Morphological Evolution and Modification of Alluvial Fan Terraces in the Eastern Himalayan Siwalik and Piedmont Sections

ABSTRACT River terrace systems are complex geological, tectonic and hydro‐geomorphological features of the Himalayas. This study focuses on the morphological evolution and modification of the Nagrakata, Matiali and Chalsa fan terraces of the eastern Himalayas, using geospatial (1984–2024) and field data (2020–2024). A combined application of remote sensing, GIS, and statistical techniques was employed to identify the role of fluvial, tectonic and anthropogenic activities in shaping different terrace morphologies over the study period. The results reveal that the multiple terraces are formed due to the incision and depositional action of rivers onto the alluvial fan. Ten terrace sequences along the rivers have been studied, which are cut‐and‐fill types of terraces influenced by climatic and tectonic activity. The results of cross‐sectional profile analysis denote that the maximum terraces are two‐level with attached floodplain, but the Jaldhaka and Jiti terraces have recorded the signature of three‐step incision within the Quaternary (Holocene) coarser sediments. The Jaldhaka River bed is incised approximately 77 m by three‐phased downcutting of the previous river valley. However, the expansion of built‐up areas (mainly construction for tourism and households) and the reduction of forests and orchards influence the Terrace Zone (TZ) modification and instabilities over time. River terraces enhance water retention as well as proper land use planning to measure soil fertility and productive agricultural space. Work highlighted the requirement of integrated approaches of terrace agricultural practices, micro watershed management, planning for human habitation and drainage and optimisation of soil erosion in the terrace sequences with environmental sustainability.