New research suggests that the geological role of rivers from the southern peninsular hills of India has been far more important in terms of contributing water and sediments to the Indo-Gangetic plains than ever estimated. It also shows that the present course of river Yamuna through Delhi is as young as 6000 years1 .
The vast stretch of the Indo-Gangetic plains, bound by Himalayas in the north and peninsular hills or cratons in the south, has ben built over a long geological period. The modern configuration of the Ganga plains suggest an obvious dominance of the Himalayan rivers draining from the north. The role of the cratonic rivers joining the Ganga and the Yamuna from the south has remained underestimated in creating one of the largest and thickest alluvial plains in the world.
Based on lithology and petrography, alluvial architecture and age data of sediments, the researchers have demonstrated for the first time that the peninsular hill rivers were a major source of sediment and water supply to the Ganga plains and the Bay of Bengal until as late as 60,000 years before present.
They also suggest that the drainage system of the Ganga plains were reorganised in a major way after this period. This reduced the contribution of the cratonic rivers in filling up the foreland basin.
An important event of this reorganisation was the eastward shift of Yamuna river 4000-6000 years before present. "This means that the present course of the Yamuna through Delhi is as young as 6000 years," says lead researcher Rajiv Sinha from the Indian Institute of Technology, Kanpur.
The researchers believe this westward course formed an important tributary to the Vedic Saraswati River during the Indus Valley Civilisation. "The demise of the Indus Civilisation has often been linked to drying up of the ancient Saraswati and, although the debate is still on, we believe that such drainage reorganisation must have played an important role in changing the hydrological budget of the river systems in the Ganga basin," Sinha says.
This could have been further amplified by monsoonal fluctuations during the Holocene period (last 10, 000 years).
This comprehensive and integrated study on sub-surface sediments of the Ganga plains reopens the debate on the role of drainage reorganisation in the Ganga basin.
