Geomorphic characteristics of landscape development and formation of lakes in the zone of Munsiari Thrust, Garhwal Himalaya, Uttarakhand, India

The present investigation is focused on imprint of tectonic perturbation along a major tributary, known as the Lastar Gad (Gad = River), in southeast catchment of the Mandakini river in Garhwal Himalaya. Based on morphotectonic studies across the watershed, we recognized evidences of neotectonics along the Jutogh/Munsiari Thrust (MT/MCT-II). Various geomorphic features, e.g., ponding of streams and formation of lakes, fluvio-lacustrine deposits, waterfalls, stabilized and active landslide fans, dissected hills, paired and unpaired terraces, fault scarps and offsetting of drainage pattern etc., point to the active tectonics. The field investigations are supported by computation of morphometric indices, such as, low values of Valley Floor Width to Valley Height (Vf), Asymmetry Factor (Af), elongated and tilted river basin with narrow valley floor as well as higher Bifurcation Ratio (Rb) and sinuosity parameters, immature topography and landscape, elongation and tilting of drainage basin and accelerated erosion-all show that the basin is structurally controlled. The uplift along the valley is further manifested by two significant indexes, e.g., Stream Length (SL) and Steepness Index (Ks). Also, the mud deposits, lacustrine flats and swampy and soggy land within the valley suggest the existence of an ancient lake basin, named as Gethina palaeolake, downstream of present day Badanital lake. The 14C AMS radiocarbon chronology of the Badanital lake core as well as Gethina-Jagtoli palaeolake profiles indicate the formation of these lakes around 5–6 ka BP due to tectonic upheaval along the MT. We assume that a huge landslide activity blocked the course of a small stream (Badani Gad), forming an ancient lake at Gethina locality, followed by deposition of a landslide fan on which was created the Badanital. Subsequently, the palaeolake basin was filled with sediments due to accelerated erosion from almost vertical hills in the catchment. Owing to this, the Gethina (ancient) lake was disappeared and the modern Badanital lake shrank to its present state.