions for a delta to form?
- Enough load (Enough tributaries, less lakes along the way, active erosion along the river
- Load can settle (Tidal currents are not too strong, sea is not too deep)
