Bronze Age Indus civilization thrived on seasonal monsoon-fed waters

A recent study revealed that people of the Bronze Age Indus Valley civilisation relied on seasonal monsoon flooding and the rich, water-trapping clays of the old river valley, rather than – in contrary to popular belief – on major glacier-fed rivers. These are now said to have dried up more than 3000 years before the peak of the Indus Valley civilisation’s  development.

Map of northwestern India and Pakistan with locations of ancient Indus settlements – most of the villages sit in areas not fed by major rivers (by NASA Goddard Space Flight Center, U.S. Geological Survey, Philippa J. Mason & Sanjeev Gupta)

The Bronze Age civilisation developed almost 5000 years ago in what is today Northwest India and Pakistan and is often put side by side with ancient powers that rose in Mesopotamia and Egypt. A few of these cities, including the famed sites of Harappa and Mohenjo-Daro, sit along major glacier-fed rivers. But the bulk of the Bronze Age Indus villages that have been found so far sit far from flowing water, north of the Thar Desert and between the Ganges-Yamuna and the Indus river systems. The assumption was that the settlements first grew alongside the river, and then dried up when the river did. In fact, the river that once filled the dry channel dried up more than 3,000 years earlier and the villages relied on seasonal monsoon flooding and the rich, water-trapping clays of the old river valley for a flourishing system of agriculture.

Bronze Age settlement of Kalibangan with a palaeochannel visible in the background (by Sanjeev Gupta & Imperial College London)

A group of researchers have spend over a decade to unravelled the role of local geology and a major feature such as the palaeochannel, called the Ghaggar in India and the Hakra in Pakistan. They combining various satellite views of the region with radar imagery to build detailed topographical maps of the dry channel, and later took sediment samples from the palaeochannel at the Indus site of Kalibangan, which sits right alongside the dry channel. The team drilled down 40 meters in the sandy soil to extract five unbroken cores of sediment – each drill took about a week. Later, within a lab, researchers sliced the cores in half to analyse the types of sediment and to reveal ages. They revealed that the palaeochannel was, indeed, once a river, with dark-brown and grey sands washed down from the rugged mountains. The sediments matched one river alone – the Sutlej, which now flows in a westerly direction across the Punjab region.

A Landsat 5 composite satellite image of Ghaggar-Hakra paleochannel (dark blue)
(by NASA Goddard Space Flight Center, U.S. Geological Survey, Philippa J. Mason & Sanjeev Gupta)

Using a technique called optically stimulated luminescence the researchers dated the five cores, along with six other cores from other locations along the former Sutlej path, revealing that that in the period from 4,800 to 3,900 years ago, when the Indus villages were at their peak the sediments were dominated by fine sands and muds, typical for low-energy river environments or lakes. The surprise was that the The Sutlej once ran through the old channel, washing down glacial sediments and probably bringing raging seasonal floods to the region,  between 15000-8000 years ago, the river changed course, leaving behind a low-lying river valley, rich in groundwater and likely fed by small, seasonal monsoon rivers that would inundate the valley in fertile mud. The researchers believe that in this environment, agriculture of the Indus valley civilisation developed.

(after Ajit Singh, Kristina J. Thomsen, Rajiv Sinha, Jan-Pieter Buylaert, Andrew Carter, Darren F. Mark, Philippa J. Mason, Alexander L. Densmore, Andrew S. Murray, Mayank Jain, Debajyoti Paul, Sanjeev Gupta, NASA Goddard Space Flight Center, U.S. Geological Survey, Imperial College London & Live Science)

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