Drainage refers to the flow of water through a definite channel. Drainage system tells us about origin and evolution of rivers and their tributaries (Antecedent, Subsequent, Sequent etc). Drainage pattern tells us about geometrical arrangement/shape which major rivers and its tributaries make (Dendritic, trellis, radial etc.).
It is related to period of formation and their relation to original slope, underlying bedrock & structures. Genetically (based on genesis or origin), streams have been divided into following types:
SEQUENT DRAINAGE SYSTEM
Those streams which follow regional slope and adjusted to local geological structure.
- Consequent stream (Dip Stream): Streams whose course is a direct consequence of original slope of surface upon which it developed, i.e., streams that follow slope of land over which they originally formed. These are also called dip streams. Ex. Godavari & Kaveri rivers of Peninsular India follow the normal slope of the land and drain into Bay of Bengal.
- Subsequent stream: These streams originated after master consequent stream and following the axis of anticlines or ridges and strikes of beds are called subsequent streams. Ex. Asan River, a tributary of Yamuna River & Song River, a tributary of Ganga River in Dehradun valley (in filled alluvial plain) are examples of subsequent streams while Yamuna and Ganga are master consequents.
- Obsequent stream: A stream that flows in the opposite direction to a consequent stream, often against the direction of dip. They are also consequents because they also follow the slopes of the ranges. Ex. Mahabharat Range of Lesser Himalaya has issued several streams from its northern slopes. These northward flowing streams join subsequent stream Kosi which runs west to east, as obsequent streams because these are opposed to directions of master consequents like Ganga and Yamuna.
- Resequent stream: Tributary streams flowing in the direction of master consequents are called resequents. These are originated at much later date in comparison to master consequents. Since they are of recent origin, and hence they are called resequent.
INSEQUENT DRAINAGE SYSTEM
- The streams which do not follow the regional slopes and drain across the geological structures are called insequent or inconsequent streams.
- Antecedent and superimposed streams are the best representative examples of insequent drainage systems:
- Superimposed stream: Superimposed stream means a river which, flowing on a definite geological formation and structure, has inherited the characteristics of its previous form developed on upper geological formation of entirely different structural characteristics. Ex. Son River flowing across Khainjua ridges in southern part of Rewa plateau (Madhya Pradesh) is a fine example of superimposed river.
- Antecedent stream or Inconsequent Drainage System: Antecedent streams are those which are originated prior to upliftment of land surface. An antecedent stream is a stream that maintains its original course and pattern despite the changes in underlying rock topography. Ex. Indus, Satluj and Brahmaputra rivers have cut transverse gorges across the Himalayan mountains. Uplift of Himalayas increased their gradients, and their increased erosive action was able to maintain their courses across the mountains.