The formation of vast lakes in the arid basins of the western United States marks one of the most profound hydroclimatic changes of the Quaternary. These ancient lakes,
serving as natural rain gauges, record dramatic fluctuations in water availability, evident from shorelines carved high above modern valley floors. Despite more than a century of
investigation, the climatic drivers of these lake high stands are debated, primarily due to disagreements over their timing. The prevailing hypothesis, based mainly on radiocarbon-dated lake carbonates, now holds that these lake high stands occurred early in the deglaciation, between 18 and 15 ka, during Heinrich Stadial 1, an interval of rapid
global warming and pronounced northern hemisphere seasonality. However, potential biases in radiocarbon dating and calibration uncertainty leaves these age determinations
and climatic interpretations open to question.

To address the limitations, my collaborators and I applied the uranium-series method to date lake and groundwater carbonates from the Mono Basin in east-central California. Our results corroborate previous studies linking the high stands to Heinrich Stadial 1, but the increased precision of our uranium-series ages refines the timing of the
high stand to a narrower interval, between 16.1 and 15.9 ka. This high-resolution chronology reveals the brevity of the wet forcing and suggests a more precise correlation
to Heinrich Event 1—a sudden, short-lived surge of ice and meltwater that was discharged from the Laurentide Ice Sheet into the North Atlantic about 16 ka. This
raises the intriguing possibility that the transient but extreme climatic impact of Heinrich Event 1 may be the precise cause for the dramatic wetting that lifted western U.S. lakes to levels higher than any attained in tens of thousands of years.