Tracing Geomorphic Variability and Forcing Mechanisms of a Highly Regulated Tropical River System in India

Tropical regions cover 19% of the world's landmass and account for more than 40% of the world's population.  Rivers in these zones exhibit significant hydrological and geomorphic dynamism, primarily due to the enormous variability in rainfall and the associated energy regimes, while also supporting some of the world's most productive biological systems.  Furthermore, tropical rivers remain among the most heavily regulated, with flow modification, dam construction, and floodplain encroachment all causing significant deviations from natural channel behaviour.  The downstream geomorphic consequences of these regulatory pressures are yet poorly constrained, emphasizing the need for large-scale, process-based studies of river variability and its governing mechanisms.
To address this research gap, the present study applies the River Styles Framework, a process-based approach and reach-scale geomorphic classification method, to the Narmada River basin (98,796 km²; 1,312 km), the largest west-flowing system in Peninsular India. The study aims to: (i) classify geomorphically distinct river styles; (ii) identify hydrological, geological, and morphological controls governing transitions along the longitudinal profile; and (iii) formulate reach-specific insights to support sustainable river-management strategies. Geomorphic characterization integrates multi-source remote-sensing datasets, SAR-based floodplain delineation, and field validation of key geomorphic units, including floodplains, riffles, pools, barforms, and planform metrics such as sinuosity. Hydrological variability is quantified through Gumbel flood-frequency analysis of four decades of discharge records to determine spatial and temporal patterns in stream power. Sedimentological assessments combine AI-assisted photogrammetry for coarse fractions with laboratory-based particle-size analysis of finer sediments.
The results show 17 distinct River Styles along the Narmada River continuum. Excluding segments affected by reservoir backwater, approximately 64% of the channel length occurs within confined valley settings, 31% within partly confined reaches, and only 5% within laterally unconfined valley environments. Valley slopes, stream power distribution, tributary confluences, and anthropogenic activities, such as dam construction, emerge as the primary controls on spatial variations in channel form and process.
Overall, the study offers a comprehensive, process-based understanding of geomorphic variation along the Narmada River and identifies reaches with high geomorphic sensitivity that require priority management intervention. By combining geomorphic, hydrological, and sedimentological assessments, the findings provide a robust scientific basis for designing economically viable and sustainable management strategies. Narmada's diverse landscapes, geological discontinuities, and significant climatic gradients make it an ideal natural laboratory for developing approaches applicable to major tropical and monsoon-dominated river systems worldwide.