A hydromorphic reevaluation of the forgotten river civilizations of Central Asia

Abstract

The Aral Sea basin in Central Asia and its major rivers, the Amu Darya and Syr Darya, were the center of advanced river civilizations, and a principal hub of the Silk Roads over a period of more than 2,000 y. The region's decline has been traditionally attributed to the devastating Mongol invasion of the early-13th century CE. However, the role of changing hydroclimatic conditions on the development of these culturally influential potamic societies has not been the subject of modern geoarchaeological investigations. In this paper we report the findings of an interdisciplinary investigation of archaeological sites and associated irrigation canals of the Otrār oasis, a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage site located at the confluence of the Syr Darya and Arys rivers in southern Kazakhstan. This includes radiometric dating of irrigation canal abandonment and an investigation of Arys river channel dynamics. Major phases of fluvial aggradation, between the seventh and early ninth century CE and between 1350 and 1550 CE coincide with economic flourishing of the oasis, facilitated by wet climatic conditions and higher river flows that favored floodwater farming. Periods of abandonment of the irrigation network and cultural decline primarily correlate with fluvial entrenchment during periods of drought, instead of being related to destructive invasions. Therefore, it seems the great rivers of Central Asia were not just static "stage sets" for some of the turning points of world history, but in many instances, inadvertently or directly shaped the final outcomes and legacies of imperial ambitions in the region.

Keywords: OSL dating; Otrar; Syr Darya; floodwater farming; fluvial geomorphology.

Fig. 1.

Research area in Central Asia. Transoxanian historical timeline: Achaemenid Empire (AE), Hellenic rule: initial conquest and successor states (HR), Sassanid Empire (SE), Turkic rule (TR), Mongol Empire and later Khanates (MR), Timurid Empire (TE), Uzbek and Kazakh rule (UR/KR), Russian Empire, its successors and the current Republic of Kazakhstan (RE/RK). Black bars indicate nomadic pastoralist cultures; white bars indicate sedentary cultures. The period of migration refers to the displacement of Saka, Kangju, Yuezhi, and Wusun nomadic confederacies in the region (SI Appendix, S1.1).

Fig. 2.

Geomorphology and archaeology of the Arys river catchment and Otrār oasis. (A) The Arys river catchment and research areas; (B) Otrār oasis with main irrigation canals and archaeological sites; (C) the Arys River at low flow; (D) the Badam River during peak discharge; (E) aerial view of irrigation canal bifurcations southeast of Altyn (Reproduced from ref. , with permission from the copyright holders, IWA Publishing.); (F) aerial view of Otrār (Reprinted with permission from ref. 20); (G) irrigation canal bifurcation with a fortified settlement (outline in red: Pchakshi) (46); (H) ancient irrigation canal near Arys town; (I) abandoned canals (white arrows) and abandoned agricultural field plot (black arrows) east of Kuik Mardan (46).

Fig. 3.

Schematic representation of Late Pleistocene and Holocene river terraces in the Badam and Arys valleys. Inset pictures: (Left) Stratigraphy of Tb1-Tb3; (Middle) laminated sandy sequences of terraces Tb4, Tb5, Ta3, and Ta4; (Right) the coarse-grained ∼1350 CE Late Medieval flood unit (Tb4, Tb5, and Ta3).

Fig. 4.

Compilation of hydrological, climatic, and archaeological data for the last 1,600 y. (A) Aral Sea levels (35); (B) Aral Sea salinity (36); (C) Pollen-based precipitation reconstruction of the Aral Sea basin (36); (D) Central Asian arid zone decadal moisture index (37); (E) Tree-ring–based temperature reconstructions from the Altai Mountains (38); (F) NAO index series (39); (G) Settled area in Otrār oasis (44); (H) radiometric dates for canal abandonment in Otrār oasis (this study): squares refer to OSL dates; circles to calibrated radiocarbon dates; a black fill refers to a TPQ date; a gray fill to a terminus ante quem date; a white fill to an accurate date for the onset of canal abandonment. Background: The gray shading highlights main phases of fluvial aggradation in the Arys catchment (this study); asterisks on the right-hand axis indicate OSL-dated extreme flood events in the Arys river catchment.