The Himalayan drainage system mainly comprises the basin areas of the Indus, the Ganga and the Brahmaputra. Here most of the rivers are perennial in nature and are fed by rains during monsoon season as well as by the melting of the snow during summer season.
These rivers are, in their youthful stage carving out a number of erosion features like deep gorges, V-shaped valleys, rapids and water falls in the Himalayan Rivers, which now belong to the three principal systems (the Indus, the Ganga and the Brahmaputra), have evolved through a long period of geological history.
They originate on the southern slopes of the Tibetan Highlands and first flow parallel to the main axis of the mountains in longitudinal troughs.
They take a sudden bend towards the south carving out deep gorges across the mountain ranges to reach the northern plains of India. Such deep gorges by the Indus, Satluj, Alaknanda, Gandak, Kosi and Brahamaputra suggest that they are older than the mountains themselves and have antecedent characteristics. E.H.
Pascoe (1919) and G.E. Pilgrim (1919) have attempted to give a tangible explanation to the problem of the Himalayan drainage. According to them the Tibetan plateau was drained to the west by mighty Tsangpo-Indus-Oxus combine called the Tibetan river.
Similarly another big river called Indobrahma (Pascoe, 1919) or the Siwalik (Pilgrim, 1919) traversed the entire longitudinal extent of the Himalaya from Assam to Punjab (from east to north-west to empty into the Sind Gulf near lower Punjab during the Miocene period. The former was disrupted by the headward erosion of its left-hand tributaries, the Proto-Sind, the Proto- Satluj, the Proto-Brahmaputra etc., and later captured by the present lower Irrawaddy-Chindwin, the Lower Brahmaputra, the lower Satluj-Indus which is the probable remnants of the Siwalik River.
Later the mighty Siwalik River was dismembered into three main systems: (1) the Indus and its tributaries in the western part, (b) the Ganga and its Himalayan tributaries in the central part, and (c) the stretch of the Brahmaputra in Assam and its Himalayan tributaries in the eastern part.
The dismemberment is attributed to the Pliestocene upheavals in the western Himalayas including the uplift of the Potwar Plateau (the Delhi Ridge) which acted as water divide between the Indus and the Ganga systems. Similarly the down thrusting of the Malda Gap area between the Rajmahal Hills and the Meghalaya Plateau during the mid-Pleistocene period (5 lakh years ago) attracted the Ganga and the Brahmaputra systems to flow towards the Bay of Bengal.
These developments brought about a reversal in the direction of flow in the middle section of the severed stream, i.e. the Ganga of today taking a southerly course and eventually annexing the Yamuna and its tributary. Earlier the Yamuna had a south-westerly course, and was a tributary of the Indus. This entire event was completed by the late Pleistocene period leading to the evolution of the present drainage systems of the northern India.
This theory has been criticised on the following grounds: (1) it is not necessary to postulate a stream of the size of the Siwalik River flowing all along the longitudinal extent of the Himalayas to explain the occurrence of the Siwalik alluvial deposits and the boulder beds.
They might represent a succession of alluvial fans deposited by rivers descending from the Himalayas which have coalesced over time. (2) The evidence furnished by the depositional history in the Ganga delta and in Assam does not fit well with this theory. In fact the alluvium should have been laid down over a much longer period of time than suggested in this concept. (3) The
evidence of the Tipam sandstones of Assam, which were deposited in an estuary situated too close to the source of the Indobrahma River also puts doubts over the acceptance of the theory.
E. Ahmad (1965 and 1971) has given following description to the evolution of the Himalayan drainage. According to him 'The Tethys remained as a basin of sedimentation from the Cambrian to the Eocene period but the major portion of the Himalayan region was occupied by the Gondwana landmass.
During the first Himalayan upheaval in Oligocene period part of the Tethyian geosyncline and probably the Gondwana shield covering the major portion of the Himalayan area was uplifted. This probably marks the initiation of the Himalayan drainage.
The Tethys was raised into a landmass with a median mass of high Tibetan plateau in the centre and two bordering ranges (KunLun and Himadri) on either side. It was from the southern edge of the median mass or a 'geanticlines' that drainage started and flowed south towards the fore deep.
As the formation of east-west ranges created east-west valleys the rivers partly flowed along these valleys as indicated by the upper course of the Indus, the Satluj, the Brahmaputra, the Shyok, and the upper Arun etc. Since whole of the Tethys was not fully raised to become land surface, there existed patches of sea along the margins and the drainage lines were not fully defined.
The second Himalayan upheaval of the mid- Miocene period increased the altitude of the median mass and the bordering ranges. It also raised the remnant of the sea into land.
A greater relief and vaster land surface meant a greater and more invigorated drainage. But along with these changes the region to the south of the first Himalayan range i.e., the proto-type of the Great Himalayas, was probably raised as the Lesser Himalayan range, across which earlier streams on the southern margin of the Tibetan plateau cut down deep valleys to maintain their courses. Along the southern slopes of the Lesser Himalayas a number of consequent streams also emerged which drained into the southern for deep.
By the third Himalayan upheaval during the Pleistocene period the Siwalik for deep was folded into hill ranges. This also raised the height of the earlier ranges and the Tibetan plateau. The rise of the Tibetan plateau blocked the streams that had gone northward into the Tibetan sea.
These streams were diverted east or west leading presumably to the formation of the trans-Himalayan master stream. This master stream was broken into two (the proto- Indus and the proto-Brahmaputra) by the formation of the Kailas Range. The uplift of the Siwalik range gave rise to the last set of consequents originating on the crest of the range and emptying into the older streams' (Ahmad, 1965 and 1971).
Except the major transverse streams like the Arun, Kosi, Karnali, Gandak and the Brahmaputra which are either consequent upon the general southerly slope of the Himalayan wall or antecedent or in sequent to its uplift, most of the Himalayan Rivers reflect striking structural control.