| Summary: | Thesis (Ph.D.)--University of Washington, 2022 This thesis examines last-glacial chronologies of the Okanogan lobe of the Cordilleran Ice Sheet and explores whether this glacier released floods. The findings include age ranges for ice advance and retreat across the Columbia River, and a map of a subglacial channel network tributary to Moses Coulee. The Okanogan lobe advanced across the Columbia River between 19 and 17 ka (thousand years ago), and this piedmont glacier retreated north of the Columbia between 15 and 13.5 ka. This timing is inferred from new exposure ages that deviate somewhat from previous estimates based on varves, tephra, and radiocarbon. Independent chronologies of the same history agree broadly but differ by as much as 2,000 years for the ages of both advance and retreat. Last-glacial floods down Moses Coulee could have come from glacial Lake Missoula, the Okanogan lobe, or both. Five such floods are known. Subglacial channels tributary to the coulee cross high divides and radiate towards the ice lobe’s margins. Subglacial drainage through these channels, if catastrophic, may thus explain the floods. Also noted are several enigmatic features of a last-glacial flood bar, downstream from the Okanogan lobe terminus and beside a Moses Coulee scabland. These features include a basal, clast-supported, flood or alluvial gravel into which calcrete, 0.5 m thick, has formed; and an overlying, last-glacial flood gravel partly supported by a silty matrix. The calcrete, and the gravel beneath it, predate the last glaciation, but it is unclear how each of the two gravel units was deposited.
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