River Processes and Landform Development

How rivers reshape landscapes

A river does more than carry water downhill. It wears away rock and sediment, moves this material downstream and deposits it elsewhere. These are fluvial processes: processes involving flowing river water. Erosion, transportation and deposition work together to change river landscapes in the UK.

Changes happen at different scales. Collisions can alter individual stones, erosion can deepen a plunge pool, and repeated erosion and deposition can reshape an entire valley. Some changes, such as a bank collapsing during high flow, happen quickly. Larger landforms develop through repeated events over much longer periods.

The amount of change depends partly on the river's flow and the resistance of the material it encounters. Discharge is the volume of water passing a point each second; velocity is its speed. Greater discharge and faster flow generally give a river more energy for erosion and transportation, although much of its energy is used in overcoming friction with the bed and banks.

Four ways rivers erode

Erosion is the wearing away and removal of material. Rivers do this through four main processes:

• Hydraulic action: the force of flowing water loosens and removes material from the bed and banks.
• Abrasion: stones and other sediment carried by the river scrape and strike the bed and banks, wearing them away.
• Attrition: pieces of sediment collide with one another during transport. They become smaller, smoother and rounder.
• Solution: river water dissolves soluble rock. Slightly acidic water can gradually remove rock such as limestone.

These processes affect different things. Abrasion wears away the river channel, whereas attrition wears down the material travelling through it. That transported sediment can then become a tool for further abrasion.

Erosion also has a direction. Vertical erosion cuts downwards, deepening the channel and valley. Lateral erosion cuts sideways, widening them. Vertical erosion is usually dominant in the upper course, nearer the river's source; lateral erosion becomes more important in the middle and lower course.

Moving sediment and depositing it

A river's load is the material it transports. Different parts of the load travel in different ways:

• Large stones roll along the bed by traction.
• Smaller particles bounce along the bed by saltation.
• Fine particles, such as silt, are carried within the water by suspension.
• Dissolved material travels within the water by solution.

Solution therefore describes both an erosion process and a transportation process: rock first dissolves, then the dissolved material is carried downstream.

Deposition occurs when the river can no longer transport part of its sediment load. Slower flow has less ability to carry sediment, so material is dropped. This can happen on the inside of a bend, where flow is slower, or when floodwater spreads out across surrounding land. Larger, heavier particles generally settle before finer particles.

Erosion, transport and deposition are connected: material removed from one part of a river landscape can be carried downstream and help build another landform.

Cutting a V-shaped valley

In the upper course, downward erosion deepens the river channel. Hydraulic action and abrasion help the river cut into its bed.

The river does not carve the whole valley shape by itself. Weathering breaks down rock on the valley sides in place. Gravity then moves loosened material downslope through mass movement. Material reaching the channel can be carried away by the river, exposing the slopes to further weathering.

Repeated downcutting, weathering and downslope movement produce a narrow valley with steep sides: a V-shaped valley. Its shape records the combined action of river and slope processes over time.

Waterfalls retreat and leave gorges

A waterfall often develops where a river crosses more resistant rock overlying less resistant rock. The less resistant rock erodes faster, creating a drop in the riverbed.

1. Hydraulic action and abrasion erode the less resistant rock at the foot of the fall, forming a deep plunge pool.
2. Continued erosion undercuts the more resistant rock above, leaving an unsupported overhang.
3. The overhang eventually collapses. Fallen rock can be moved around in the plunge pool, causing further abrasion and deepening it.

As the waterfall retreats, it leaves a narrow, steep-sided gorge downstream. The plunge pool is a smaller feature within the waterfall system; the gorge is the larger landform produced by repeated retreat. A collapse may be sudden, but the gorge develops through many such events.

Meanders: erosion and deposition on the same bend

A meander is a bend in a river. In the middle and lower course, lateral erosion helps bends grow and migrate across the valley.

On the outside of a bend, flow is faster. Hydraulic action and abrasion erode and undercut the bank. The bank can collapse, creating a steep river cliff and moving the outside edge of the bend outwards.

On the inside of the same bend, flow is slower. The river deposits sediment, building a gently sloping slip-off slope. Erosion on one side and deposition on the other gradually shift the channel's position.

The river cliff and slip-off slope are therefore related features, not separate accidents. Their contrasting shapes result from different flow conditions across one bend.

Building a floodplain

A floodplain is the broad, relatively flat land beside a river that can be covered by floodwater. Both lateral erosion and deposition contribute to its development.

As meanders migrate, sideways erosion widens the valley floor. During high discharge, water may overflow the banks and spread across this land. Contact with the land surface increases friction, slowing the water and causing it to deposit sediment called alluvium.

Repeated floods add layers of alluvium, gradually building up the floodplain. One flood can leave a fresh layer of sediment, while repeated flooding and channel migration create a much larger landscape over time. This shows how short-lived events combine to produce long-term landform change.

Erosion and transport

• Hydraulic action: water force removes bed and bank material.
• Abrasion: carried sediment wears away the channel.
• Attrition: sediment collisions make particles smaller and rounder.
• Solution: soluble rock dissolves; dissolved material is transported.