Why does course shifting of the Kosi river increase flood problems in Bihar?

Correct answer: Because heavy sediment can make the channel shallow and shift course

The direct answer is C: heavy sediment can make the Kosi channel shallow and cause it to shift course. The Kosi carries a very large load of sediment from its mountainous catchment. When this sediment is deposited in the river channel, the bed rises and the channel becomes shallower. Water then seeks a lower or easier route and may break through its banks, creating a new channel. Because the new course can spread across a wide, flat alluvial plain, flooding affects many areas of Bihar. Option C correctly explains the physical mechanism and the connection with Kosi floods. Option A is wrong because an estuary at the sea does not explain the Kosi’s repeated course changes in Bihar. Option B is wrong because the Kosi does not rise in the Western Ghats; its catchment is associated with the Himalaya. Option D is wrong because the Kosi’s water is not completely saline; it is mainly a freshwater river, though its sediment load is high. The fact chain is Himalayan erosion → heavy sediment → channel aggradation and shallowing → bank overflow or course shift → floods. Remember the phrase ‘Kosi, the sorrow of Bihar,’ but understand that sediment and shifting channels are the reason, not merely the nickname.