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Class 11 Drainage System: cheat sheet
CBSE Social Science. How rivers drain the land: drainage patterns and basins, and the Himalayan and Peninsular river systems with their sources, tributaries and lengths.
Key ideas
- Drainage patterns: dendritic (like tree branches), trellis (parallel streams joined at right angles), radial (outward from a hill, as from Amarkantak) and centripetal (inward to a lake).
- About 77% of India’s drainage area drains to the Bay of Bengal and 23% to the Arabian Sea.
- Himalayan rivers are perennial (snow and rain), carve gorges, meander and shift course in the plains, and build deltas.
- Peninsular rivers are older and seasonal, with fixed courses and broad, graded valleys.
- Most Peninsular rivers flow east because the block tilts from north-west to south-east and the Western Ghats form the main divide.
- The Narmada and Tapi flow west in rift valleys and form estuaries, not deltas.
- The Bhagirathi and Alaknanda join at Devprayag to form the Ganga, whose longest tributary is the Yamuna.
Remember
| Basin size | major over 20,000 sq km; medium 2,000–20,000 sq km; minor under 2,000 sq km |
| River lengths | Ganga 2,525 km; Godavari 1,465; Krishna 1,401; Narmada 1,312; Mahanadi 851; Kaveri 800; Tapi 724 |
| Catchments | Godavari 313,000 sq km (largest Peninsular, the Dakshin Ganga); Mahanadi 142,000 |
| Share draining to a sea | 77% (Bay of Bengal) or 23% (Arabian Sea) of the area |
| Gradient | fall in m ÷ length in km |
| Nicknames | Kosi “sorrow of Bihar”; Damodar once the “sorrow of Bengal” |
Watch out
- Saying the Narmada builds a delta: it forms an estuary.
- Mixing up radial (flowing out) and centripetal (flowing in).
- Treating Peninsular rivers as perennial and snow-fed.
Worked example
Is each river basin major, medium or minor? (a) 392,000 sq km (b) 14,500 sq km
Working: major over 20,000; medium 2,000–20,000; minor under 2,000 sq km
Answer: (a) major (b) medium
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