Rivers are essential for water supply, agriculture, hydropower, and biodiversity, yet they face increasing threats from climate change and pollution globally. The Koshi River Basin, spanning China, Nepal, and India, is a transboundary basin that includes glacier-fed headwaters and densely populated downstream regions. This study investigates the hydrogeochemistry and water quality dynamics of the basin, focusing on seasonal and spatial variations and their implications for drinking and irrigation suitability. A total of 117 water samples from different segments of the basin were analyzed across premonsoon, monsoon, and post-monsoon seasons using in-situ, laboratory tests, and multivariate statistical methods. The results indicated that carbonate and silicate weathering were the dominant sources of solutes in the study region, with Ca2+ and \(\text{HCO}_{3^{-}}\) being the major ionic constituents. Seasonal variations revealed higher ionic concentrations in pre- and post-monsoon periods due to increased mineralization, while monsoonal dilution significantly reduced solute loads. Spatially, the higher hydrogeochemistry index in the upstream and lower values in downstream tributaries indicate intensive weathering in the headwaters but are further diluted by melting water and precipitation downstream. Inversely, higher pollution risks only appeared in downstream areas, where \(\text{NO}_{3^{-}}\) suggested agricultural runoff and wastewater intrusion. Water quality index < 50 classified most river segments as suitable for drinking, while sodium adsorption ratio < 1 confirmed irrigation suitability, except in some segments of the downstream zones with higher anthropogenic influence based on major ions. This study also supports Sustainable Development Goal 6 (clean water and sanitation), 13 (climate action), and 15 (life on land) by providing critical insights into water quality dynamics and informing sustainable water management strategies applicable to other glacier-fed river basins worldwide.
