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ABSTRACT This study uses a hydrologic‐balance model to evaluate the range of precipitation and temperature (P‐T) conditions required to sustain Lake Bonneville at two lake levels during the late Pleistocene. Intersection with a second set of P‐T curves determined from glacial modelling in the nearby Wasatch Mountains places tighter climatic constraints that suggest gradually increasing wetness from ~21 to 15 ka. Specifically, during the latter part of the Last Glacial Maximum (LGM) (~21–20 ka), Lake Bonneville approached its highest level under conditions roughly 9.5°C colder but only 7% wetter than modern. As the lake reached its pre‐flood Bonneville level (~18.2–17.5 ka), climate conditions were ~16% wetter and ~9°C colder than modern. Byca. 15–14.8 ka, Lake Bonneville abandoned the overflowing Provo level under conditions that were ~21% wetter and ~7°C cooler. These results suggest that regional LGM highstands were not caused by large increases in precipitation, but rather by a climatic optimum in which moderate wetness combined with depressed temperatures to create a positive hydrologic budget. Later highstands during Heinrich I from 17 to 15 ka were likely achieved under gradual increases in precipitation, prior to a transition to drier conditions after 15 ka.