Drainage Patterns: Concordant, Discordant for UPSC

Drainage Systems (Drainage Patterns)

Drainage systems, also known as river systems, are the patterns formed by the streams, rivers, and lakes in a particular drainage basin.

They are governed by tectonic irregularity, nature of underlying rock strata, and the gradient of the land.

Based on the correlation between the topology and the direction of flow, drainage patterns are classified into concordant drainage and discordant or insequent drainage.


1. Concordant drainage patterns

A drainage pattern is described as concordant if it correlates to the topology and geology of the area.

In simple words, in a concordant drainage pattern, the path of the river is highly dependent on the slope of the river and topography.

Concordant drainage patterns are the most commonly found drainage patterns and are classified into many consequent, subsequent, obsequent and resequent.

# Consequent Rivers

The rivers which follow the general direction of the slope are known as the consequent rivers.

Most of the rivers of peninsular India are consequent rivers.

For example, rivers like the Godavari, Krishna and Cauvery, descending from the Western Ghats and flowing into the Bay of Bengal, are some of the consequent rivers of Peninsular India.

# Subsequent Rivers

A tributary stream that is formed by headward erosion along an underlying rock after the main drainage pattern (consequent river) has been established is known as a subsequent river.

The Chambal, Sind, Ken, Betwa, Tons and Son meet the Yamuna and the Ganga at right angles. They are the subsequent drainage of the Ganga drainage system.

# Obsequent Rivers

After the valley development of consequent and subsequent rivers, obsequent rivers may form at right angles to the subsequent rivers and flow opposite to the direction of flow of the original consequent river.

#Resequent Rivers

A resequent river flows in the same direction as that of the initial consequent drainage.
Resequent rivers originate at a much later stage (hence they are called resequent) in comparison to the master consequent rivers.


2. Discordant or Insequent drainage patterns

A drainage pattern is described as discordant if it does not correlate to the topology (surface relief features) and geology of the area.

In simple words, in a discordant drainage pattern, the river follows its initial path irrespective of the changes in topography.
Discordant drainage patterns are classified into two main types: antecedent and superimposed.

Usually, rivers in both these drainage types flow through a highly sloping surface.



A.) Antecedent Drainage or Inconsequent Drainage

A part of a river slope and the surrounding area gets uplifted, and the river sticks to its original slope, cutting through the uplifted portion like a saw (vertical erosion) and forming deep gorges. This type of drainage is called antecedent drainage.

Example: Indus, Sutlej, Brahmaputra and other Himalayan rivers that are older than the Himalayas themselves. There are usually called antecedent rivers (rivers older than the existing land itself).



B.) Superimposed or Epigenetic (Discordant) or Superinduced Drainage

When a river flowing over a softer rock stratum reaches the harder basal rocks but continues to follow the initial slope, it seems to have no relation with the harder rock bed. This type of drainage is called superimposed drainage.

Explanation
Usually, the drainage patterns (dendritic, trellis, etc.) are strongly influenced by the hardness and softness of the rock and patterns of faults or fractures.

Sometimes, however, the land rises rapidly relative to the base level of the stream. This increases the gradient of the stream and therefore, gives the stream more erosive power.

The stream has enough erosive power that it cuts its way through any bedrock, maintaining its former drainage pattern.

You get a situation, then, where the drainage pattern does not correspond to the hardness or softness of the bedrock or the locations of faults and fractures.

In other words, it is a drainage pattern which exhibits discordance with the underlying rock structure because it originally developed on a cover of rocks that has now disappeared due to denudation.

Consequently, river directions relate to the former cover rocks and, as the latter was being eroded, the rivers have been able to retain their courses unaffected by the newly exposed structures.

The stream pattern is thus superposed on or placed on structural features that were previously buried.

The Damodar, the Subarnarekha, the Chambal, the Banas and the rivers flowing at the Rewa Plateau present some good examples of superimposed drainage.
[In simple words, the river flow becomes independent of present Topography. It flows in its initial paths without being influenced by changing topography].

Antecedent Drainage: cuts through the newly formed landform and maintains the same path. E.g. Himalayan Rivers.

Superimposed Drainage: cuts deeper through the existing landform and maintains the same path. E.g. some medium-scale rivers of the Northern and Eastern peninsular India.

Antecedent Drainage: The soil formed is weak (mostly weak sediments), and the rivers easily erode it.

Superimposed Drainage: The rivers have high erosive power so that they can cut through the underlying strata.