Geomorphologic study of the valley floor in different tectonic segments along Kosi River valley between South Almora Thrust and Himalayan Frontal Thrust: Kumaun Himalaya, India

摘要

> In the Kumaun Himalaya, a portion of the Kosi River valley of ~90?km in length is chosen to study the fluvial morphology that provides first‐order information about the dynamic response of bedrock channels to tectonic impulse. The Kosi River flows across/along major tectonic boundaries such as the South Almora Thrust, Ramgarh Thrust, Main Boundary Thrust, and the Himalayan Frontal Thrust, and local transverse and longitudinal faults. Varied fluvial landforms correspond to different tectonic settings, lithologies, bedrock channels, hillslopes, large landslides, terraces, and fans. The longitudinal valleys are also the sites for thick aggradational landforms. Some portion of these valleys fall in the areas of active extensional tectonics and is characterized by one of the widest valley floor sections in the Lesser Himalaya. In contrast, the transverse valley sections are incised by deep‐cut v‐shaped valleys and the narrowest valley section. Swerving of the Kosi River is observed in the Ramgarh Thrust and Amel Fault zones and also in the Main Boundary Thrust zone. Recent tectonic activity is evident from the presence of the faulted Quaternary deposits, linear fault scarps, abandoned channels, incised meandering, and multiple levels of terraces/strath terraces. Field observations and the computed ratio of valley floor width to valley height (V _f ) corroborate each other. Valleys developed parallel to the strike of faults and bedrocks have relatively broader valleys with higher V _f values whereas in contrast, the valleys developed across the bedrock strike are narrow with smaller V _f values. The results of computed stream length gradient (SL) and steepness (Ks) indices show considerable correlations between the obtained SL and Ks data and the field evidences; high values of SL and