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

Rectangular Drainage Pattern

• Rectangular drainage pattern is 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

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

• 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.