Features of Erosion and Deposition

Features of Erosion and Deposition

IGCSE Geography

Learning Objectives:

• Explain the main erosional and depositional features associated with rivers

• Describe the key features of the following:

  • Floodplains

  • Levées

  • Braided channels

  • Deltas

IGCSE Geography

Open Ended

How was Niagara Falls formed?

Features of erosion

Localised erosion by hydraulic action and abrasion especially by large pieces of debris, may lead to the formation of potholes.

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Occurs in the upper course of the river where load is larger and more rugged.​

Hydraulic action - the force of air and water on the sides of rivers and in cracks

Abrasion​ (corrasion) - wearing away of the bed and bank by the reload carried by a river

​River Potholes

Waterfall

an area where water flows over a vertical drop, along the course of a stream or river

Waterfalls occur on horizontally bedded rocks

• Soft rock is undercut by hydraulic action and abrasion

• Weight of the water and the lack of support cause the waterfall to collapse and retreat

Gorge

a narrow cleft with steep, rocky walls

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also called as 'small canyons'​

Waterfalls to Gorges

• ​Over thousands of years, the waterfall may retreat enough to form a gorge of recession

• Where there are small outcrops of hard and soft rock, rapids may develop rather than a waterfall

Gorges and waterfalls Presentation

Meanders

extreme U-bend in the course of a stream, usually occurring in a series

Meanders

• Water does not follow a straight line but takes a curving route

• Speed variation across a river (velocity): fastest on the outside bank (erosion) and slowest on the inside bank​ (deposition)

Meanders

• This produces a steep river cliff on the outer bank of a meander and gentle slip-off slop on its inner bank.

Meanders

Presentation

Oxbow lakes

U-shape lake or pool that forms when a wide meander of a river is cut off, creating a free-standing body of water.

Oxbow lakes

• a result of erosion and deposition

• Lateral erosion - caused by fast flow in the meanders (concentrated on the outer, deeper bank)​

Oxbow lakes

• during times of flooding, erosion, increases

• the river break through and creates a new, steeper channel

• in time, the old meander is closed off by deposition to form an oxbow lake​

oxbow lake

Presentation

Floodplains

a flat area of land next to a river or stream

Floodplains

• the area covered by water when a river floods is known as its floodplain

• when the river discharge exceeds the capacity of the channel, the water rises over the river bank and floods surrounding low-lying area​

Floodplains

• sometimes a floodplain will itself eroded following a fall in the sea level

• when this happens, the remnants of the old floodplain are left behind as river terraces​

Floodplains

• floodplain terraces are useful for settlement as they are above new level of the floodplain and are free from flooding

floodplains

Presentation

Levées

a natural wall that blocks water from going where we don't want it to go

Levées

• when the river floods its speed is reduced, slowed down by friction caused by contact with the floodplain

Levées

• with velocity is reduced the river has to deposit some of its load

• coarser and heavier ​materials (sand and gravel) are deposited first to form raised banks (levées) at the edge of the river

levées

Presentation

Braided channels

consists of a network of river channels separated by small, often temporary, islands called braid bars (aits or eyots)

braided channels

• occurs when a river transports a very heavy load in relation to its velocity

• common in rivers that experience seasonal variations

braided channels

• when the discharge falls, its competence and capacity are reduced

• this forces the river to deposit large amounts of its load and multi-channels, or braided channels, are formed​

braided channels

Presentation

Deltas

landforms created by deposition of sedimen that is carried by a river as the flow leaves it mouth and enter slower-moving or stagnant water​

Deltas

• for deltas to be formed, a river needs to:

  • carry a large volume of sediment

  • enter a still body of water which causes the velocity to fall (water loses its capacity and competence); the heaviest particles deposited first and lighest last

Deltas

• deposition is increased if the water is salty, as this causes salt particles to group together, become heavier, and be deposited

• vegetation also increases the rate of deposition by losing down the water​

Deltas

Presentation