CHARACTERIZATION OF SEDIMENTS OF THE ALAKNANDA RIVER, GARHWAL HIMALAYA, INDIA

Abstract

The transportation and transformation of sediments by the fluvial system is an important process which influences the continental landscape, global climate, river water quality and flow dynamics. The Himalayan Rivers viz. Ganga and Brahmaputra transport ~25% of the global sediment flux and are considered to have a large influence on the global climate and vice versa. The previous studies showed that the Ganga River transports ~72.9×107 tons of sediments dominantly sourced from the Higher Himalayas. Therefore, it is essential that the sediments from the Upper Ganga basin should be characterized for their physical and chemical characteristics. The Alaknanda River is a headwater stream of the Ganga River flowing in the Garhwal Himalayas in India. The Alaknanda River basin is a unique sub-basin of the Ganga River due to the high rate of physical weathering processes governed by the extreme seasonal rainfall and characteristic bedrock lithology. The sediment erosion rate in the Alaknanda basin is ~863 ton/km2 The study of sediment load and its size and composition is important in understanding the river processes related to hydraulics, geomorphology, flow dynamics, heavy metal pollutants, siltation in reservoirs and contamination by pathogens. The major factors responsible for grain size characteristics and load vary widely including climatic conditions, lithology, discharge, relief, basin area, river energy and anthropogenic activities. Grain size exerts a strong control on the chemical composition of sediments leading to preferential enrichment of specific nutrients, contaminants, pathogens and heavy metals in certain grain-size fractions. Identification of sediment source holds importance for understanding the sediment dynamics and sediment budgeting of a catchment. Previous studies on the Alaknanda River basin dealt with major, trace, rare earth elements (REEs) and isotopic composition of sediments in the middle and lower Ganga River Basin. However, comprehensive studies on the characteristics of the Alaknanda River sediments were lacking. /year, which is five times higher than the average global physical denudation rate. The Alaknanda River transports an enormous amount of sediments downstream which are responsible for the formation of Ganga alluvial plains. In recent years, owing to the large hydropower potential and projects in the Alaknanda basin, erosion processes and fluvial transport of sediments have become a focus of attention. With the objective of characterizing the sediments of the Alaknanda River and its tributaries, water and channel sediment samples were collected during three seasons: pre-monsoon, monsoon, and post-monsoon. The biggest advantage of studying recent sediments is

iv that the sediment composition and its characteristics can be unambiguously correlated with the real-time physiographic and climatic conditions. Quantitative analysis of the sediment load, grain size distribution, major oxide composition, rare earth elements, clay mineralogy and magnetic minerals of the Alaknanda River were conducted. The thesis mostly discusses the spatial and temporal variation of sediment load, grain size distribution, weathering intensity in the basin and the provenance of sediments. In addition, since transportation of carbon by the Himalayan Rivers is a major factor in the biogeochemical cycle of carbon, therefore, a preliminary database of recent total organic carbon (TOC) transported by the Ganga River in its upstream region was also generated; the organic carbon was estimated in the form of the dissolved and particulate load. The present study on the climatic, hydrological, topographic, geological and anthropogenic activities on a temporal scale showed precipitation as a major factor controlling the amount of sediment load carried by the Alaknanda River. During the monsoon months of June-September, >85% of the annual sediment load is transported by the Alaknanda River. The sediment yield during the non-monsoon and monsoon season at Srinagar and Devprayag varied from 0.61-55.19 tons/km2/day and 0.52-38.06 tons/km2 The results showed that sediment load and grain size distribution in the Alaknanda River are influenced by a combination of factors that include topography, hydrology, lithology, confluence with tributaries and anthropogenic activities (dams and reservoirs). The suspended sediments were found to be poorly sorted and the mean grain size (Ms) varied from 8.9-56.3 μm to 25.3-87.3 μm during the post-monsoon and monsoon season respectively, whereas the channel sediments were found to be mostly unimodal, medium to coarse sand (56.4-930 μm) and fine to symmetrically skewed in distribution. /day respectively. The clay mineral and major oxide indices such as illite crystallinity index, kaolinite/illite ratio, chemical index of alteration (CIA) and weathering index of parker (WIP) provided crucial insights to understand the physical and chemical weathering rate in the basin. A similar CIA value and clay mineral assemblages (illite followed by kaolinite and chlorites) in all the three seasons showed that high physical weathering during the monsoon does not correspond to high chemical weathering. The low amount of Al2O3, P2O5, CaO present in the channel sediments and the ratios SiO2/Al2O3 and K2O/Al2O3 The provenance of sediments (suspended and channel) was determined by applying multiple geochemical and mineralogical indices. Application of multiple parameters and indices are considered crucially important for the basin having a complex lithology, such as the varying from 1.0-3.1 and 0.9-1.4 respectively showed the immature nature of river sediments. v Alaknanda River basin. The source of suspended sediment was determined by analyzing the concentration of rare earth elements (REEs). The suspended sediments showed high HREE and inconsistent europium anomaly. The ratios La/Lu and La/Yb >1 and negative Europium anomaly indicated a felsic source of the sediment, whereas the presence of positive Europium and negative Cerium anomaly was attributed to the carbonate and mafic weathering. The principal component analysis (PCA) on the major oxide composition of channel sediments revealed that weathering of felsic, mafic, and carbonate rocks are responsible for the high sediment load carried by the Alaknanda River. The values of low magnetic field susceptibility (χlf) highlighted the role of local lithology and sediment mixing. Magnetic parameters and ratios, e.g., χlf, χARM, B(0)CR The variation in the organic carbon content in the Alaknanda River showed that seasonal erosivity and physiography of the basin are the key parameters which control the mode of transport, residence time, and oxidation of the organic matter. During the monsoon season, at Devprayag, the Alaknanda River contributes ~66% of the total dissolved organic carbon (DOC) flux to the Ganga River. , SIRM, HIRM, S-ratio and the heavy mineral assemblages indicated a predominance of ferrimagnetic minerals in all the samples derived from the low to high grade metamorphic rocks, igneous intrusions, and volcanics of the Alaknanda valley. The results of REEs, major oxides, clay mineral assemblages, magnetic mineral parameters and heavy mineral assemblages were all in accordance with each other and revealed that Inner Lesser Himalayas (Garhwal Group) enriched in carbonates does not only dominantly source the dissolved load but also a large part of the sediment load carried by the Alaknanda River. The present thesis has been organized into six chapters. Chapter 1 outlines a brief introduction and motivation to carry out the study in the Alaknanda basin and the aim and objectives. Chapter 2 summarizes the literature review on global rivers and studies on the Alaknanda River. Chapter 3 deals with the details of the study area (Alaknanda basin) including geographic location, climate, geology, etc. Chapter 4 deals with the methodology formulated to achieve the aims and objectives of the thesis. Chapter 5 includes results and discussion section and Chapter 6 listed the major conclusions of the present study and future scope.