What is a drainage system?
Drainage is the flow of water through clear channels. A drainage system is the network of a main river and all the streams that join it (tributaries). A stream that leaves the main river is a distributary, common in deltas.
Drainage patterns
- Dendritic: like the branches of a tree; forms on rock of the same hardness. Rivers of the northern plain.
- Radial: rivers flow outward in all directions from a high central point. Rivers from the Amarkantak range (Narmada, Son, Mahanadi).
- Trellis: main rivers flow parallel, and tributaries join at right angles; forms where hard and soft rocks lie side by side. Upper parts of Himalayan rivers.
- Centripetal: rivers flow inward into a lake or low basin. Loktak lake, Manipur.
River basin, catchment and watershed
- Catchment area: the land from which a river collects its water.
- River basin: the whole area drained by a big river and all its tributaries (e.g. the Ganga basin).
- Watershed (water divide): the high land or ridge that separates two drainage systems. Rain on one side goes to one river, on the other side to another. The Western Ghats are a big water divide. Small drainage units are also called watersheds in planning.
A basin is managed as one unit because what happens upstream (a dam, a factory) affects everyone downstream.
Himalayan and peninsular rivers
| Himalayan rivers | Peninsular rivers |
|---|---|
| Indus, Ganga, Brahmaputra systems | Mahanadi, Godavari, Krishna, Kaveri, Narmada, Tapi |
| Perennial: snow/glacier melt + rain | Seasonal: rain only |
| Young; cut deep gorges (e.g. Indus, Brahmaputra) | Old; shallow, wide valleys, graded courses |
| Long, large basins | Shorter, smaller basins |
| Meander in the plain; shift course (Kosi) | Fixed courses, few meanders |
| Big deltas (Ganga–Brahmaputra) | East-flowing: deltas; west-flowing Narmada, Tapi: estuaries, flow in rift valleys |
Main systems
Indus: rises near Mansarovar; tributaries Jhelum, Chenab, Ravi, Beas, Satluj. Ganga: Bhagirathi and Alaknanda join at Devprayag; Yamuna joins at Prayagraj; also Ghaghara, Gandak, Kosi, Son. Brahmaputra: called Tsangpo in Tibet, enters India as the Dihang. Godavari is the largest peninsular river ("Dakshin Ganga").
River linking, water use problems and pollution
Uneven water
Rain falls mostly in 3–4 monsoon months and more in the east and north-east. So some basins flood while others face drought.
River linking
The plan is to join surplus basins to deficit basins by canals (e.g. Ken–Betwa link). Benefits: irrigation, drinking water, less flood and drought, hydropower, inland waterways. Concerns: high cost, people displaced, forests drowned, changes to river ecology, disputes between states.
Water use problems
Over-pumping groundwater, wasteful flood irrigation, dams reducing flow downstream, and sharing disputes between states.
Pollution
Untreated sewage from cities, factory waste, farm chemicals, and dumping of waste make rivers like parts of the Ganga and Yamuna unfit to drink or bathe in. Low dissolved oxygen kills fish. Solutions: treat sewage before release, treat factory waste, less chemical farming, programmes like Namami Gange, and people not dumping waste.
Key formulas and definitions
- Dendritic = tree; Radial = spokes; Trellis = right angles; Centripetal = inward to a lake
- Watershed (ridge) divides basins; catchment = land that feeds a river
- Himalayan: snow + rain → perennial; Peninsular: rain → seasonal
- East-flowing peninsular → deltas; Narmada, Tapi (west) → estuaries
- More sewage treated → more dissolved oxygen → more river life
Worked examples
1. Why are the Narmada and Tapi different from other peninsular rivers?
They flow west into the Arabian Sea through rift valleys (long cracks in the crust) and end in estuaries, while most peninsular rivers flow east and make deltas.
2. Rain falls on the Western Ghats. Where can it go?
The Ghats are a water divide. Rain on the west side runs down short, fast streams to the Arabian Sea; rain on the east side joins long rivers like the Krishna flowing to the Bay of Bengal.
3. Why does the Kosi change its course often?
It brings huge amounts of silt from the Himalaya and drops it on the flat plain. The bed rises and the river breaks out into a new channel, causing floods; it is called the 'sorrow of Bihar'.
4. Give one benefit and one concern of river linking.
Benefit: water from a surplus basin can irrigate drought areas. Concern: canals and dams may drown forests and displace people.
Common mistakes
- Thinking catchment and watershed mean the same. Catchment is the area; watershed is the dividing ridge.
- Saying all peninsular rivers flow east. The Narmada and Tapi flow west.
- Saying peninsular rivers are perennial like Himalayan ones. Most are rain-fed and seasonal.
- Mixing radial and centripetal. Radial goes OUT from a hill; centripetal comes IN to a basin.