Hydrological impacts of large reservoirs on alterations in the river flow regimes

Abstract

This study evaluated the cascading impacts of large dams Rani Avanti Bai Sagar (RABS), Indira Sagar (IS), Omkareshwar, and Sardar Sarovar (SS) on the water flow regimes of the Narmada River Basin in central India. The construction of these dams has significantly altered hydrological patterns, especially affecting downstream ecosystems. To assess dam-specific impacts, the basin was divided into six units—two for each dam: RABS (Manot+Mohgaon+Bamni and Barmanghat-Patan-Belkhedi), IS (Handia and Mandleshwar-Kogaon), and SS (Mandleshwar and Garudeshwar)—representing upstream and downstream flows. Streamflows were adjusted for tributary contributions by adding at upstream stations (Burnher and Bamni) and excluding at downstream stations (Hiran, Sher, and Kundi) to accurately quantify dam impacts. Critical change points were identified with dam commissioning: RABS (1988), IS (2006), and SS at two stages—SS1 (2006, 121.92 m) and SS2 (2017, 138.68 m), as well as using satellite imagery. Streamflow variability was analysed for pre- and post-dam periods using Indices of Hydrological Alteration (IHA) to evaluate changes in magnitude, duration, timing, frequency, and rate of extreme flows. The Range of Variability Approach (RVA) results and Flow Duration Curves (FDCs) showed significant reductions in downstream mean streamflow, especially during the post-monsoon period (November–May), with substantial declines in cumulative minimum flow affecting water availability. The overall alteration in streamflow was high for RABS (68.5%), moderate for IS and Omkareshwar (≈58%), and for SS, moderate at SS1 (≈60%) and high at SS2 (≈70%). These changes, driven by reservoir operations such as water regulation and diversion, highlight the need for balanced management strategies to optimise reservoir function, preserve natural flow regimes, and enhance flood control in the Narmada Basin. Keywords: Streamflow Variability, Indices of Hydrological Alteration (IHA), Range of Variability Approach (RVA), Cascading Impact, Narmada River Basin.