Himalayas play an important role in the drainage system of the North India. This is because the rivers of North India have their sources in these mountains and beyond. These rivers differ from those of South India as they are still deepening their valleys rather rapidly.
The debris eroded by these rivers are carried to the plains and seas and deposited there. This deposition is caused by the reduced velocity of river waters in the plains and deltas for want of necessary slope.
The Great North Indian plain has been formed by the silt brought down by these rivers. Some of the Himalayan rivers are older than the Himalayas themselves. As the ranges of the Himalayas had been rising upwards, these rivers were equally busy in downward cutting forming deep gorges and valleys.
Consequently, parts of the valleys of these rivers are very deep and gorges have been formed. The depth of the Indus gorge near Bunji (Jammu & Kashmir) is 5200 metres. Sutlej and Brahmaputra have also formed such gorges.
The drainage system of Northern India can be further sub-divided into three subsystems - Indus System, Ganga System and Brahmaputra System.
The major rivers of Indus basin are the Indus, Jhelum, Chenab, Ravi, Beas and Sutlej. The Ganga basin includes Ramganga, Ghaghra, Gomti, Gandak, Kosi, Yamuna along with its southern tributaries, Son and Damodar rivers. The major rivers of Brahmputra basin are Dibang and Lohit in Arunachal Pradesh and Assam, Tista in Sikkim, West Bengal and Bangladesh and Meghna, draining northeastern part of Bangladesh.