Drainage | Social Science (Geography) Grade 9 CBSE

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The language of drainage

A river system has its own vocabulary. Tap each term to see what it points to and how the pieces of India's drainage fit together.

Explore · The words of a river systemtap a term

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The three big ideas

Drainage — the river system that drains an area; how the water of a region is carried away by rivers and streams.
Drainage basin — the land drained by a river and all its tributaries. The Ganga basin is the largest river basin in India.
Tributary — a smaller stream that joins a larger (main) river, adding its water to it.
Watershed — the upland boundary (a mountain or highland) that separates two neighbouring drainage basins.

Common mistake: a tributary JOINS the main river, while a distributary BRANCHES OFF it (as in a delta, where the river splits into several channels before reaching the sea). They are opposites — one adds water to the river, the other carries water away from it.

The three great Himalayan river systems are the Indus, the Ganga and the Brahmaputra.
They are perennial — they flow all year round because they are fed by both rainfall and melting snow from the high mountains.
In the mountains they cut deep gorges; on the plains they form wide loops (meanders), and near the sea they build large deltas.
The Ganga basin is the largest river basin in India.

The main Peninsular rivers are the Godavari, Krishna, Kaveri, Narmada and Tapi. The Godavari, the largest, is called the "Dakshin Ganga".
They depend mainly on rainfall, so their flow rises in the rainy season and falls in the dry season — most are seasonal, not perennial.
Most flow east into the Bay of Bengal, but the Narmada and Tapi flow west into the Arabian Sea through rift valleys.
Lakes store water, help control floods and support life. Most are freshwater; some, like Sambhar in Rajasthan, are salt water used to make salt.
River pollution: growing untreated sewage and industrial waste lower water quality, so clean-up programmes such as the Ganga Action Plan were launched.

Worked example. Why do Himalayan rivers flow all year while many Peninsular rivers do not?

Himalayan rivers: they get water from snowmelt AND rain, so even in the dry season the melting snow keeps them flowing — they are perennial.

Peninsular rivers: they depend mostly on seasonal rainfall, so when the rains stop their flow drops sharply and many shrink to a trickle — they are largely seasonal.

Where you'll meet it

Drainage, at work

Drinking water & irrigation

Rivers and lakes are the main source of fresh water for homes and farms. Towns draw drinking water from them, and canals carry river water to fields, making irrigation possible across the plains.

Hydroelectric power

Fast-flowing rivers can be dammed to spin turbines and generate hydroelectric power. The same dams and lakes also store water, help control floods and support fishing and tourism.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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