Answer: Because they are fed by the melting snow and glaciers of the Himalayas as well as by rainfall.
- The source regions of these rivers lie in the high Himalayas, where snow accumulates and glaciers form.
- In summer the snow and ice melt and feed the rivers, so they carry water even in the dry season.
- During the monsoon, rainfall adds to their water and they are often in flood.
- Therefore the Indus, Ganga and Brahmaputra flow throughout the year. Peninsular rivers depend only on rainfall, so their flow becomes very low in summer and many smaller ones dry up.