The Big Lost River flood in southeastern Idaho occurred 20,500 calibrated yr B.P., on the basis of dates derived from cosmogenic 3He and 21Ne measurements of samples from flood-deposited boulders and from scour features. This date corresponds to a date of 16,90014C yr B.P. and is close in age to several other cataclysmic flood events in western North America; it may mark evidence for widespread warming at the end of the Pleistocene in western North America. The Big Lost River flood was smaller than some other late Pleistocene floods, such as the Bonnevi1le flood and the Missoula floods; thus, some samples exposed after the flood had significant amounts of cosmogenic 3He and 21Ne that was acquired before the flood occurred.

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