Evolution of valley-fill terraces in the Alaknanda Valley, NW Himalaya: Its implication on river response studies

Abstract

The present study attempts to understand the importance of place and local factors in the process of valley filling in an active mountain belt. The accumulation of sediments in a valley stretch depends upon the relationship between sediment carried from upstream by the main river entering into the stretch (qi), locally derived sediments (i.e., from local streams (qt), hillslopes (qh), and glacial and periglacial processes (qg)), and sediment exiting that stretch (qo); these are in turn governed by other processes like tectonic activity, intensity of precipitation, and lithology.As an example a valley stretch in the NW Himalaya is chosen in the Alaknanda River, one of the two uppermost tributaries of the Ganga River. This stretch is called as Pipalkoti Valley after the name of the main town; it is located close to the Main Central Thrust (MCT) and falls in the zone of orographic precipitation. Detailed geomorphic mapping reveals four surfaces and two terraces. Three debris-flow surfaces suggest their deposition by local mass flows and stream flows with very small contributions from the Alaknanda River. Terrace T2 occurs as a strath in some parts and is made up of fluvial and lacustrine sediments in others. The presence of lacustrine sediments suggests local damming of the river. Thick fluvial sediment is deposited in stretches of the rivers where it becomes wide due to the joining of a tributary. Synsedimentary deformation has been noted indicating that tectonic activity has affected sedimentation.The results suggest that local processes played a dominant role in the accumulation of sediments in this valley stretch. A model is proposed showing the role of glacial activity, high intensity meteorological events, river width, active faults and landslide damming in the origin of valley-fill deposits of the Pipalkoti Valley. It is concluded that before correlating valley-fill deposits along the lengths of the Himalayan valleys, it is important to evaluate the role of various local processes that have operated in particular stretches.