Around 5000 years from now, in the plain of Indo-Gangetic basin, the great cities of Indus civilization flourished. The common wisdom is that major Himalayan-based rivers nourished this civilization and sustained the life in it. However, at the divide of Ganga-Yamuna and Indus, in the Ghaggar-Hakra region, multiple urban settlements were found without a major Himalayan-fed river flowing through it. This had been the subject of an intense debate: what caused the development of civilization in the Ghaggar-Hakra region located in the North-Western part of India and Pakistan, far away from any active river. It is believed that once a big perennial river flowed through this part of the land and departure of this river led to the decline of the urbanization. Recently, a study published in Nature Communications by a group of scientists from Indian Institute of Technology, Kanpur tried to find an answer .
For mapping the ‘lost rivers’ in the Ghaggar-Hakra region, the researchers used NASA’s Landsat program that takes satellite images of Earth from its orbit 700 km above the sea level. Analysis of these satellite-based images enabled them to identify the ancient rivers once flowed in this region. Guided by these images, the researchers drilled several sites across the river channel to obtain ‘sediment core’.
The ‘sediment core’ obtained by the research team contained coarse-grained sands that indicated the existence of a big river in this region. Now, where did this sand grains come from? To answer this question, the researchers used radioisotopic ‘fingerprinting’ method. Sand grains contain many minerals like zircon, mica etc. that has radioactive isotopes. For example, zircon samples have a fraction radioactive potassium isotope (K40) which spontaneously decays to Ar40 (argon) with a defined rate. Estimating how much of radioactive isotopes left in the sample, one can calculate its age. Provenance analysis of zircon and mica present in the sand grain reveals that the source of the sand grains is Sutlej river which now flows 150 km west of Ghaggar-Hakra site. But, when did Sutlej change its course?
To find the timing of the migration of Sutlej river, they used a technique called Optically Stimulated Luminescence (OSL) dating. The minerals present in the sediment, e.g., feldspar, quartz etc. once buried beneath the ground store energy due to the background environmental radiation. But when the minerals are exposed to the daylight, they release this stored energy. Hence, estimation of this stored energy in minerals can reveal when the sediment got buried and therefore when the river was active. To their surprise, the OSL data showed that Sutlej river migrated from the Ghaggar-Hakra region around 15000 and 8000 years from now which was way before the Indus valley settlements.
This study unequivocally showed that Sutlej river once flowed in the Ghaggar-Hakra region. The migration of the river happened several thousand years before the urban colonization in this place. So, this clearly rules out the possibility that drying up rivers declined the civilization here. In contrary, abandonment of Sutlej triggered the urbanization in the Ghaggar-Hakra region. According to this study, the departure of Sutlej river created many topographical lows that aided to form many monsoon-fed seasonal rivers and water reservoirs. This probably helped the citizen of Indus civilization providing fresh water for drinking and irrigation. Also, the absence of a major Himalayan-fed river eliminated the events of catastrophic floods.
We always discerned that the rivers helped to build up the civilizations. This study prompts us to think in a different light. It suggests that migration of a river can also assist the development of early civilizations.