Hot-spring waters near the Main Central thrust in the Marsyandi River of central Nepal have Sr concentrations to 115 μM with 87Sr/86Sr to 0.77. Small amounts of hydrothermal water (≤1% of total river discharge) have a significant impact on the solute chemistry and the budget of radiogenic Sr in the Marsyandi. In the upper Marsyandi, river chemistry reflects carbonate weathering, with 87Sr/86Sr ≤ 0.72. As the Marsyandi flows across the dominantly silicate High Himalayan Crystalline terrane, both 87Sr/86Sr and [Sr] increase, associated with increases in the concentration of Na+, K+, and Cl−, all of which are high in the hydrothermal waters. Cation concentrations decrease along the Lesser Himalayan reach of the river. Hot-spring dissolved CO2 has a δ13C value to +5.9‰, indicating that metamorphic decarbonation reactions contribute CO2 to the fluids. Hydrothermal CO2 is partially neutralized in high-temperature weathering reactions, which generate alkalinity and yield abundant radiogenic Sr. Radiogenic hydrothermal carbonate can form from these solutions and later weather, releasing silicate Sr but imparting carbonate characteristics to the overall water chemistry.