Geomorphological significance of the morphometric characteristics of first-order basins in the Siwalik Hills in the Himalayas, Nepal

First-order basin morphology is important for the functioning of the whole hydrologic system. This study examines the geomorphology of three areas in the tectonically active Siwalik Hills, Nepal, using morphometric variables. Twenty variables for 11,200 first-order basins were obtained from maps and Digital Elevation Models (DEMs) of three areas (the Machheli, Banganga and Binai river basins). Principal component analysis of each area condensed these into 6 major components, which accounted for 78-82% of the total variance in each data set. Rotated principal components were remarkably consistent between the three areas. The average slope, area, elevation range, valley angle, shape index, hypsometric integral, basin asymmetry factor and drainage density were identified as the eight most important morphometric variables defining these components. Ordinal grouping and k-means clustering led to recognition of eight basin types, distinguished predominantly by elevation range, average slope, area and valley angle. Shallow and steep basins are predominant in all three areas. The spatial distribution of first-order basin types exhibits a distinct pattern of geomorphic regions. The study demonstrated that a first-order basin is a suitable unit to demonstrate the morphological characteristics of a geologically complex area, and the study approach can be applied more broadly.