Journal of Geoscience and Eco Agricultural Studies

Abstract

Geologists and geomorphologists have employed various morphometric methods, techniques, and indices to study landform evolution, their origin, and spatial distribution. One of the widely used morphometric methods for the qualitative interpretation of slope–length gradient and drainage characteristics of rivers was proposed by Hack [1,2]. The present study applies the SL Index to analyse the morphotectonic framework of the Rispana River, located in the Dehradun district of the Garhwal Himalaya, India. The methodology is based on a high-resolution Digital Terrain Model (DTM) generated from 1:50,000 scale Survey of India topographic sheets with 20 m contour intervals. On the basis of slope elements, the longitudinal profile of the river was divided into 14 segments. SL Index values were calculated for each segment and represented through tabular and graphical methods to identify spatial anomalies along the river course. The SL Index values range from 1 to 11.48. Out of the 14 segments, three exhibit first-order anomalies, seven show second-order anomalies, and the remaining four segments display no detectable anomalies. High SL values (11.00, 11.44, and 11.48) are associated with structurally resistant lithologies such as quartzite and dolomite, steep gradients due to tectonic uplift, and the presence of waterfalls and knickpoints. The SL Index shows an abrupt decline at segment 11 and subsequently fluctuates within moderate values (≈2–4), indicating gentler slopes and uniform sediment deposition in the downstream reaches within the Doon Valley. A distinct contrast is observed between the two physiographic regions—the mountainous terrain and the Doon Valley Plain. The SL Index is consistently higher in the Lesser Himalayan zone and lower in the Doon Valley Plain. The application of the Hack Index proves to be highly effective for the geo-morphic analysis of Himalayan rivers, particularly in understanding the influence of lithology, tectonic control, and slope gradient.