River Landscapes » River Profiles and Processes

The long profile of a river

A river starts at its source, often high up in the hills. It ends at its mouth, where it flows into a sea or lake. The river runs in a channel, which sits in a larger valley.

Imagine slicing along the river from source to mouth and looking from the side. That view is the long profile. It is a smooth curve that starts steep and gets flatter.

• Upper course: near the source. The gradient (how steep the slope is) is steep, so water flows downhill fast.
• Middle course: the slope becomes gentler and the river grows wider.
• Lower course: near the mouth. The gradient is very gentle and the land is low and flat.

Tributaries join the river on the way, so the amount of water in the channel gets bigger as you go downstream.

Key terms:

• Source: where a river begins.
• Mouth: where a river ends, at a sea or lake.
• Channel: the path the water flows along, between its banks and bed.
• Long profile: the gradient of a river from its source to its mouth.
• Cross profile: the shape of the river channel and valley when seen from one side to the other.

The cross profile of a river and its valley

Now imagine slicing across the river and valley, from one side to the other. That view is the cross profile. It changes a lot as the river flows downstream.

In the middle course the valley and channel are in between. The channel gets wider and deeper, the valley sides become gentler and the bed has smaller stones.

Notice the pattern. The river gets bigger downstream, with a wider and deeper channel, a gentler gradient and a flatter valley. The next lesson, Waterfalls, Gorges and Interlocking Spurs, shows the landforms that appear along the way.

How rivers erode

Remember, erosion is the wearing away of the land. Abrasion and attrition work in rivers in a similar way to the coast. Rivers erode in four main ways.

• Hydraulic action: the force of the water alone. Water is forced into cracks in the bed and banks. Air in the cracks is squeezed and the rock is slowly broken apart.
• Abrasion: the river's load (the stones and sand it carries) scrapes along the bed and banks and wears them away.
• Attrition: stones carried by the river knock into each other and break. They become smaller and more rounded as they travel downstream.
• Solution: some rocks, such as limestone and chalk, are dissolved by slightly acidic river water.

Vertical and lateral erosion

Rivers can erode in two directions.

• Vertical erosion: erosion downwards, which deepens the channel. It is strongest in the upper course, where the river has a steep gradient.
• Lateral erosion: erosion sideways, which widens the channel and valley. It is strongest in the middle and lower courses.

This is why the upper valley is a narrow V shape and the lower valley is wide. Early on the river cuts down. Later it cuts sideways.

How rivers transport their load

Rivers carry the material they have eroded. This is transportation. The size of the material decides how it is moved.

• Traction: large boulders are rolled along the river bed.
• Saltation: pebbles and gravel are bounced along the bed in a series of hops.
• Suspension: small particles of silt and clay are carried within the water. This makes a river look muddy.
• Solution: minerals are dissolved in the water. You cannot see them.

A fast, deep river after heavy rain can carry larger stones than a slow one. This is why a flooded river often looks brown and can roll boulders along its bed.

Why rivers deposit sediment

Deposition happens when a river no longer has enough energy to carry its load. Rivers deposit sediment when:

• the water becomes shallower, which slows the river;
• the volume of water falls, for example after a dry period;
• the river carries more sediment than it can transport, for example after heavy erosion upstream;
• the river reaches its mouth, where it slows down as it meets the sea or a lake.

The biggest and heaviest stones are dropped first. The smallest particles are carried the furthest. The landforms made by deposition are covered in Meanders, Ox-bow Lakes, Levees and Flood Plains.

Exam-style question

(a) Describe how the cross profile of a river valley changes from the upper course to the lower course. [2 marks]

(b) Explain why a river deposits sediment near its mouth. [2 marks]

(c) Describe how a river transports large boulders. [1 mark]