Assertion (A): The frequency of floods in North India plains has increased during the last couple of decades. Reason (R): There has been reduction in the depth of river valleys due to deposition of silt.

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

Silt raises riverbeds, so channels hold less water and overflow more often.

The idea

A river channel has a limited carrying capacity. When sediment raises its bed, less space remains below the banks for floodwater. For the conditions described in this 2000 question, this supplies a causal link between silting and more frequent overflow; it is not a dated claim about flood trends today.

River carries sediment → Flow slows in the plain → Sediment settles → Bed level rises → Channel holds less floodwater → High discharge overtops the banks

Why the others are wrong

(b) The key links them — silting directly reduces carrying capacity.

(c) Silting is well documented (Kosi, Ganga).

(d) Flood frequency has risen.

The trap

Rain and deforestation also matter, but silting is the stated cause and it does explain A.

Elimination technique

Ask whether R supplies an actual link, rather than merely another true fact. A raised bed reduces channel capacity, so the given reason explains the assertion in the exam setting.

Also know

Kosi is known for sediment deposition and channel shifts. Embankments can contain ordinary floods but may also confine deposited sediment within a channel. Flood risk also depends on rainfall, drainage blockage and settlement on floodplains; one mechanism does not explain every flood.