erosional landforms

Question 1

Waterfall and gorges

Answer 1

A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock).

1. The soft rock is eroded quicker than the hard rock and this creates a step.
2. As erosion continues, the hard rock is undercut forming an overhang.
3. Abrasion and hydraulic action erode to create a plunge pool.
4. Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
5. This process continues and the waterfall retreats upstream.
6. A steep-sided valley is left where the waterfall once was. This is called a gorge.

Question 2

Interlocking spurs

Answer 2

In the upper course there is more vertical erosion. The river cuts down into the valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs of land which link together like the teeth of a zip.

Question 3

vertical erosion

Answer 3

When the land is eroded or worn away in a downwards direction, eg a river that is high up will erode vertically because gravity pulls the water downwards.

Question 4

Meanders

Answer 4

As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.

As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope.
The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).

Question 5

Oxbow lakes

Answer 5

Due to erosion on the outside of a bend and deposition on the inside, the shape of a meander will change over a period of time. Erosion narrows the neck of the land within the meander and as the process continues, the meanders move closer together. When there is a very high discharge (usually during a flood), the river cuts across the neck, taking a new, straighter and shorter route. Deposition will occur to cut off the original meander, leaving a horseshoe-shaped oxbow lake.

Question 6

Floodplain

Answer 6

A floodplain is an area of land which is covered in water when the river bursts its banks.

They form due to both erosion and deposition. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river. During a flood, material being carried by the river is deposited (as the river loses its speed and energy to transport material). Over time, the height of the floodplain increases as material is deposited on either side of the river.

Floodplains are often agricultural land, as the area is very fertile because it’s made up of alluvium (deposited silt from a river flood). The floodplain is often a wide, flat area caused by meanders shifting along the valley.

Question 7

Levees

Answer 7

Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
Sediment that has been eroded further upstream is transported downstream.
When the river floods, the sediment spreads out across the floodplain.
When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.

Question 8

estuary

Answer 8

An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.

Question 9

Deltas

Answer 9

Deltas are found at the mouth of a river, as it enters a sea or lake. The sediments the river has been transporting by bedload and suspended load are deposited as velocity decreases as the river flows into the sea. Deltas are usually made up of three types of deposit:

The larger and heavier sediments are the first to be deposited as the river loses its energy. These form the topset beds.
Medium-sized sediments travel little further before they are deposited, forming the foreset beds.
The finest sediments travel furthest into the lake before deposition and form the bottomset beds.

Question 10

Upper course
Middle course
Lower course

Answer 10

Erosion
Transportation
Deposition