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River Drainage Patterns & Erosion Processes

River Drainage Patterns

Dendritic Drainage Patterns

  • If the rocks are composed of homogeneous beds of uniform resistance to erosion, the tributaries will join the main stream obliquely as insequent streams
  • The drainage pattern so evolved will be tree like in appearance & therefore termed as Dendritic drainage
  • Most of the rivers of the Indo-Gangetic Plains are of dendritic type

Dendritic Drainage Pattern, Rectangular Drainage Pattern

Rectangular Drainage Pattern

  • Rectangular drainage patternis found in regions that have undergone faulting.
  • Rectangular drainage develops on rocks that are of approximately uniform resistance to erosion, but which have two directions of jointing at approximately right angles.
  • The joints are usually less resistant to erosion than the bulk rock so erosion tends to open the joints and streams eventually develop along the joints.
  • The result is a stream system in which streams consist mainly of straight line segments with right angle bends and tributaries join larger streams at right angles.
  • A typical example of this drainage pattern is found is the Vindhyan Mountains of India.

 

Trellis Drainage Pattern

  • Trellis drainage develops in folded topography like that found in the Appalachian Mountains of North America.
  • Rocks are made up of alternate layers of hard & soft rocks, hence tributaries tends to follow the pattern of rock structure
  • A trellis drainage pattern occurs when tributaries join a main stream and erode a valley at approximately right angles in a pattern that resembles a garden trellis.
  • The old folded mountains of the Singhbhum (Chotanagpur Plateau) have a drainage of trellis pattern

Trellis Drainage Pattern, Radial Drainage Pattern

Radial Drainage Pattern

  • It is a pattern characterized by outflowing rivers, away from a central point, analogous with the spokes of a wheel.
  • It tends to develop on the flanks of a dome or a volcanic cone.
  • A good example of a radial drainage pattern is provided by the rivers originating from the Amarkantak Mountain.
  • Rivers like Narmada, Son and Mahanadi originating from Amarkantak Hills flow in different directions and are good examples of radial pattern.
  • Radial drainage patterns are also found/in the Girnar Hills (Kathiawar, Gujarat), and Mikir Hills of Assam

 


Processes of river action

  • When a river flows it carries with it eroded materials which can be divided into 3 distinct types

 

Materials in Solution Minerals which are dissolved in water
Materials in Suspension Suspension of Sand, Silt & Mud in water
The Traction Load Coarser materials such as pebbles, stones, rocks & Boulders

 

  • The ability of river to move the various grades of materials depends greatly on the volume of the water, velocity of the flow & size, shape and weight of the load
  • It is said that by doubling the velocity of a river, its transporting power is increased by more than 10 times.

 


River Erosion & Transportation Processes

Corrasion / Abrasion

  • Mechanical grinding of river’s traction load against the banks & bed of the river.
  • The rock fragments hurdle against the sides as well as bottom of the river leading to lateral & vertical corrasion
  • Lateral corrasion is sideways erosion which widens the V shaped valley.
  • Vertical corrasion is the downward action which deepens the river channel.

 

Corrosion / Solution

  • Chemical action of water on soluble or partly soluble rocks with which river come into contact
  • For ex. in case of Calcium carbonate

 

Hydraulic Action

  • Mechanical loosening & sweeping away of materials by river water
  • Mainly by surging into the crevices & cracks of rocks & disintegrating them

 

Attrition

  • Wear & tear of transported material among them when they roll and collide into one another

 

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