Impact of neotectonics on drainage network evolution reconstructed from morphometric indices: case study from NW Indian Himalaya

Abstract

Summary. The Spiti river basin in North West Indian Himalaya (31–33° E; 77–79° S), is tectonically unstable, exhibits a complex topography, landscape relief and varied Quaternary sedimentation. The major geomorphic landforms viz., alluvial terraces, strath terraces, alluvial fans, debris cone, fluvio-lacustrine deposits, scree and talus cone are present throughout the valley. An attempt has been made by using the geomorphometry approach to access the area in terms of its neotectonic instability, as this basin feeds the river Sutlej. We tried to see the Spiti and the Parachu basin of draining into Sutlej valley, which have been in news several times in the last decade causing floods downstream, due to the outburst of glacial lakes and the formation of temporary lakes during the sliding events, even by the slight disturbance in the monsoon pattern and tectonic activity. Thus the landforms were mapped and assessed in the valley to elucidate spatio-temporal scale dependencies of surface processes active in this region. The Spiti river (constituting of Spiti and Parachu basins) was examined using geomorphic field methods (Spiti basin) and OSL dating. The Quaternary saw ubiquitous mass movements and catastrophic landslides which transported material from steep slopes to valley bottoms and were responsible for the formation of lakes (preserved as thick sequences of fine sediment), while the outburst floods redistributed sediment downvalley affecting life and property downstream. The morphometric approach such as Basin Asymmetry (AF), Topographic Symmetric Factor (T), Stream length Gradient Index (GI) Hydauralic Sinuosity Index (HSI), Topographic sinuosity Index (TSI) and Standard Sinuosity Index (SSI) have helped in understanding tectonic and climatic perturbation. Our observations point towards a tectonically active region with enormous piles of loose, unconsolidated sediment cover which could be disastrous during the slight shift of the climatic and tectonic forces operating in this area.