Assertion (A): Floods are a recurring feature in the Indo-Gangetic Plain. Reason (R): The Indo-Gangetic Plain is built of porous and soft alluvium.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Soft alluvium silts up riverbeds and banks erode easily, so rivers overflow and shift — recurring floods.

The idea

A flood occurs when incoming water exceeds what a channel and its floodplain can safely carry. Heavy rainfall, a low gradient, sediment deposition and channel shifts can combine on an alluvial plain. Soft alluvium makes banks easier to erode, but porosity alone is not a sufficient cause of flooding.

Heavy rain and upstream runoff → River discharge rises → Low-gradient channel has limited capacity → Sediment and bank erosion alter the channel → Water overtops banks or finds a new course → Floodwater spreads across the plain

Why the others are wrong

(b) The key links the two.

(c) The plain is soft alluvium.

(d) Floods recur (Kosi, Brahmaputra).

The trap

Monsoon rain is the obvious cause, but the question asks whether soft alluvium explains it — the key says yes.

Also know

Kosi is known for channel shifts across its alluvial fan in Bihar. Ghaghara and Gandak also bring Himalayan water and sediment to the Ganga plain. An oxbow lake is a cut-off meander; a levee is a raised bank deposit formed by repeated floods.