River Landforms: Erosional and Depositional Features

How rivers shape their valleys

A river can wear away its bed and banks, carry the loosened material and deposit it elsewhere. These processes produce different landforms depending on where erosion occurs and where sediment builds up.

Vertical erosion cuts down into the riverbed and is especially important in upland, upper-course valleys. Lateral erosion wears away the banks, allowing the river to widen its channel and move sideways across its valley. This becomes more important in lowland areas. Deposition occurs when the flow slows and can no longer carry as much sediment. Some landforms are mainly erosional; others depend on both erosion and deposition.

V-shaped valleys: downcutting and changing slopes

In the upper course, vertical erosion deepens the river channel. However, the river does not carve the entire V shape by itself: processes acting on the valley sides also matter.

Weathering breaks down rock on the slopes. Mass movement then carries loosened material downhill into the channel, where the river can transport it away. Together, downcutting and the movement of material from the sides produce a valley with steep slopes and a narrow floor.

Imagine looking across the valley rather than along it: the slopes descend towards the river from both sides, giving the cross-section its characteristic V shape.

Waterfalls and gorges: erosion, collapse and retreat

A waterfall is a sudden drop in the riverbed. One common way it forms is where more resistant rock lies above less resistant rock. The less resistant rock wears away more quickly, so an uneven step develops in the bed.

At the foot of the fall, hydraulic action, the force of moving water, and abrasion, the scraping action of rock fragments carried by the water, erode the softer rock. A deep plunge pool develops, and erosion undercuts the resistant rock above it.

The undercutting leaves an unsupported overhang. Eventually, this collapses into the plunge pool. The fallen fragments provide more material for abrasion, helping to deepen the pool.

This sequence repeats: undercutting creates another overhang, which then collapses. The position of the waterfall therefore moves upstream. Its retreat leaves a narrow, steep-sided gorge downstream. The waterfall and gorge are connected landforms: the gorge records the route along which the waterfall has retreated.

Meanders: erosion and deposition on opposite banks

A meander is a bend in a river. In lowland areas, lateral erosion and deposition allow bends to enlarge and migrate across the valley.

Water flows fastest around the outside bend. Hydraulic action and abrasion erode and undercut this bank. Material collapses into the channel, leaving a steep river cliff and moving the bank outwards.

On the inside bend, the flow is slower. The river deposits sediment, building a gently sloping slip-off slope. Erosion on one bank and deposition on the other gradually change the position and shape of the bend.

A cross-section across a meander is therefore uneven: the channel is deeper beside the eroding outside bank and shallower beside the depositional inside bank.

Ox-bow lakes: cutting off a meander

As a meander becomes more pronounced, erosion on the outside banks can bring neighbouring parts of the channel closer together. The narrow strip of land between them is the meander neck.

During a flood, the river can cut through this neck. Water then follows the shorter, straighter channel rather than travelling around the long loop.

Flow into and out of the old loop slows, so sediment is deposited at its entrances. These deposits eventually seal the loop off from the main channel, leaving a curved ox-bow lake. Erosion creates the shortcut, but deposition completes the separation.

Floodplains: widening a valley and building its surface

A floodplain is a broad, relatively flat area beside a river. Its formation involves both lateral erosion and deposition.

As meanders migrate across the valley, they erode its sides and help widen the valley floor. When river discharge is high enough, water overflows the banks and spreads across this low-lying land.

Outside the channel, increased contact with the land surface creates friction and slows the water. Sediment is deposited across the flooded area. Repeated floods add further layers, gradually building up the floodplain surface.

Levees: raised banks built by floods

Floodwater does not deposit all its sediment in the same place. As it leaves the channel and slows, the larger, heavier particles are deposited first, close to the riverbank. Finer sediment travels further across the floodplain before settling.

Over many floods, the deposits nearest the channel accumulate into raised natural embankments called levees. Picture a cross-section from one side of the valley to the other: a relatively flat floodplain rises to a levee beside the channel, drops into the river, then rises to another levee on the opposite bank.

Floodplains and levees therefore form together, but they have different shapes: the floodplain is the broad, flat surface, while levees are the higher ridges immediately beside the river.

Mainly erosional landforms

• V-shaped valley: vertical erosion deepens the channel; weathering and mass movement bring material down the valley sides; the river removes it.
• Waterfall: less resistant rock erodes faster → plunge pool and undercutting → unsupported resistant-rock overhang collapses.
• Gorge: repeated undercutting and collapse make the waterfall retreat upstream, leaving a narrow, steep-sided valley.

Erosion and deposition together

• Meander: faster flow on the outside bend → erosion and a river cliff. Slower flow on the inside → deposition and a slip-off slope. The bend enlarges and migrates.
• Ox-bow lake: outside-bank erosion narrows the neck → a flood cuts through → water follows the shortcut → deposition seals off the old loop.

Floodplain and levees

• Floodplain: migrating meanders widen the valley floor; floods spread out, slow down and deposit sediment in layers.
• Levees: larger sediment settles beside the channel first; finer sediment travels further. Repeated floods build raised natural banks.

Key distinction: vertical erosion deepens the channel; lateral erosion widens it. Deposition builds landforms from sediment.