Hello steemit friends, in my most recent post I explained on how and where a river deposits. Today I will be writing on the landforms which evolve as a result river (fluvial) deposition. There are four major landforms which result from fluvial deposition and found principally at the lower course of running water. These are alluvial fans, flood plain and associated features, estuary and delta.

Alluvial fans

Tributary flash floods or streams carve deep gullies on the sides hills suddenly losing velocity when they descend to the flat floor of the plain, they deposit their load of coarse materials, spreading them in gently sloping, fan-shaped features known as alluvial fans. The fan thins out towards the edges while the running water divides into several channels or rills on it. As the water in the rills percolates down these deposits, the rills progressively dwindle in size and eventually die of alluvial suffocation.

alluvial fan

Flood plain and associated features

A flood plain or alluvial plain is that part of the lower course of a river valley, generally near the coast, flat or gently sloping and periodically flooded. It is carpeted with thick alluvium deposited by the now tired and lazy meandering river in its old age.
There major processes give rise to a flood plain, these are lateral corrasion, meander sweep, and river aggradation.

Lateral corrasion: it enlarges the meander scar sideways, thereby widening the valley sides. By so doing, it carves out and broadens the valley of the would be flood plain.

Meander sweep: as the meander swing from one side of the valley to the other, they get accentuated and migrate downstream in a process called meander sweep. The sweep further cuts and elongates each individual scar downstream until successive ones coalesce to form a continuous cliff called river cliff or river bluff. It is this cliff or bluff that defines the outer boundaries of the flood plain.

Aggradation: the meander sweep is also accompanied with flooding and massive aggradation, and this accounts for the thick alluvial deposits at the floor of the flood plain.

A flood plain has a number of micro-topographical features associated with it. These features are explained below.

• The lazy river is often chocked with its alluvium, thereby causing it to become braided or divided into many channels. A braided stream is therefore one of the features associated with a flood plain.
  
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• When the flood water recedes and the river shrinks to its normal size, it leaves behind pools of water which eventually get colonised by reeds and turn into swamps and marshes.

• Also, constant low floods deposit alluvium at the immediate banks close to the river. This raises the banks to form a pair of parallel ridge like features on either side of the river. These ridge-like features, normally exposed after the floods have subsided, are called levees.
  
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Estuary and delta

Both features are found at the mouth of rivers where they enter a sea or lake. Where the river enters the water body through one mouth, it is called an estuary, but where it enters through several mouths, the feature is called a delta. Where the river enters a sea, we have a marine estuary or a marine delta as the case may be, but where it enters a lake the feature is correspondingly called lacustrine estuary or a lacustrine delta. A delta is often colonised by vegetation, has cut-off backwater lagoons and its several river mouths are called distributaries.

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Classified by shapes, deltas are of three types; arcuate, bird's foot, and estuarine.

• Arcuate delta: an arcuate delta has many distributaries and the outline of an arc with the typical shape of the Greek letter delta. It is made up of deposits of both coarse and fine grain materials. An example is the Nile and the Niger Deltas.

• Bird's foot delta: a bird's foot delta is shaped like the foot of a bird, with few long distributaries pushin fine materials into a relatively cslm sea.

• Estuarine delta: an estuarine delta is a previously elongated delta which has subsequently been submerged to make it look like an estuary. It however has few islands of deposited materials at the mouth which are remnant of the previous delta.

For the conditions and mode of formation of deltas, the river must carry a large load to deposit. It must also flow with low velocity near its mouth so that the deposits are not pushed too far away into the sea. The sea should be calm and saline as this condition enables the sediments to settle and bind together undisturbed near the river mouth.

Under these conditions, the river deposits its load at its mouth, and in stratified layers or beds. At low tides, the deposits get exposed and force the river to split, first into at least two distributaries. Each distributary pushes its own load further into the sea and similarly splits into other distributaries, while growing deposits cut off parts of the sea behind them to form the backwater lagoons. Meanwhile, the entire delta itself grows and pushes itself seawards.

Reference
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My geomorphology lecture notes.