Major-ion chemistry, strontium isotope ratios (⁸⁷Sr/⁸⁶Sr), stable isotope ratios (δ¹⁸O, δ²H), and tritium were analyzed for water samples from the southern Bear Lake Valley, Utah and Idaho, to characterize the types and distribution of groundwater sources and their relation to Bear Lake’s pre-diversion chemistry. Four ground-water types were identified: (1) Ca-Mg-HCO₃ water with ⁸⁷Sr/⁸⁶Sr values of ~0.71050 and modern tritium concentrations was found in the mountainous carbonate terrain of the Bear River Range. Magnesium (Mg) and bicarbonate (HCO₃) concentrations at Swan Creek Spring are discharge dependent...