The evolution of the Indian rivers is related to the evolution of the Himalayas and the Peninsula. Due to the different evolution of the Himalayas and the Peninsula, the nature of their rivers is also different.
The Evolution of Himalayan Rivers.
Most of the rivers which rise in the Himalayas are antecedent in nature. These antecedent rivers existed and flowed before the Himalayas were formed. Many rivers arise in the Tibetan Plateau.
They usually flow in longitudinal valleys parallel to the main axis of the Himalayas. These rivers later cross the Himalayas by making deep gorges and then end up in the Arabian Sea or the Bay of Bengal.
When the Himalayas started rising from the Tethys sea about 70 million years ago, rivers like Brahmaputra, Sindh, Sutlej, Alaknanda of the Ganges, Gandak, etc. started cutting down the rising mountains and like a saw, continued to cut them down almost as fast as the mountains rose.
Thousands of metres deep valleys are a proof of the antecedence of these rivers. The consequent rivers flow according to the slope of the land, the Ghaghra is one such river.
The Siwalik River.
A river which existed in the Himalayan upliftment is called Siwalik or Indo-Braham. The Siwalik range is composed of alluvial sediment, i.e., clay, sand, boulder, conglomerate, etc.
The extent of the Siwalik can be seen from Assam towards west along the Himalayas in their foothill areas. Pilgrim, G.E. says that this sedimentary deposit is caused by this ancient river.
This river flowed from Assam to Punjab in a longitudinal valley. Pascoe, E.H. named it Indo-Braham.
The Direction of the Siwalik.
Pascoe thinks that this river continued depositing the sediment from mid-Miocene to Pliocene (a duration of about 1.8 to 1.9 million years).
In Assam its direction was north-western which made it flow from Assam to the Punjab, and then it turned to south. In Miocene there existed a gulf over the present Sindh Province (Pakistan) and the southern Punjab. The Siwalik River ended up in this gulf.
The Deposition by the Siwalik River.
The sedimentary nature of the Siwalik is an indication of the great deposition made by this river over such a vast area and its transformation into a plain.
Evidences Favouring the Existence of the Siwalik River
1. The Alluvial Extent of the Siwaliks.
The Siwalik range appears as a long arc along the southern periphery of the Greater and Lesser Himalayas. It makes us to imagine of a river flowing in the past along this route.
2. The Nature of the Deposits.
A study of the deposits reveals that the deposits are not old but are comparatively young. The sediment from the Himalayas has been found deposited in this section.
This proves that this deposit of sediment could only be brought by a fast flowing and very long river.
3. The Effect of the tributaries of Tsang-po.
There are many westward flowing tributary rivers of Tsang-po. This supports the hypothesis of Siwalik River.
4. The Siwalik River Evolution.
The high Himalayan area lies in the north. The rivers descending down this area have been found to be having intense headward erosion.
The fast erosional activity has cut down this area into many sections. This gives rise to many drainage patterns- (i) The Sindh drainage pattern, (ii) the drainage pattern of the tributaries of the Sindh and (Hi) the drainage pattern of the Ganges and its tributaries.
There are two more causes of the split of the Siwalik River. The movements that took place in the Pleistocene raised many parts of western Himalayas and faults developed in them.
The Potwar plateau was uplifted in that very period. The Sindh area of the river was cut away and the direction of the Ganges was reversed and it began to flow towards the east. Previously the Yamuna was a tributary of the Sindh but this movement turned it to become the tributary of the Ganges.
In this way the tributaries of the Ganges and the Sindh made many such exchanges of their tributaries. All the tributaries of the northern plain came into their new pattern after the split of the Siwalik.
Evidences against the Existence of the Siwalik River
1. The Depositional Features.
To explain such a long and big alluvial deposits of the Siwalik range, it is not necessary to imagine a river like the Siwalik. The rivers flowing down the Himalayas have produced a large number of the alluvial fans.
These alluvial fans by the process of coalescence could bring about such huge deposits which form this range.
2. The Gap between the Rajmahal Hills and the Shillong Plateau.
This gap is filled with such deep deposits that this Siwalik River in her brief life could never have filled it.
3. The Teepam Sand Stone.
This sand stone is deposited in an estuary. This estuary then existed at the source of the river. These deposits do not support this situation of the river.
In fact the Siwalik hypothesis can neither be easily abandoned nor accepted because the proofs in favour and against are very substantive.