Downstream portion of river: landforms

Downstream portion of river: landforms

River Channel: channel is at its widest and deepest.

River also has largest volume of water and load.

Load consists of gravel, sand grains, silt, clay and dissolved substances.

Gradient of land is very gentle.

Lateral erosion is rampant


River Valley: wide and flat bottomed.

Lateral erosion by river and weathering and erosion helps to widen valley.

Work of river: lateral erosion, especially along the concave banks of meanders.

Main work is deposition.


A) Flood plains and levees

Cross-section of a floodplain



• a floodplain is a gently sloping/level plain built up of successive layers of alluvium deposited by a river in times of flood.

• During heavy rainfall, amount of water in the river channel increases.

• As the river continues to rise, the water will cover any adjacent flat land.



• land susceptible to flooding in this manner is known as the floodplain.

• Once out of the channel, water encounters more friction because of increased wetted perimeter.

• This reduces speed of water, resulting in deposition.



• When a river overflows its bank, the larger and coarser materials will be deposited first.

• This forms a small, natural embankment alongside the channel.

• This embankment is called a levee.

• Smaller sized and finer materials will be deposited further away from the river.



• The thin veneer of silt deposited by each flood increases the fertility of the floodplain.

• Successive flooding causes the floodplain to build up in height.

• The floodplain may also be made up of materials deposited as point bars on the inside of meanders.



Stages in the formation of flood plains and levees



Note: although flood plains and levees are better developed in the lower course of the river, they can also occur between the upper course and lower course.


B) Ox-bow lakes.

• horse-shoed/crescent-shaped lake.


B) Ox-bow lakes.

• river in the lower course meanders widely across the low-lying floodplain.

• Lateral erosion and undercutting occur on the concave banks where stream flow is fastest.


B) Ox-bow lakes.• deposition occurs on the convex banks because of slack water flow.

• Over time, the meander becomes very pronounced (a)

• Two neighbouring concave banks will get closer.


B) Ox-bow lakes.• narrow neck of land between the two neighbouring concave banks will eventually be cut through either by lateral erosion on the concave banks or strong currents during a flood.


B) Ox-bow lakes.• a new straighter river channel is created through which the river now flows.

• An abandoned meander loop called a cut-off is formed (b).


B) Ox-bow lakes.• deposition can now take place next to the river bank.

• When enough deposition has taken place, the cut-off is sealed off from the main river channel.

• An ox-bow lake is now formed. (c)


B) Ox-bow lakes.Note: An ox-bow lake may not be a permanent feature as the water may dry up and it will be then filled with sediments and vegetation.


C) Braided streams

• when river’s level falls rapidly, competence and capacity of the river is compromised.

• Load will be deposited.

• These deposits will obstruct he flow of the river and cause it to split or divide into smaller channels.

• The river will divide into a series of diverging and converging segments.


C) Braided streams


C) Braided streams


C) Deferred Streams

• streams on the flood plain may not be able to join the main river directly because of the presence of levees.

• They end up flowing down the valley and joining the main river further downstream.

• These are known as deferred streams.


C) Deferred Streams


E) Deltas

• flat alluvial depositional plain produced by a river when it enters calm water such as a lake or tideless sea.


E) Deltas

Conditions for formation:

1. Active erosion along the course of the river.

2. River flows across resistant rocks so that the weathered material and eroded rock particles can add on to the load.

3. Tributaries are present to add to the load.


E) Deltas


4. There are few/no large lakes along the river to trap the load.

5. Gentle offshore gradient with shallow water.

6. Coastal waters are calm/tides are weak - no strong currents/waves to remove deposited sediments.


E) Deltas


7. Removal of sediments is slower than deposition.

8. Climate of are favours fast growth of vegetation that binds and stabilises the sediment.


E) Deltas

Formation process:

• When a river enters the sea, it mixes with the surrounding water and its speed is reduced.

• The salty seawater causes the silty particles to aggregate/accumulate into larger particles (flocculation).


E) Deltas

Formation process:

• As these deposits are heavier, they are deposited first at the river’s mouth.

• The main river channel will be silted and blocked up.

• River overflows and splits into many small channels called distributaries.


E) Deltas

Formation process:

• As layers upon layers of alluvial materials are deposited, a platform of alluvium is built up and rises above the water.

• This flat alluvium is called delta.


E) Deltas

Formation process:


E) Deltas

Formation process:

• The distributaries formed will build up their own levees.

• Vegetation (mangroves) that later grows on the alluvium helps to bind the alluvial deposits together and stabilizes the delta.

• Deltas typically have triangular shapes.


E) Deltas

Formation process:

• As the delta grows larger and seawards, it will eventually merge with the river’s flood plain and no longer possess the appearance of a delta.


E) Deltas

Types of deltas


E) Deltas

Types of deltas: Arcuate delta

• triangular/fan-shaped.

• water enters the sea through many distributaries.

• formed when alluvial deposits are evenly spread out.

• Longshore currents help to form spits and lagoons at the mouths of the distributaries

• Eg.: Nile delta


E) Deltas

Types of deltas: Bird’s foot delta

• has distributaries that extend far into the water.

• Formed when river discharge is high and load is enormous.

• Sediments deposited far exceeds those removed by tides and currents.

• Eg.: Mississippi delta


E) Deltas

Types of deltas: Estuarine delta

• sediments are deposited in a long narrow submerged estuary.

• Delta does not usually grow beyond the general coastline because sediments deposited outside the estuary would be washed away by waves and currents.

• Eg.: Seine delta


References:

Chong, M. (2001) Aspects of Physical Geography, Singapore Pekoe Books.

Waugh, D (2000) Geography – An Integrated Approach, UK: Nelson

Documents

Downstream portion of river: landforms