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Rivers and Drainage Basins » River Erosion, Transportation and Deposition

What you'll learn this session

Study time: 30 minutes

Cambridge spec: 1.1.5

The four ways a river erodes: hydraulic action, abrasion, attrition and solution
The difference between vertical and lateral erosion
The four ways a river transports its load, and how particle size and energy decide which one happens
When and why a river deposits its load

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A river is a moving machine

A river does three jobs all the time. It wears away its bed and banks (erosion), it carries material along (transportation) and it drops material (deposition). Which job wins depends on the river's energy. A fast, powerful river erodes and carries. A slow river drops what it is carrying.

Remember, the Bradshaw model showed that rivers change downstream. This lesson explains the processes behind those changes.

Erosion: wearing things away

Key terms:

Erosion: the wearing away of the river bed and banks.
Hydraulic action: the force of moving water pushing into cracks and breaking rock away.
Abrasion: the load scraping and hitting the bed and banks and wearing them away.
Attrition: pieces of load knocking against each other and breaking into smaller, rounder pieces.
Solution (erosion): soluble rock dissolved by the river water.

There are four erosion processes. Two attack the bed and banks, one breaks up the load, and one is chemical.

🌊 Hydraulic action

Water is forced into cracks in the bed and banks. Air in the cracks is squeezed, and the pressure widens the crack until pieces break off. It attacks the bed and banks.

🔨 Abrasion

The river picks up rocks and scrapes and hits them against the bed and banks, like sandpaper. It attacks the bed and banks.

⚡ Attrition

Pieces of load knock against each other. The edges are knocked off, so the load becomes smaller and rounder. It acts on the load, not on the bed and banks.

🧪 Solution

Some rocks, such as limestone and chalk, are dissolved by slightly acidic river water. This is a chemical process. It attacks the bed and banks and can dissolve the load.

Vertical and lateral erosion

Erosion can cut in two directions.

Vertical erosion: erosion downwards, which deepens the river channel.
Lateral erosion: erosion sideways, which widens the channel and the valley.

Near the source the gradient is steep, so the river cuts downwards. Further downstream the gradient is gentler and the river is bigger, so more of its energy goes into cutting sideways.

Transportation: carrying the load

A river carries its load in four ways. The way depends on the size of the particle and on the river's energy. A bigger, faster river can carry bigger particles.

Process	What happens	Size of load
Traction	Rolled or dragged along the bed	Largest: boulders and large stones
Saltation	Bounced along the bed in short hops	Medium: pebbles, gravel and coarse sand
Suspension	Carried along within the flowing water	Smallest: fine sand, silt and clay
Solution	Dissolved in the water, so it cannot be seen	Dissolved minerals

Key terms:

Transportation: the movement of the load by the river.
Traction: large particles rolled along the river bed.
Saltation: medium particles bounced along the river bed.
Suspension: small particles carried within the water.
Solution (transport): minerals dissolved in the water and carried invisibly.

Energy changes the size

In a flood the river has much more energy, so it can move much bigger particles. Stones that sat still for months can start to roll. When the river slows down again, the biggest particles stop first.

Deposition: dropping the load

Deposition is when a river drops its load. This happens when the river loses velocity and so loses energy, and can no longer carry the material. The biggest, heaviest particles are dropped first and the smallest last.

Rivers lose velocity and deposit in these places:

The inside bend of a meander: the water is shallow and slow here.
Where the river enters a lake or the sea: the water spreads out and slows down.
After a flood: when the water level falls, discharge and velocity drop.
In dry weather: low discharge means low energy.

Where is each process most important?

Course	What dominates
Upper course	Vertical erosion by hydraulic action and abrasion. Large, angular load moved by traction.
Middle course	Lateral erosion increases. More saltation and suspension. Some deposition.
Lower course	Lateral erosion and deposition. The load is small and rounded after attrition, and is carried mostly in suspension and solution.

The landforms these processes build are covered in the next two lessons, Upper Course River Landforms and Lower Course River Landforms.

Common mistakes

1. Mixing up abrasion and attrition. Abrasion is the load wearing away the bed and banks. Attrition is the load wearing itself down. 2. Writing that a river transports "by erosion". Erosion wears material away. Transportation carries it. 3. Saying deposition happens because the river has too much energy. It happens because the river has too little.

Exam-style question

Fig. 1.1 shows made-up measurements of the load on a river at three sites.

Site	Distance from source (km)	Average pebble length (cm)	Shape of pebbles
A	2	18	Very angular
B	15	7	Slightly rounded
C	40	2	Well rounded

(a) Describe how the pebbles change between Site A and Site C. [2 marks]

(b) Explain why the pebbles change in this way. [3 marks]

Model answer

(a) The pebbles get smaller, from 18 cm to 2 cm (1). They also become more rounded, from very angular to well rounded (1).

(b) The pebbles knock against each other and against the bed as they are carried (1). This is attrition, which breaks them into smaller pieces (1) and knocks off the edges so they become rounder (1).

(c) Top level (5-6 marks): A river transports its load by traction, where large boulders are rolled along the bed, by saltation, where pebbles bounce along the bed, by suspension, where fine silt and clay are carried in the water, and by solution, where minerals are dissolved. The size of particle carried depends on the energy of the river, so in a flood the river can move bigger particles. Deposition happens when the river loses velocity and energy, so it can no longer carry its load. This happens on the inside of a bend where the water is slow, where the river enters the sea or a lake, and after a flood when the water level falls. The heaviest particles are dropped first. A lower-level answer (1-2 marks) would only name one or two processes with no link to particle size or velocity.

Exam tip

In part (c), name all four transport processes, link traction, saltation and suspension to particle size, and say that solution carries dissolved minerals. Then give two places where deposition happens and say why the velocity falls there.

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