Drainage system refers to the origin and development of streams through time. The examples of drainage systems are consequent, subsequent, and antecedent and superimposed streams, etc.
The origin and subsequent evolution of any drainage system in a region are determined and controlled by two main factors:
- The nature of the initial surface and slope and
- The geological structures (e.g. folds, faults)
Types of Drainage System:
Drainage systems are divided into two broad categories based on the adjustment of the streams to the initial surface and geological structures e.g. (1) Sequent Drainage Systems, and (2) Insequent Drainage Systems.
Sequent Drainage System:
In this drainage system, streams follow the regional slope and are well adjusted to the geological structures e.g. consequent and subsequent streams.
(1) Consequent Streams are the first stream to be originated in a particular region, and many streams remain consequent throughout their evolutionary development.
These streams have their courses in accordance with the initial slope of the land surface. In other words, the consequent streams follow the regional slope.
In a region of folded structure consequent streams are formed in the synclinal troughs. These streams also become the master consequent streams of trellis drainage pattern at much later date.
Most of the streams draining The coastal plains of India are examples of consequent streams.
(2) Subsequent Streams develop along zones of structural weakness. They may excavate their channels along an outcrop of weak bedrock, or perhaps follow a fault zone and join the master consequent at an almost right angle.
Insequent Drainage System:
In this drainage system, streams do not follow the regional slope and are not adjusted to the geological structures e.g. antecedent and superimposed streams.
(1) Antecedent Streams are those which are originated before the upliftment of land surface.
If a river has fully developed its valley and course in a region of the almost flat topographic surface.
At a later date, there is upliftment of land area across the river. The land is rising in the form of a ridge with a slow rate of upliftment. This local upliftment of the land rejuvenates the river due to which it deepens its valley with an accelerated rate of downcutting.
If the rate of downcutting equals the rate of uplift the bed of the river valley remains constant and the river maintains its usual flow direction.
The river continued to deepen its valley through active downcutting so long as the upliftment continuous. Thus, the river develops very deep and narrow gorges across the uplifted land area wherein the valley sides are of convex slope and rise almost vertically from the valley floor.
It may be pointed out that tributaries of the master antecedent streams cannot deepen their valleys at par with there master streams and hence their valleys are at a higher level than the valleys of their master streams. Thus, the tributary valleys become hanging valleys.
Examples of antecedent streams are found in almost all of the folded mountains of the world. Many of the major Himalayan rivers are the examples of antecedent streams e.g the Indus, the Satluj, and the Brahmaputra, etc.
(2) Superimposed Streams develop on a non-resistant mass covering a buried, resistant bedrock, and if the base level falls enough, the drainage will cut into the bedrock in the established pattern.
The consequent streams are developed on the almost flat ground surface of the horizontally bedded sedimentary rocks. These streams develop their valleys through vertical erosion (downcutting). Over time, the lower structure is exposed to the river which continues its cutting and extends its valley downward on the lower geological structure and thus the valley developed on the upper structure is superimposed on the lower structure.
The lower structure has to accept the form of the valley already developed on an entirely different structure. Thus, the river maintains the form of its valley, the flow direction and its drainage patterns as usual. Such rivers are called superimposed rivers.
Most of the rivers of the Deccan trap region of Peninsular India is an example of superimposed streams.