Introduction to Drainage Systems
In geomorphology, 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 topography of the land, whether a particular region is dominated by hard or soft rocks, and the gradient of the land. Wherever these channels are ill-defined or choked, flooding is a common phenomenon.
- Drainage: It is the flow of water through well-defined channels.
- Drainage system: It means the networks or the patterns formed by the streams, rivers, and lakes in a particular drainage basin. It is an outcome of geological time period, nature and structure of rocks, topography, slope, amount of
water flowing and the periodicity of the flow.
- Catchment area: A specific area from where water is collected and drained by a river.
- River basins: These are the catchments of large rivers and cover larger areas.
- Watersheds: These are the catchments of small rivulets and rills are. These are smaller in area than basins.
- Drainage basin: It is an area drained by a river and its tributaries. It is a topographic region from which a stream receives runoff, through flow, and groundwater flow.
- Watershed: It is the boundary line separating one drainage basin from the other.
- Perennial stream: It is a stream or river channel that has continuous flow in parts of its stream bed all year round during years of normal rainfall.
- Intermittent stream: It is a stream or river channel which normally cease flowing for weeks or months each year.
- Ephemeral stream: It is a stream or river channel that flows only for hours or days following rainfall.
Division of Indian Drainage System
On the basis of orientations to sea
- Arabian Sea drainage: Around 23 per cent comprising Indus, Narmada, Tapi, Mahi and Periyar systems discharge their waters in the Arabian Sea.
- Bay of Bengal drainage: Nearly 77 per cent of the drainage area consisting of Ganga, Brahmaputra, Mahanadi, Krishna, etc. is oriented towards Bay of Bengal. Large rivers flowing on the Peninsular plateau have their origin in the Western Ghats and discharge their waters in the Bay of Bengal except Narmada and Tapi.
Both systems are separated from one another through Delhi ridge, Aravalis and Sahyadris (water divide).
On the basis of size of watershed
- Major river basins: These have more than 20,000 sq. km of catchment area. It includes 14 drainage basins such as the Ganga, the Brahmaputra, the Krishna, the Tapi, the Narmada, the Mahi, the Pennar, the Sabarmati, the Barak, etc.
- Medium river basins: These have catchment area between 2,000-20,000 sq. km incorporating
44 river basins such as the Kalindi, the Periyar, the Meghna, etc.
- Minor river basins: These have catchment area of less than 2,000 sq. km
include fairly good number of rivers flowing in
the area of low rainfall.
On the basis of the mode of origin, nature and characteristics
- Himalayan drainage: It mainly includes the Ganga, the Indus and the Brahmaputra river basins.
- Peninsular drainage: It is older than the Himalayan one and Mahanadi, Godavari, Krishna and Kaveri systems are the important ones.
Although this scheme has the problem of including Chambal, Betwa, Son, etc. which are much older in age and origin than other rivers that have their origin in the Himalayas, it is the most accepted basis of classification.
Sequent and Insequent drainage system
Sequent Drainage Systems
The streams which follow the regional slopes are called sequent streams.
- Consequent Stream: These are the first streams to be originated in a particular region. These streams have their courses in accordance with the initial slope of land surface. It means, the consequent streams follow the regional slope. Most of the streams draining the coastal plains of India are the examples of consequent streams.
- Subsequent Stream: When the master consequent stream is joined by its tribuatary at right angles it is called subsequent stream. River Son - a tributary of the Ganga is a subsequent stream.
- Obsequent Stream: It is the stream which flows down the slope opposite to master consequent stream. For example: Several streams originating from the northern slopes of the Siwalik Range drain due northward to join east-west subsequent streams of the southward flowing master consequents of the Ganga and the Yamuna
- Resequent Stream: Such stream follows the direction of master consequent stream that meets the subsequent stream at right angles.
Insequent Drainage System
The streams which do not follow the regional slopes and drain across the geological structure are called insequent or inconsequent streams.
- Antecedent Drainage: The stream which originated before the upliftment of the surface on which they flow. For example, Indus, Sutlej and Brahmaputra are antecedent rivers as they originated before the upliftment of Himalayan Range and hence create deep Gorge, along the Mountain Range.
- Superimposed Drainage: It is formed when the nature and characteristics of the valley and flow direction of a consequent stream develop on the upper geological formulation and structure are superimposed on the lower geological formation of the entirely different characteristics. For example, river Subarnarekha is superimposed on Dalma hills on the west of Chandil in the Chhotanagpur plateau region of Jharkhand.
Accordant drainage patterns
A drainage system is described as accordant if its pattern correlates to the structure and relief of the landscape over which it flows.
- A pattern of drainage which is branching, ramifying or dichotomising, thereby giving the appearance of a tree.
- Most of the rivers of the Indo-Gangetic Plains are of dendritic type.
- The drainage pattern in which the rivers flow almost parallel to each other.
- The small and swift rivers originating in the Western Ghats and discharging their water into the Arabian Sea creates a parallel drainage pattern in India.
- It is a rectangular pattern formed where two sets of structural controls occurs at right angles. In a trellis pattern, the river forms a net like system and the tributaries flow roughly parallel to each other.
- The old folded mountains of the Singhbhum (Chotanagpur Plateau) have drainage of trellis pattern
- This drainage pattern is marked by right-angled bends and right-angled junctions between tributaries and the main stream.
- It differs from the trellis pattern in so far as it is more irregular and its tributary streams are neither as long, nor parallel as in trellis drainage.
- A typical example of this pattern is found is the Vindhyan Mountains of India.
- It is a pattern characterised by out flowing 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.
- Rivers like Narmada, Son and Mahanadi originating from Amarkantak Hills flow in different directions and are good examples of radial pattern.
- This pattern is also found in the Girnar Hills (Kathiwar, Gujarat), and Mikir Hills of Assam.
- This is an uncoordinated pattern of drainage characteristic of a region recently vacated by an ice-sheet.
- This is probably due to the irregularities produced by glacially deposited materials, e.g., Kame and Kettle, and by the fact that there has been insufficient time for the drainage to become adjusted to the structures of the solid rock underlying the glacial drift. The picture is one of numerous water courses, lakes and marshes; some inter-connected and some in local drainage basins of their own.
- This type of drainage is found in the glaciated valleys of Karakoram.
- The subsequent streams follow curving or actuate courses prior to joining the consequent stream. This results from a partial adaptation to an underground circular structure; a dome like igneous intrusion (batholith). The subsequent streams find it easier to erode the concentric, less resistant strata.
- This is not a very common drainage pattern in India. Some examples of this are however found in Pithoragarh (Uttarakhand), Nilgiri Hills in Tamil Nadu and Kerala.
- A pattern of drainage in which the confluence of a tributary with the main river is characterised by a discordant junction—as if the tributary intends to flow upstream and not downstream. This pattern is the result of capture of the main river which completely reverses its direction of flow, while the tributaries continue to point in the direction of former flow.
- The Arun River (Nepal), a tributary of the Kosi is an interesting example of barbed drainage pattern.
Discordant Drainage Patterns
A drainage pattern is described as discordant if it does not correlate to the topography and geology of the area. Discordant drainage patterns are classified into two main types: antecedent and superimposed.
- An antecedent stream is a stream that maintains its original course and pattern despite the changes in underlying rock topography.
- Here, a river's vertical incision ability matches that of land uplift due to tectonic forces.
- Here, stream retains its course and pattern as it proceeds to erode the underlying rocks despite their different character.
- Superimposed drainage develops differently: initially, a drainage system develops on a surface composed of 'younger' rocks, but due to denudative activities this surface of younger rocks is removed and the river continues to flow over a seemingly new surface, but one in fact made up of rocks of old geological formation.