Drainage System

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

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