Morphometric and sediment source characterization of the Alaknanda river basin, headwaters of river Ganga, India

Abstract Erosion and the resulting sediment load is a silent natural hazard that can affect the hydraulic processes in a fluvial system. The physical erosion rate in the Alaknanda basin is five times higher than the global average, and Alaknanda River is a major supplier of sediments to the Ganga River. Anthropogenic intrusion in the form of construction of dams and reservoirs is influencing the natural landscape of the basin. Thus, it is necessary to prioritize erosion prone areas, understand the weathering intensity and identify source bed rocks contributing to the sediment load. The present study displays a combined approach of morphometry and geochemistry for erosion risk estimation. Nineteen morphometric parameters were evaluated for Alaknanda main channel, Mandakini, Pinder, Nandakini, Birahi Ganga and Dhauli Ganga sub-catchments. Suspended sediment samples collected during non-monsoon and monsoon seasons of the year 2014 were analyzed and quantified for sediment load, grain size distribution, clay mineralogy and rare earth elements composition. The results showed the dominance of structural, lithological and climatic control on the erosion processes. The eastern side of the Alaknanda basin was found to be more vulnerable to fluvial erosion. The mean grain size varied from 8.9 to 56.3 μm and 25.3 to 87.3 μm in the post-monsoon and monsoon season, respectively. The clay mineral assemblages, low values of kaolinite/illite ratio, illite chemistry index and illite crystallinity index along with inconsistent Eu and Ce anomaly indicate that physical and chemical weathering of felsic, mafic and carbonate rocks contributes to high sediment load carried by the Alaknanda River.