River Landforms

Potholes are rounded depressions caused by vertical erosion in the solid bedrock of the river's upper course.

Pebbles and gravel collect in a depression on the river bed.

As the water flows past, it creates turbulence which causes the sediment to rotate in the hollow.

This causes a type of abrasion called drilling, and the pebbles drill down into the bedrock.

Over time, more pebbles and gravel become trapped in the deepening hollow and the process continues, forming a pothole.

A river often flows over a variety of different rock types as it makes its way from source to mouth.

Tougher, more resistant rocks are less easily eroded than weaker rocks, and they will often form irregular steps in the long profile of a river.

On a small scale, these irregularities may create rapids.

A waterfall is formed by a more pronounced step in the long profile.

Meanders form where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along the strech of a river.

The river channel is deeper in pools, so it has more energy and greater erosive power. Energy is lost as the river flows over riffles because of friction.

The spacing and distane between riffles and pools causes the river's flow to become uneven and the thalweg (max. flow) to be concentrated on one side of the river.

Turbulence increases in and around pools, causing helicoidial flow (corkscrew-like currents in the river) which spiral from bank to bank between pools.

The helicoidial flow causes eroded material to be deposited on the inside of the next bend, where the river loses energy.

The combination of erosion and deposition exaggerates the bends until large meanders are formed.

Oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding.

Deposition dams off the loop, leaving an oxbow lake.

Braided channels are multiple-thread channels.

Channel division is caused by: deposition of sediment within the channel caused by:

- excessive bedload due to poorly vegetated surfaces and abundant coarse debris

- easily eroded banks of gravel and sand, which cause localised overloading of rivers with coarse sediment

- high and variable discharge with high peak flows

- steep channel gradients

Deposition within the channel (aggradation) steepens the channel gradient and restroes the river's competence to transport its load.

Floodplains are the result of both river erosion and deposition.

Lateral erosion and the downstream migration of meanders widen the valley.

Fluvial deposition infills the valley with alluvium. This involves:

Lateral accretion: mainy comprises point bars and channel sedimets, abandoned as the river shifts its course through lateral erosion

Vertical accretion: occurs when floodwater spills out of the channel and spreads acorss the valley floor.

Low ridges of alluvium that run parallel to and flank both sides of the river channels.

They form when water spills out of the channel at bankful and quickly loses energy, so sediment is deposited.

A river loses energy very rapidly when it enters the sea or a lake.

As a result, vast amounts of silt and clay are deposited in a fan shape where the two meet.

Over time, the build-up of deosited material breaks through the water surface and forms news land - a delta.

Often a river flowing across a delta is forced to split into separate channels, called distibutaries. There are four types of delta:

Arcuate: triangular in shape - Nile Delta

Bird's foot: long thin shape - Mississippi Delta

Cuspate: 'V' shape with curving sides - Tiber River Delta

Estuarine: river flows into an estuary that will fill with sediment - Seine River (France)