Assertion: Ganga, the Brahmaputra, and the Indus have huge catchment areas. 
Reasoning: Much of the annual water flow in south Indian rivers like the Godavari, the Krishna, and the Kaveri has been harnessed.

Both Assertion (A) and reasoning (R) are correct, but R is not the correct explanation of A.

You also have studied in the textbook that some of the rivers in the country like the Ganga, the Brahmaputra, and the Indus have huge catchment areas. Given that precipitation is relatively high in the catchment areas of the Ganga, the Brahmaputra and the Barak rivers, these rivers, although account for only about one-third of the total area in the country, have 60 per cent of the total surface water resources. Much of the annual water flow in south Indian rivers like the Godavari, the Krishna, and the Kaveri has been harnessed, but it is yet to be done in the Brahmaputra and the Ganga basins.