Fluvial landscapes of the Harappan civilization PNAS PLUS Liviu Giosana,1, Peter D. Cliftb,2, Mark G. Macklinc, Dorian Q. Fullerd, Stefan Constantinescue, Julie A. Durcanc, Thomas Stevensf, Geoff A. T. Dullerc, Ali R. Tabrezg, Kavita Gangalh, Ronojoy Adhikarii, Anwar Alizaib, Florin Filipe, Sam VanLaninghamj, and James P. M. Syvitskik aGeology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; bSchool of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom; cInstitute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, United Kingdom; dInstitute of Archaeology, University College London, London WC1H 0PY, United Kingdom; eDepartment of Geography, University of Bucharest, Bucharest, 70709, Romania; fDepartment of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom; gNational Institute of Oceanography, Karachi, 75600, Pakistan; hSchool of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom; iThe Institute of Mathematical Sciences, Chennai 600 113, India; jSchool of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, AK 99775-7220; and kCommunity Surface Dynamics Modeling System (CSDMS) Integration Facility, Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309-0545 Edited by Charles S. Spencer, American Museum of Natural History, New York, NY, and approved March 20, 2012 (received for review August 5, 2011) The collapse of the Bronze Age Harappan, one of the earliest urban Brief Harappan History civilizations, remains an enigma. Urbanism flourished in the wes- The Harappan cultural tradition (2–4) evolved during an Early tern region of the Indo-Gangetic Plain for approximately 600 y, but Phase (5,200–4,500 y ago) from antecedent agricultural commu- since approximately 3,900 y ago, the total settled area and settle- nities of the hills bordering the Indus alluvial plain to the west and ment sizes declined, many sites were abandoned, and a significant reached its urban peak (Mature Phase) between approximately shift in site numbers and density towards the east is recorded. We 4;500 and 3,900 y ago. The Harappans were agrarian but devel- report morphologic and chronologic evidence indicating that flu- oped large, architecturally complex urban centers and a sophis- vial landscapes in Harappan territory became remarkably stable ticated material culture coupled with a robust trade system. In during the late Holocene as aridification intensified in the region contrast to the neighboring hydraulic civilization of Mesopotamia after approximately 5,000 BP. Upstream on the alluvial plain, the (14) and to Egypt (15), Harappans did not attempt to control large Himalayan rivers in Punjab stopped incising, while down- water resources by large-scale canal irrigation (16). Deurbaniza- tion ensued after approximately 3;900 y ago and was character- stream, sedimentation slowed on the distinctive mega-fluvial ridge, EARTH, ATMOSPHERIC, ized by the development of increasingly regional artefact styles AND PLANETARY SCIENCES which the Indus built in Sindh. This fluvial quiescence suggests a gra- and trading networks as well as the disappearance of the distinc- dual decrease in flood intensity that probably stimulated intensive tive Harappan script (2–4; 17). While this is often referred to as agriculture initially and encouraged urbanization around 4,500 BP. “collapse,” archaeological evidence indicates a protracted and However, further decline in monsoon precipitation led to conditions regionally varied process (2, 4, 17). Many settlements exhibit adverse to both inundation- and rain-based farming. Contrary to continuity, albeit with reduced size, whereas many other riverine earlier assumptions that a large glacier-fed Himalayan river, identi- sites are abandoned, especially along the lower Indus and the fied by some with the mythical Sarasvati, watered the Harappan Ghaggar-Hakra course (3–5; SI Text). Between 3,900 and 3,000 y heartland on the interfluve between the Indus and Ganges basins, ago, there was a proliferation of smaller, village-type settlements ENVIRONMENTAL SCIENCES we show that only monsoonal-fed rivers were active there during (2–4, 6, 18), especially in the Himalayan foothills and the western the Holocene. As the monsoon weakened, monsoonal rivers gradu- part of the Ganges basin along the Yamuna River and on the ally dried or became seasonal, affecting habitability along their Yamuna-Ganga interfluve (Fig. 3B). courses. Hydroclimatic stress increased the vulnerability of agricul- Socio-economic theories have been invoked to address the tural production supporting Harappan urbanism, leading to settle- collapse of urban Harappan society, including foreign invasions, ment downsizing, diversification of crops, and a drastic increase in social instabilities, and decline in trade (4). Alternatively, envir- settlements in the moister monsoon regions of the upper Punjab, onmental factors were suggested to play a decisive role in the Haryana, and Uttar Pradesh. decline (1, 2, 8); among these, regional aridification, hydrological changes such as the drying or capture of the Ghaggar-Hakra Indus Valley ∣ floods ∣ droughts ∣ climate change ∣ archaeology system (3, 4, 7, 19, 20), as well as human-induced environmental degradation (21) have been advanced. Despite almost a century he Harappan or Indus Civilization (1–8) developed at the arid of research, a clear perspective on the role played by fluvial Touter edge of the monsoonal rain belt (9, Fig. 1) and largely dynamics in influencing the fate of the Harappan civilization has depended on river water for agriculture (10). The Harappans been hampered by a lack of high-resolution topographic data and settled the Indus plain over a territory larger than the contempor- sedimentary chronologies. Shuttle Radar Topography Mission (SRTM) data (22) combined with field surveys and radiocarbon ary extent of Egypt and Mesopotamia combined (Figs. 2 and 3). and optically stimulated luminescence dating offer us a way to Between the Indus and Ganges watersheds, a now largely defunct analyze fluvial landforms and date deposits of the Indo-Gangetic smaller drainage system, the Ghaggar-Hakra, was also heavily Plain. In this context, we reexamine archaeological site distribu- populated during Harappan times (4, 5). Controlled by the Indian monsoon and the melting of Himalayan snow and glaciers (2, 11, 12), the highly variable hydrologic regime, with recurring droughts Author contributions: L.G., P.D.C., M.G.M., and D.Q.F.designed research; L.G., P.D.C., M.G.M., and floods, must have been a critical concern for Harappans, as it is D.Q.F., S.C., J.A.D., T.S., G.A.T.D., A.R.T., K.G., R.A., A.A., F.F., S.V., and J.P.M.S. performed research; L.G., P.D.C., M.G.M., D.Q.F., S.C., J.A.D., T.S., G.A.T.D., A.R.T., K.G., R.A., and J.P.M.S. today for almost a billion people living on the Indo-Gangetic Plain analyzed data; and L.G. and D.Q.F. wrote the paper. in Pakistan, northern India, and Bangladesh. In such challenging The authors declare no conflict of interest. environmental conditions, both the development and the decline This article is a PNAS Direct Submission. of the Harappan remain equally puzzling (13). We investigate how 1To whom correspondence should be addressed. E-mail: [email protected]. climate change affected this civilization by focusing on fluvial 2Present address: Department of Geology and Geophysics, Louisiana State University, morphodynamics, which constitutes a critical gap in our current Baton Rouge, LA 70803. understanding of the Harappan in the way it affects habitability This article contains supporting information online at www.pnas.org/lookup/suppl/ and human settlement patterns near rivers in arid regions. doi:10.1073/pnas.1112743109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1112743109 PNAS Early Edition ∣ 1of7 Downloaded by guest on September 26, 2021 200 Karakoram m Punjab Plain B A 150 Indus (Sindh) Plain Ravi II-II’ Indus Jhelum 100 IV Chenab Himalayas Indus IV-IV’ Ravi I-I’ III-III’ Beas 50 Togarh Indus Delta Range Sutlej 0 0 500 1,000 (km) III Yamuna 1,500 Sulaiman Ganga Range Ramganga Ghaghara IV’ Himalayas Brahui III’ Range nge a g Kirthar Thar Desert k t II’ I II ravalli R A 2,000 C /sec) Indus 3 1,000 I’ Vindhya Range Satpura Range 700 Panjnad 600 500 Discharge (m J MMJ SN Month D Indus Indus 2.8 Indus Jhelum Chenab Ravi Sutlej 20 30 2.3 Chenab 3.9 200 2.1 Indus 0.9 20 2.6 10.0 10.4 10 120 3.6 0.8 10 180 0 110 Profile I-I’ (150 km) Profile II-II’ (130 km) Profile III-III’ (205 km) 100 Profile IV-IV’ (420 km) 160 Indus (Sindh) Plain Punjab Plain Fig. 1. Large-scale morphology of the Indo-Gangetic Plain (for altitudes, pattern of colors repeats every 10 m to 300 m in height; higher landscape in gray). Convex downslope, aggradational landscapes (e.g., Tista ¼ t; Kosi ¼ k; Gandak ¼ g) have a lighter mask, whereas incisional landscapes have a red mask. (B) Along-channel longitudinal profiles for the Indus and its Punjab tributaries. (C) Hydrographs for the Indus and confluent Punjab tributaries (Panjnad River). (D) Profiles across the extended Indus plain (in meters above sea level). River channel locations are identified on the profiles, as are locations and ages of studied fluvial sedimentary deposits in Sindh and Punjab (ages in thousands of years). tion to understand how climate-controlled changes in river dy- of the Sutlej River (12), the easternmost tributary of the Indus, namics affected the Harappans (SI Text). century-long phases of sediment load decline caused by weak monsoons were responsible for incision, primarily in the early Morphodynamics of the Indo-Gangetic Plain Holocene between approximately 10;000 and 8,700 y ago. The The Indo-Gangetic Plain (Fig. 1) was built during the Cenozoic presence of Harappan and even earlier settlements within these with sediments derived primarily from the Himalayas (23).