River deposition (left bank). Image credit Panoramio.
ZIMSEC O Level Geography Notes: River deposition
Deposition occurs when a river no longer as sufficient energy to transport its load.
When its velocity begins to fall and has less energy, a river’s competence (maximum size of material which a river is capable of transporting) and capacity ( maximum amount of load that a river is capable of transporting) falls and therefore deposition begins.
Deposition occurs when:
Discharge is reduced after a period of low precipitation.
Velocity is reduced upon the river reaching the dam, lake,sea or ocean resulting in the formation of deltas.
Shallow water occurs on the inside section of a meander for example.
The load is suddenly increased for example in the event of a landslide for instance when a portion of bank collapses into the river.
When the river overflows its banks so that the velocity outside the channel is reduced resulting in the formation of a floodplain.
During floods, especially in the lower course rivers spread to the sides of the channel.
Frictional drag and the reduced gradient slow down the flowing water resulting in deposition.
Deposition occurs along the entire course of the river:
On the channel bed.
The river valley floor especially during floods.
On the river’s banks as in a meander.
At the river’s mouth when it empties into the sea.
NB Deposition occurs at any part of a river’s course depending on a river’s energy and velocity. The division of a river into stages is therefore useful but by no means conclusive.
When the river loses its energy to any of the reasons pointed out above the following happens.
The heaviest material/load is deposited first this is why rivers are littered with boulders in the upper course.
This is because traction load and siltation loads require more energy to transport.
The finest material is deposited last and may reach the sea where it is deposited onto and to form deltas.
The dissolved load which is in solution water is deposited at all but transported to the sea where it maintains the saltiness of oceans.
The deposition of sand and silt leads to the development of a gently sloping plain known as a flood plain.
Deposition can result in aggredation where the river’s bed and gradient are increased. This can happen at deltas and on alluvial fans.
To learn more about the features formed by river erosion and to access more topics go to the Geography Notes page.