Abstract

Loess deposits adjacent to flood coulees on the Columbia Plateau in eastern Washington State record multiple episodes of pre-late Wisconsin cataclysmic floods. Evidence in the loess consists of flood-cut unconformities, upward fining in individual loess strata, and flood deposits that consist of basaltic gravel, fragments of calcified paleosols, and slackwater sediment. At least six episodes of floods during the Pleistocene are indicated by paleomagnetic measurements, tephra correlations, and the stratigraphic position of flood indicators. One episode of the six occurred prior to 790 ka; the most recent episode before the last glacial floods occurred at 36 ka, at the latest.

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