Sediment records as archives of the Late Pleistocene–Holocene hydrological change in the alluvial Narmada River basin, western India

Abstract

The rivers of western India are monsoon dominated and have been so throughout the late Quaternary. Sediment accumulation in these river basins has been controlled by climatic and tectonic changes over a time span from the Late Pleistocene to the recent. The lithofacies assemblages associated with the various sediment archives in the Narmada basin range from the boulders of the alluvial fans to overbank fines on the alluvial plains. Estimates, based on clast size, of stream power and competence, bed shear stress and discharge reveal that hydrological conditions during the Late Pleistocene (∼90 ka) were comparable to the present day. The size of the transported clasts and the thickness of the accumulated sediment indicate the influence of basin subsidence rather than an increase in discharge. Discharge estimates based on sedimentary structures preserved in the alluvial-plain facies suggest that the channel had a persistent flow, with a low width–depth ratio and large meander wavelength. The hydrological changes during the Holocene are more pronounced where the early Holocene is marked by a high-intensity hydrological regime that induced erosion and incision of the earlier sediments. The mid-Holocene stream channel was less sinuous and had a higher width–depth ratio and a higher meander amplitude in comparison with the present-day channel. Palaeo-fluvial reconstructions based on the sediment archives in the alluvial reach of the river basin are important tools in understanding the long-term hydrological changes and the intricate fluvial architecture preserved in the Narmada River basin ensures scope for detailed studies to identify phases of weak and enhanced hydrological regimes.

Introduction

The present-day fluvial processes in the alluvial river basins (the term ‘alluvial’ river is used here to describe a lowland river in which the bed and banks are made up of mobile sediment and soil) of western India are closely connected to the periodicity and intensity of the southwest monsoon. These fluvial systems (Fig. 1a) have been sensitive to climate change during the late Quaternary and the corresponding hydrological alteration is manifest in the altered fluvial styles and unique sedimentary environments. The monsoon dominated rivers conform more closely to environments with high variability in discharge, where large floods are the formative discharges (Gupta, 1998). Sediment archives preserve the evidence of past flow regimes; sand-sized particles reflect the average flow conditions, whereas coarser sediments yield information on the maximum flow and the related stream competence (Wohl and Enzel, 1995). Thick sediment sequences forming the wide alluvial plains of western India represent a time span from the Late Pleistocene to the recent (Chamyal and Juyal, 2008, Juyal et al., 2006, Chamyal et al., 2003, Jain and Tandon, 2003). The intriguing lithofacies diversity in these deposits can be inferred in terms of the changes in the flow regime (i.e. the competence of the flow to transport a particular sediment size and related discharges) and channel geometry. The flow regime of a river basin may be directly related to the character of the monsoon; however, channel behaviour and sediment dynamics are dependent on channel bed characteristics and geometry. This paper synthesizes sediment records in order to compare the Late Pleistocene and Holocene hydrological regimes in the alluvial Narmada River basin.