On the existence of a perennial river in the Harappan heartland

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river's uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar's perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using ⁴⁰Ar/³⁹Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.