Shuttle Radar Topography Mission (SRTM) data combined with field surveys and radiocarbon and optically stimulated luminescence dating offer us a way to analyze fluvial landforms and date deposits of the Indo-Gangetic Plain. We investigate how climate change affected this civilization by focusing on fluvial morphodynamics, which constitutes a critical gap in our current understanding of the Harappan environment. Controlled by the monsoon and the melting of Himalayan snow, the variable hydrologic regime must have been a concern for Harappans, as it is today for a billion people living on the Indo-Gangetic Plain. Between the Indus and Ganges watersheds, a now largely defunct drainage system, the Ghaggar-Hakra, was also heavily populated during Harappan times. The Harappans inhabited the Indus plain at the arid edge of the monsoonal belt and developed one of the earliest urban civilizations over a territory larger than the contemporary extent of Egypt and Mesopotamia combined ( Fig. P1 A). ![]() The collapse of the Bronze Age Harappan or Indus Civilization remains an enigma ( 1). Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. ![]() The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma.
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