Late Holocene chronology and geomorphic development of fluvial-tidal floodplains in the upper reaches of the lower Columbia River Valley, Washington and Oregon, USA

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