Riverscape dynamics in the northwestern Himalaya and its non-contemporaneity to the Harappan Civilization

A large perennial river system (Ghaggar-Hakra), comprising palaeo-Sutlej and Yamuna Rivers, has been argued to have sustained the Bronze Age Harappan urban settlements (~۴.۶-۳.۹ thousand years before present, ka) in northwestern Indian plains. Based on a comprehensive chronostratigraphic, sedimentological and sediment provenance data, our work has shown that palaeo-Sutlej was already defunct in this region by ~۸ ka. The Harappan urban settlements thus developed along an abandoned river valley rather than an active Himalayan River. However, the role of the paleo-Yamuna as an important feeder to the Ghaggar-Hakra system remained weakly documented and enigmatic which has eluded the complete understanding of the impact of local hydrological conditions on the growth and eventual demise of these settlements. Our new data on high-resolution chrono-stratigraphy from six sediment cores (~۵۰ m deep) across the palaeochannels of Yamuna document multistoried sand bodies deposited by a mobile channel belt in a large fluvial fan system. Based on extensive luminescence dating, we infer a major drainage reorganization of the palaeo-Yamuna River at ~۱۸ ka, at least ۱۰ ka before the westward switch of the paleo-Sutlej River. This means that the Ghaggar-Hakra system was already flowing with a much-reduced discharge during the Early Holocene and was completely defunct by ~۸ ka, much before the decline of the Harappan Civilization started.