Disentangling relative controls of landscape evolution and spatial heterogeneity in the Vaigai River Basin, southern India: Regional implications and applicability

Deciphering tectono-climatic-lithological and other controls over landscape evolution at variable spatiotemporal scales is paramount in natural resources management and development. We have systematically documented and analysed selected critical quantitative morphometric data, namely, asymmetry factor, hypsometric integral, hypsometric curve, drainage divide migration and normalised steepness index, in addition to the assessment of longitudinal profiles and knickpoints, for a unique morphological river basin, the Vaigai River Basin, located in southern India. The spatial variability of various controls of geomorphological characteristics of the basin is interpreted using these morphometric parameters and field data. Notable results include, predominance of a concave-up nature of the majority of the sub-basins, occurrence of a distinct convex zone in the upper part and a distinctly concave zone in the lower part of the basin, a range of symmetric to asymmetric topographic nature with very prominent asymmetry in the middle of the basin, highest normalised channel steepness in the northernmost sub-basin and insignificant and lesser steepness in the majority of the basin, evidence of north-easterly migration of drainage divides within the river basin, manifested by sub-basin boundaries and the alignment of the majority of the knickpoints parallel to the NE–SW trending Karur-Kambam-Painavu-Thrissur shear zone. The results cumulatively indicate that the Vaigai River that runs on an antecedent basin continues evolving, principally under the controls of regional Precambrian structures. Nevertheless, the evolution has been episodic and spatially varied. The resurgent tectonic influence is more pronounced in the northern and north-western parts of the basin, while the river capture and drainage divide migration are pronounced in the northeastern part, signifying the collective influence of tectonics and climate. On the contrary, the middle and lower reaches of the basin lack a soil-sediment-weathered zone, signifying the prevalence of an intense erosional phase of the river. This information, together with previously documented catastrophic flooding events and sediment accumulation, categorise the river basin as episodically active, and in a transient phase of evolution. While providing affirmative evidence for the previous regional geomorphological model, the interpretation of the continuum of inherited Precambrian structures in the present study implies that there are subtle spatial differences of lithological-climatological and structural controls within and among each of the sub-basins that need to be considered for any natural resources management at local to regional scales.