The Quaternary geologic history of Lake Thatcher, southeastern Idaho, is pivotal in determining when the Bear River carved its through-flowing channel into the Bonneville Basin. Bright's reconstruction of the history of Lake Thatcher was based on 14C dates that are now known to be inaccurate. Lava Creek B ash found interbedded with lacustrine sediment suggests that Thatcher Basin contained a lake well before Bright's original estimate of 34 ka. D/L ratios, which measure the extent of racemization in fossil gastropods, in conjunction with paleosols and tephra, were used to correlate between four localities across Thatcher Basin to reassess lake-level chronology. Lab-heating experiments were performed on four genera of molluscs to obtain integeneric conversions of D/L, which aided in chronostratigraphic correlation between beds that contained different genera. An age calibration curve for Lymnaea was developed using D/L ratios associated with the proto Mount St. Helens tephra (90±30 ka; D/L=0.21 [Valvata]) and Lake Bonneville sediment (20±5 ka; D/L=0.09 [lymnaea]). D/L ratios from non-dated lake deposits were assigned ages using this calibration curve, providing the basis for the late-Pleistocene Lake Thatcher chronology. Based on high D/L ratios in Valvata (~0.7) contained in the lower Thatcher sediment, lake level through early-middle Pleistocene was shallow, evidenced by numerous paleosols and organic-rich beds. A gap in the Lake Thatcher stratigraphic record exists for much of the middle Pleistocene. Lake Thatcher existed at its highest elevation from ~110 until ~80 ka, separated from the final Lake Thatcher high-water episode at ~40 ka by a middle-Wisconsin soil. This provides a maximum age on the breaching of the divide at Oneida Narrows and unimpeded flow of the Bear River into Bonneville Basin. By ~17 ka, Lake Bonneville backed up into Thatcher Basin, during its all-time highstand, which Bright believed was owned in part to the added inflow of the Bear River. However, evidence from this study suggests that the Bear River may have been spilling over the divide at Oneida Narrows into Bonneville Basin at ~110 ka.