Geography · Ch 3 — Drainage System
Introduction
Introduction
The photograph shows a river in its youthful, mountainous stage. The water is confined within a narrow, V-shaped valley, cutting directly through forested slopes. Hills rise in the background, and the dense vegetation on both banks makes the channel clearly visible as it winds downhill. There are no floodplains, meanders, or wide flat banks — the river is still actively eroding downward, carving its valley rather than depositing sediment across a plain.
This image is the visual anchor for the chapter's opening idea: drainage is the flow of water through well-defined channels. In the mountains, that channel is sharply defined because the river has high energy and steep gradient. The forested valley walls also show why such channels matter — they collect and funnel runoff from the surrounding catchment area. Every drop of rain that falls on those slopes eventually finds its way into this single thread of water.
The photograph also sets up the contrast that runs through the entire chapter. A Himalayan river in its mountainous course behaves very differently from the same river downstream on the northern plain. Here the river is swift, narrow, and erosive; later it will slow, widen, and deposit. So the picture is not just scenery — it is the first evidence that a river's character changes along its length, and that the drainage system of an area is shaped by slope, structure of rocks, and the amount of water flowing.
Water that flows through rivers, nalas and channels during the rainy season is not random — it is excess water being drained away. If these channels did not exist, large-scale flooding would be a regular occurrence. Wherever channels are ill-defined or choked, flooding is indeed a common phenomenon. This is the starting point for understanding what drainage actually means.
The flow of water through well-defined channels is known as drainage, and the network of such channels is called a drainage system. The drainage pattern of any area is not accidental; it is the outcome of several factors working together — the geological time period, the nature and structure of rocks, topography, slope, the amount of water flowing, and the periodicity of the flow.
Think about a river near your village or city. Is it perennial, meaning it always has water, or ephemeral, meaning it has water only during the rainy season and is dry otherwise? Rivers generally flow in the same direction, and the reason lies in the slope of the land. You have already studied slopes in your other geography textbooks this year. That slope explains why water flows from one direction to another — and why rivers originating from the Himalayas in northern India and the Western Ghats in southern India flow towards the east and discharge their waters into the Bay of Bengal.
A river drains the water collected from a specific area, which is called its catchment area. The area drained by a river and its tributaries together is called a drainage basin. The boundary line separating one drainage basin from another is known as the watershed.
There is a slight difference between a river basin and a watershed. The catchments of large rivers are called river basins, while those of small rivulets and rills are often referred to as watersheds. Watersheds are small in area, while basins cover larger areas. Both river basins and watersheds are marked by unity — what happens in one part of the basin or watershed directly affects the other parts and the unit as a whole. That is why they are accepted as the most appropriate micro, meso or macro planning regions.
Important Drainage Patterns
The book identifies four major drainage patterns:
- Dendritic — The pattern resembles the branches of a tree. The rivers of the northern plain are examples of this pattern.
- Radial — When rivers originate from a hill and flow in all directions, the pattern is known as radial. The rivers originating from the Amarkantak range present a good example.
- Trellis — When the primary tributaries of rivers flow parallel to each other and secondary tributaries join them at right angles, the pattern is known as trellis.
- Centripetal — When rivers discharge their waters from all directions into a lake or depression, the pattern is known as centripetal.
You can find some of these patterns in the topo sheet given in Chapter 5 of Practical Work in Geography – Part I.
Classification of the Indian Drainage System
The Indian drainage system may be divided on various bases.
On the Basis of Discharge of Water (Orientation to the Sea)
On this basis, it may be grouped into two categories:
- The Arabian Sea drainage
- The Bay of Bengal drainage
These two are separated from each other through the Delhi ridge, the Aravalis and the Sahyadris. Nearly 77 per cent of the drainage area — consisting of the Ganga, the Brahmaputra, the Mahanadi, the Krishna, etc. — is oriented towards the Bay of Bengal. The remaining 23 per cent, comprising the Indus, the Narmada, the Tapi, the Mahi and the Periyar systems, discharges its waters into the Arabian Sea.
On the Basis of the Size of the Watershed
The drainage basins of India are grouped into three categories:
| Category | Catchment Area | Number of Basins | Examples |
|---|---|---|---|
| Major river basins | More than 20,000 sq. km | 14 | Ganga, Brahmaputra, Krishna, Tapi, Narmada, Mahi, Pennar, Sabarmati, Barak, etc. |
| Medium river basins | Between 2,000–20,000 sq. km | 44 | Kalindi, Periyar, Meghna, etc. |
| Minor river basins | Less than 2,000 sq. km | Fairly good number | Rivers flowing in areas of low rainfall |
On the Basis of Mode of Origin, Nature and Characteristics
The Indian drainage may also be classified into the Himalayan drainage and the Peninsular drainage. This classification has a problem — it includes the Chambal, the Betwa and the Son, which are much older in age and origin than other rivers that have their origin in the Himalayas. Despite this difficulty, it is the most accepted basis of classification, and hence this scheme has been followed in the book.
Looking at the map of major rivers of India, you can observe that many rivers have their sources in the Himalayas and discharge their waters either in the Bay of Bengal or in the Arabian Sea. Large rivers flowing on the Peninsular plateau have their origin in the Western Ghats and discharge their waters in the Bay of Bengal. The Narmada and Tapi are two large rivers which are exceptions — they, along with many small rivers, discharge their waters in the Arabian Sea. These include the rivers of the western coastal region from the Konkan to the Malabar coast.