Introduction: India's Drainage System

Overview and Scale

India's rivers are the arteries of its civilisation — sustaining agriculture, supporting over 1.4 billion people, generating hydroelectric power, and carrying millions of tonnes of sediment annually. The country's river network is shaped by its unique physiographic structure: the Himalayas in the north and the Peninsular Plateau in the south create two fundamentally different drainage regimes, separated by the Indo-Gangetic trough.

India has approximately 400 rivers with major basins. The Central Water Commission classifies them into 12 major basins (drainage area >20,000 sq km), 46 medium basins (2,000–20,000 sq km), and numerous minor basins. Together, India's rivers discharge about 1,869 billion cubic metres (BCM) of water annually — though utilizable water is only ~1,123 BCM.

Drainage Basin vs Watershed

Drainage basin (catchment area): the total area drained by a river and its tributaries. Watershed: the dividing line (ridge/hill) separating adjacent drainage basins. The Western Ghats serve as the principal watershed of Peninsular India, separating west-flowing (Arabian Sea) rivers from east-flowing (Bay of Bengal) rivers.

Two Broad Categories

• Himalayan Rivers: Perennial (flow year-round), glacier and snow-fed, longer, large sediment load, antecedent drainage, frequently flood prone
• Peninsular Rivers: Mostly seasonal (rain-fed), shorter, less sediment, flow through hard rock channels, consequent or subsequent drainage pattern

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