Diverted from its former course to the Pacific Ocean by basalt flows in Gem Valley, Idaho, the Bear River presently flows south into the Bonneville Basin. Constraining the timing of the river's diversion is pivotal to understanding the hydrologic budgets, and thus the climatological implications of the Bonneville Basin lakes. This study employs strontium (Sr) isotopes in mollusc fossils as a tracer of the Bear River water that entered Lake Thatcher, a small, closed-basin lake into which the redirected river flowed en route to the Bonneville Basin. The Sr ratios, combined with the temporal control afforded by amino acid geochronology and tephrochronology, were compared to mixing models constructed from the 87Sr/86Sr composition of the modern rivers draining into the basin to stimulate the Sr isotropic composition of Lake Thatcher. Strontium ratios of six fossil molluscs collected from the lower-most exposed section of the Main Canyon Formation (MCF) indicate that during the early Quaternary (>620 ka), Thatcher Basin was occupied by a locally fed, isotopically-enriched (87Sr/86Sr=0.71309) lake and did not receive input form the Bear River. Eleven fossils, collected from the uppermost exposed section of the MCF, indicate at least three course changes of the Bear River in the late Quaternary: diversion into the basin around 140 ka, diversion from the basin sometime between 140 and 100 ka, and finally diversion back into the basin around 50 ka. Hydrologic modeling of Thatcher Basin with and without the input of the Bear River suggests that water from both a Bear River-influenced or a locally fed lake is capable of filling the basin and causing it to spillover into the adjacent Bonneville Basin. Thus, the Bonneville Basin may have been receiving water from either the Bear River, or the Thatcher Basin rivers, significantly earlier than the ~30 ka previously proposed. Additional hydrologic modeling in Thatcher Basin suggests that a two-fold reduction in the effective precipitation as compared to modern conditions would be required to lower a locally fed Lake Thatcher the ~30 m necessary to account for the paleosol exposed in the uppermost MCF.