Contrary to current belief, it was the departure of a large river — not its arrival — that triggered the growth of Indus urban centres that developed in what is now northwest India and Pakistan some 5,300 years ago, says a new study.
The Indus civilisation developed at about the same time as urban civilisations developed in Mesopotamia and Egypt. Archaeological evidence shows that many of the settlements in the Indus civilisation developed along the banks of a river called the Ghaggar-Hakra.
But the new study, led by researchers from the Indian Institute of Technology (IIT) Kanpur and Imperial College London, has now provided evidence that a major Himalayan river did not flow at the same time as the development of Indus civilisation urban settlements.
“The findings challenge our current understanding of how urbanisation in many ancient civilisations began and grew in relation to natural resources,” said lead author Sanjeev Gupta, Professor at Imperial.
The study, published in the journal Nature Communications, showed that today’s Sutlej River used to flow along the trace of the Ghaggar-Hakra river but rapidly changed course upstream 8,000 years ago.
This meant that 3,000 years later, when the Indus people settled the area, there was only an abandoned large river valley occupied by seasonal monsoon river flow instead of a large Himalayan river.
The researchers said the time gap between the river shifting course and the Indus civilisation settlements appearing rules out the existence of a Himalayan-fed river that nourished Indus civilisation urban settlements along the river channel.
The team was also able to pinpoint what the original source of the river sediments had been, showing that the Sutlej had once flowed along the Ghaggar-Hakra dried river channel, or palaeochannel.
They found that after the Sutlej changed course, the scar it left in the landscape acted as a topographic low to capture river flow during the monsoon.
This meant that despite not living along a permanent river, the Indus settlements still benefited from a water source.
To determine the timing of the river, the team drilled cores through the dried Ghaggar-Hakra river bed and analysed the layers of river sediments that had built up over time. They used a method called optically stimulated luminescence dating to determine when the river last flowed.