Siwalik River also known as the Indo-Brahm River. The Himalayan Mountain system and its various sub-systems, viz Naga-Lusai of Assam and the hills of the Chittagong and chittagong hill tracts had emerged and the tethys sea completely disappeared from the subcontinent as a consequence of the violent orogenesis of the mid-Miocene time (approximately 10 million years before present). With the emergence of the Himalayas, a foredeep was also formed on its front. This foredeep was extended from Assam in the east up to Patwar of Pakistan in the west.
With the emergence of the Himalayas, numerous turbulent streams sprang up. These stream courses carried along with them sediments to the foredeep after eroding the newly emerged hill ranges diagonally. Later on, those small rivulets combined into one single large river. Sir EH Pascoe and Dr GE Pillgrim christened the river Siwalik or Indo-Brahm River. It is supposed that the Siwalik River emanating from Assam, used to flow along the foothills of the Himalayas northwestward parallel to the Himalayan mountain. At present where the Siwalik range of north India lies, the river was flowing along the Main Boundary Fault over that place. The direction of the river was diverted southward after it entered into Nanno and Patwar in Pakistan and finally fell into the Arabian sea after cruising along the course of present day Sind (Indus).
It is quite difficult to say whether the Siwalik River at that time flowed past Bangladesh. But there is no doubt that it emanated from Assam. It may be noted here that the brahmaputra and the Sind were flowing in the subcontinent as pre-antecedent river since much before the emergence of the Himalayas. It will not be wrong to anticipate that another Himalayan river- the ganges originated as a result of the creaton of the foredeep and the Main Boundary Fault following the emergence of the Himalayas during the mid-Miocene violent orogenesis.
It is supposed that the Ganges River at that time flew past the place where the present day Siwalik hill range exists. Later on, only one river turned into Siwalik or Indo-Brahm River by joining with the Sind and the Brahmaputra under process of water mugging or stream piracy. This river under the new name may have fallen into the Arabian sea after flowing over Assam, Northern India and Pakistan till the end of the Pliocene (about 2 million years before present). Afterwards, the Siwalik River failed to maintain its entity as a single river in the subcontinent due to the creation of Siwalik hill range in India and a series of mountains at Patwar region of the Punjab which were the results of the 4th Himalayan orogenesis that took place during late Pliocene. In its place, three separate rivers, namely, the Indus, the Ganges and the Brahmaputra sprang up. [Sifatul Quader Chowdhury]