Fluvial/River Landform: Depositional

Introduction:

Fluvial Landforms are created by erosional river activities. They are associated with the movement and energy of rivers and streams. These landforms are small to medium tracts. The two types of landforms created by Fluvial Landform are:

Fluvial Erosional Landforms & Fluvial Depositional Landforms

• Erosional landforms are formed due to the removal and transport of sediment due to fluvial processes. 
• Water consists of 71% of the total surface area of the earth. The running water becomes the most special agent that leads to the running away of the earth’s surface through erosion. The different ways in which fluvial erosion takes place are:

1. Corrosion or Abrasion: It is a process in which erosion takes place by mechanical grinding of a river’s coarse material, wearing the rocks down. Corrosions are of 2 types – Lateral Corrosion & Vertical Corrosion.
2. Attrition: The type of Fluvial erosion in which bedload is eroded by itself due to wear and tear of the transported material when they roll and collide into one another.
3. Hydraulic Action: A mechanical process in which the moving water current flows against the banks and bed of a river. Water enters into cracks after getting splashed against the river banks. The river water picks up the loose fragments from its banks and transports them away to the different parts.
4. Corrosion: The chemical action of water on rocks, which comes in contact with the river is known as corrosion.

River Erosion usually takes place in 3 ways:

1. Headward Erosion: erosion at the origin of a stream channel
2. Vertical Erosion: erosion of the base of a stream
3. Lateral Erosion: erosion of the walls of a stream

Three distinct ages of a stream are:

Youth, Mature & Old

Youth: The valley developed is deep, narrow and distinctly V-shaped with no floodplains or with very narrow floodplains.

• Down-cutting predominates over lateral corrosion
• Its characteristic features are Steep-sided “V-shaped’ valleys, waterfalls, and rapids
• Canyons, gorges, waterfalls, etc. are examples of features developed in this stage
• E.g. Rivers flowing in the Himalayas

Mature: They are lower-energy systems. Due to an active erosion of the banks, the valleys are still V-shaped but wide and deep.

• Erosion occurs generally on the outside of bends. Thus, they create meanders in the soft alluvium of the river plain
• Typically, on the inside of bends and river beds, deposition takes place
• Disappearance of swamps and marshes of a younger stage at flat and broad inter-stream areas take place. The stream divides get sharper

• Waterfalls and rapids disappear
• Characteristic features of this stage are meander and slip-off slopes

E.g. Rivers flowing in the Indo-Gangetic-Brahmaputra plain

Old: Sediment is deposited as the velocity of the river slows, at the mouth of the river.

• Only a few smaller tributaries are left during old age with gentle gradients
• Streams meander freely over vast floodplains. With lakes, swamps and marshes; divides are broad and flat
• Depositional features predominate in this stage
• Characteristic features of this stage are floodplains, natural levees, oxbow lakes and Delta etc
• The formation of the delta takes place when the actual mouth of the river moves away from the source into the sea or lake

• E.g. Ganga-Brahmaputra delta

Landforms created by river

River Valleys:

• A river valley is defined as the extended depression on the ground through which a stream flows throughout its course
• Depending upon the different stages of the erosional cycle, the valley obtains different profiles
• Valleys start as small and narrow rolls but will convert into long and wide gullies upon development
• These gullies will further increase in depth, width and length to give rise to valleys
• Based upon the dimensions and the shape they acquire, valleys like the V-shaped valley, gorge, canyon, etc. can be acknowledged

Potholes and Plunge Pools:

Potholes are circular depressions formed over the rocky beds of hill-streams. These are formed because of stream erosion aided by the corrosion of rock fragments.

• Once a small depression is formed, pebbles and boulders get collected in those depressions and get rotated by flowing water. In this way, the depressions grow in dimensions, consequently
• Depressions are joined which leads to deepening of the stream valley
• Large potholes are formed at the foot of waterfalls also because of the sheer impact of water and rotation of boulders. Plunge pools can be defined as the deep and large holes at the base of waterfalls
• These plunge pools are the reason behind the deepening of valleys

Incised or Entrenched Meanders:

These are found cut in hard rocks which are very deep, and created when there is a rapid cutting of the river bed such that the river does not erode to the lateral sides. 

• They are widened and deepened over a long period of time and can be found as deep gorges and canyons in the areas where hard rocks are found
• They give an indication of the status of original land surfaces over which streams have developed
• Incised meanders are an impact of river rejuvenation

River Terraces:

River terraces are defined as surfaces relating to old valley floor or floodplain levels.

• They can be found as bedrock surfaces without any alluvial cover. They also may be alluvial terraces having stream deposits
• River terraces are formed due to vertical erosion by the stream into its own depositional floodplain so they are also known as a product of erosion
• There can be so many terraces. They are found at different altitudes indicating former river bed levels
• The river terraces may form at the same height on both sides of the rivers

Drainage Basin:

• The terms that are used to describe drainage basins are catchment, catchment area, catchment basin, drainage area, river basin, and water basin
• Streams and rivers and the land surface, all are included in the drainage basin
• The drainage basin collects all the water within the basin and directs it to a single point. That is why it is also referred to as a funnel
• In closed drainage basins, water converges to a single point inside the basin, that particular point is known as a sink
• Examples of permanent lakes are Lake Aral & Dead Sea; dry lake (some desert lakes like Lake Chad, Africa), or sinkholes in Karst landforms (the point where surface water is lost underground)

Peneplain:

• This is formed due to erosion of the stream
• A peneplain is a low-relief plain which is also called an almost plain
• The peneplain is said to be the representation of a penultimate stage
• It is also known as the end product of an erosional cycle

Drainage Patterns:

  The shape of a river course when it completes one erosional cycle is known as the drainage pattern of the stream. The various types of drainage patterns are:

Dendritic drainage pattern; Trellis drainage pattern; Parallel drainage pattern; Rectangular drainage pattern; Aerial drainage pattern; Radial  drainage pattern; Centripetal drainage pattern;  Annular  drainage pattern.

Conclusion:

Fluvial landforms are formed due to erosional activities of the river. They are of the following 2 types: Fluvial erosional landforms & fluvial depositional landforms. The different ways in which fluvial erosion can take place are Corrosion, Attrition, Hydraulic action & Corrosion. Three distinct ages of a stream are Youth, Mature & Old. Some of the major landforms formed as a result of fluvial erosion are:

River Valleys, Potholes and Plunge Pools, River Terraces, Drainage basin, Peneplain.