Although the main rivers of the plains originate in the Himalayas, they form the most important physio graphic features of the Indo-Gangetic Plains. In fact, it is these river systems that have made the plains what they are: highly fertile lands supporting extensive agriculture, which have cradled some of the greatest civilizations in history.
It is remarkable that the great rivers of the northern part of the Indian subcontinent spring from almost the same territory, in fact, from within a few tens of kilometres of each other, separated by water divides. The glaciers of the Kailas Range east of the holy lake of Manasarowar in Tibet are the source of the east-flowing Brahmaputra. The Indus also rises from the same range, northwest of Manasarowar (Tso Mapham or Tso Mavang in Tibetan and Mapam Yumco in Chinese), but takes a north-westerly course from there, almost the opposite of the Tsangpo, as the Brahmaputra is called in Tibet (and which has now been renamed Yarlung Zangbo Jiang by the Chinese). The Satluj, born from the Rakas Lake (Lengak Tso in Tibetan and La’nga Co in Chinese) west of Manasarowar, also follows a westerly route. The Bhagirathi and the Alaknanda together with their tributaries, which further down grow into the Ganga, have their genesis in the snow-clad peaks of the Great Himalayan ranges, across from Manasarowar, only 100 km away.
Some Himalayan streams are the most remarkable examples of antecedent rivers, that is, streams that have maintained their original course or direction by deepening their valleys simultaneously while the uplift of the mountains was in progress, thus becoming permanently entrenched in the rising topography. These rivers have been in existence before the tectonic processes raised the Himalayas and are truly ancient. They have carved their channels through mountains with peaks that now rise high above the level of their source. The deep valleys or gorges that have been cut through the ranges have not resulted from headward erosion as in the case of the peninsular rivers. These antecedent gorges of the Himalayan Rivers, both great and small, are among the deepest cuttings in the world. In comparison, the gorges of the Swiss Alps pale into insignificance.
The Indus, Satluj and the Brahmaputra flow for long distances, the first two westward and the third eastwards, before they make sharp changes in their direction to descend to the plains after cutting step defiles right across high mountains. Some of these gorges are deep slit-like gashes cut in the bottom of V-shaped valleys, which go to show that there was an increased rate of their rejuvenation during the Pleistocene and Holocene times. For example, the Indus has cut through over 5,000 m of rock – first vertical walled gorges and then by a series of steps before it enters the plains. The Indus flows at an average level of 1,300 m above the sea north of the Nanga Parbat which rises steeply to 8,125 m within a few kilometer of the channel. The immense difference in the height of the two is due to the antecedent character of the river supplemented by uplift in very recent times. The Hunza River, a tributary of the Indus, flows at a level of about 1,800 m, while the Mount Rakaposhi rises to 7,788 m only some four kilometers away. The Satluj River gashes through the high Zaskar Range by a similar transverse gorge carving out a perpendicular defile of 1,800-2,000 m depth at Shipki. The Kali River on the Kumaon-Nepal border pierces through the mountains at a level of only 1,500 m between the Nanda Devi (7,817 m) and Api (7,132 m) peaks. Several rivers of Nepal have also made similar incisions across the strike of the mountains. The Kali Gandak River flows only at an elevation of 1,200 m at Dana between the summits of Dhaulagiri (8,172 m) and Annapurna (8,078 m) only 55 km apart. The bed level of the Arun, in eastern Nepal, is only at 1,800 m altitude between the ranges ion which the towering peaks of Mount Everest-Makalu (8,848 – 8,481 m) and the Kanchenjunga (8,598 m) preside.
The courses of most the Himalayan rivers are controlled by the strike and the structure of the mountains, except where streams cut across the grain of the mountains in their antecedent phase. The Indus, for example, flows in a northwest direction through a strike valley between the Higher Himalaya and turns southwards near Gilgit between two ranges which formed a natural defile before their uplift. Tsangpo, rising in the same area, on the other hand, takes an easterly course through a much less rugged region and follows the depression of the suture zone. The sudden change in its direction to the south around the Namche Barwa peak is controlled by the eastern syntaxis just as that of the Indus is influenced by the western syntaxis. The rivers of the Garhwal to Arunachal Himalayas flow south down the slopes of the glacier-clad ranges but receive tributaries which follow the east-west general strike of the hills. In both cases, the directions are controlled by a combination of slope and the main structures, aided sometimes by faults. Once they come down from the hills they are controlled by the morphology of the plains.