Late Holocene Chronology and Geomorphic Development of Fluvial-Tidal Floodplains in the Upper Reaches of the Lower Columbia River Valley, Washington and Oregon, USA

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Depositional rates, Floodplains, Fluvial-tidal, Late Holocene, Sea level rise


The upper reaches of the lower Columbia River Valley (125km in length) comprise an alluvial system that is transitional between fluvial and fluvial-tidal dominance. Sinuous channels separate elongate islands (1-8km in length) and floodplains (0.5-12.7km in total width). Thirty-six floodplain overbank deposits are analyzed for age and depth, which demonstrate an average sedimentation rate of 1.6mka-1 during the last 5-6ka. Older core records confirm that long-term depositional rates are controlled by relative sea level rise. Rising floodplain groundwater surfaces, which followed relative sea level rise (~1.25mka-1), submerged isolated floodplain depressions. Low sedimentation rates in the isolated depressions (0.6-1.1mka-1) maintained large ellipsoidal bullseye lakes (7-22km2 in area) dating back to 3.5-4.0ka. Increases in the widths of the floodplains and bullseye lakes are associated with broadening of the incised valley (4-13km width) in the Portland Basin. Dated basal overbank deposits (0.5-5.0ka in age) and their separation distances establish channel migration rates of 0.3-1.9kmka-1. Shallow burial rates relative to rapid channel migration rates resulted in reworking of late Holocene floodplains (50-75% erosion) since 5ka in the upper reaches of the lower Columbia River Valley.


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