The Evolution of Peninsular Drainage System

The story of how Peninsular India’s rivers came to look the way they do is a story of three great geological upheavals. Long before the present landscape took shape, the Peninsula was a stable block of ancient rock, and its rivers once flowed in a pattern that has since been completely rearranged. Three major events in the distant past — all tied to the break-up of Gondwanaland and the rise of the Himalayas — are responsible for the drainage systems we see today.

The first event was the subsidence of the western flank of the Peninsula during the early Tertiary period. The western edge of the block sank and went under the sea. This was a serious disturbance: it broke the symmetrical plan that the rivers had on either side of the original watershed. Where rivers had once drained evenly to both coasts, the western side was now lost to the ocean, and the balance of the entire system was thrown off.

The second event was the upheaval of the Himalayas, which had a direct effect on the northern edge of the Peninsular block. As the mountains rose, the northern flank of the Peninsula was pushed down, and this subsidence led to trough faulting — long, narrow cracks in the earth’s crust. The Narmada and the Tapi flow through these very trough faults. Because they occupy these structural cracks, the rivers simply fill the original cracks with their own detritus (the rock and sediment they carry). This is why the Narmada and Tapi show a striking lack of alluvial and deltaic deposits — they do not build up broad floodplains or large deltas the way most large rivers do, because their valleys are already defined by the faults rather than built by their own sediment.

The third event was a slight tilting of the Peninsular block from the northwest to the southeast, which happened during the same period. This gentle tilt gave a new orientation to the entire drainage system — it turned the rivers towards the Bay of Bengal. The tilt was enough to redirect the flow of most Peninsular rivers eastward, which is why the great majority of them today drain into the Bay of Bengal rather than the Arabian Sea. …