- These rivers are again subdivided into two groups – Trans Himalayan and Himalayan.
- The Trans-Himalayan Rivers originate beyond the Great Himalayas.
- These are the Indus, the Sutlej and the Brahmaputra rivers.
- Himalayan rivers are those which originate in the Himalayas and flow through the Northern Plains, e.g., the Ganga, the Yamuna and their tributaries.
- These rivers are useful for irrigation and navigation and the lowlands drained by them have fertile alluvial deposits.
- The Himalayan drainage system comprises of all the International Rivers of India i.e. Indus, Ganga and Brahmaputra.
- Most of these rivers are perennial in nature obtaining their water from glaciers and rains.
- These rivers are in their youthful Stage carving out a number of erosional landforms like waterfalls, cataracts, rapids, gorges etc.
Evolution Himalayan Drainage
- First theory was given by Pascoe and Pilgrim independently in 1919.
- Hypothesis: They assumed that there was an ancient mighty river flowed from Assam to Punjab. This hypothetical ancient river was called the Indo-Brahm by E.H. Pascoe who thought that the present day Indus and Brahmaputra were the severed parts of the original river.
- However, it was named as the Shiwalik River by E.G. Pilgrim who considered that the course of the primitive river is occupied by the present day Shiwalik hills.
- The river came into being due to earth movements which took place in Tertiary period and is believed to be successor of the Himalayan Sea.
- In the Eocene Epoch a gulf extended from Sind to Afghanistan and from there extended eastward and south-eastward through Kohat and Punjab to the neighbourhood of Nainital.
- This gulf gave place to a great river. With its headwater consisting of portions of the Brahmaputra, this master stream flowed along the foot of the Himalayas first westward and then north-westward as far as north-western Punjab where it turned southward more or less along the course of the modern Indus, and emptied itself into the Arabian Sea.
Later, this mighty stream got dismembered into the following systems and sub-systems:
(a) The Indus,
(b) The five tributaries of the Indus in Punjab,
(c) The Ganga and its Himalayan tributaries, and
(d) The stretch of the Brahmaputra in Assam and its Himalayan tributaries.
The dismemberment was the result of the following two events:
(i) Upheavals in the western Himalayas including the Potwar Plateau in the Pleistocene age and
(ii) Headward erosion by the tributaries of the Indobrahma River.
As a result of the above mentioned dismemberment of the Indobrahma River, the Indus and its tributaries, the Ganga and its tributaries and the Brahmaputra and its tributaries came into being. It is supposed that the Yamuna was first a tributary of the Indus.
- Second theory was developed by E. Ahmad (1965 – 71).
- He believed that the Tethys remained as a basin of sedimentation from the Cambrian to the Eocene period but the major portion of the Himalayan region as occupied by the Gondwana landmass.
- The first upheaval of Himalayas in Oligocene time initiated the Himalayan Drainage.
- As the formation of east-west ranges created east-west valleys, the rivers partly flowed along these valleys. This is indicated by the upper course of several rivers such as the Indus, the Sutluj, the Brahmaputra, the Shyok, the Arun, etc.
- Since the 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 during the mid-Miocene period increased the altitude of the medium mass and the bordering ranges.
- The remnant sea was also raised to form landmass.
- The rise in land resulted in greater and more invigorated drainage.
- The third Himalayan upheaval during the Pleistocene period resulted in the folding of the Shiwalik foredeep into hill ranges.
- Also the height of earlier ranges and the Tibetan plateau was raised.
- The rise in the Tibetan plateau blocked the streams that had gone northward into the Tibetan sea.
- These streams were diverted east or west which probably led 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 Shiwalik range gave rise to the last set of consequents originating on the crest of the range emptying into older streams.
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