The Himalayan Rivers:
According to geologists, the Shivalik hills run parallel to the main Himalayan ranges and are formed of alluvial deposits consisting of sands, clays and boulder conglomerates. These deposits were laid down by a mighty stream which flowed from Assam to Punjab and even beyond upto Sind.
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It finally emptied itself into a gulf which occupied parts of the Sind and lower Punjab in the Miocene period. This hypothetical stream is known as the ‘Indobrahma’ or the ‘Shiwalik’ river. This river carried the combined flow of the three main rivers the Indus, the Ganga and the Brahmaputra ansd deposited huge quantities of alluvium. Later, this mighty stream dismembered into the following systems and sub-systems:
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(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.
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The dismemberment was the result of the following two events:
(i) Upheavals in the western Himalayas including the Potwar Plateau in the Pliestocene age.
(ii) Headward erosion by the tributaries of the Indobrahma River.
Peninsular Rivers:
The subsidence of the land west of Sahyadri during the Cretaceous and tilting of the Sahyadri, the entire drainage of the Peninsular India got oriented towards the Bay of Bengal; leasing numerous small streams only to flow into Arabian Sea, except of course, the fault guided Narmada and Tapti. Another distortion was introduced during the Himalayan upheaval when the northern flank of the peninsular block was subjected to subsidence and consequent trough faulting.
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The Narmada and Tapti rivers flow in such trough-faults and have courses consequent to their general trend. During the course of the alluvial activity, they seem to have filled the original cracks with their detrius. This largely explains the lack of alluvial and deltaic deposits in their valleys. The peninsular rivers flowing on the Deccan peneplain viz. Godavari, Krishna, Cauvery, Chambal, Damodar and Subararekha have their channels superimposed on older structures in different stretches.
As a result of the above-mentioned dismemberment of the Indobrahma River, the Indus and its tributaries and the Brahmaputra and its tributaries came into being. It is supposed that the Yamuna was first tributary of the Indus, but it was annexed by the Ganga later on and it became the tributary of the Ganga.