Mid to late Holocene geomorphological and sedimentological evolution of the fluvial-tidal zone: Lower Columbia River, WA/OR, USA

Abstract Optically stimulated luminescence dating of lower Columbia River (LCR) sediments reveals only a small fraction of mid-Holocene (ca. 4.3–2.0 ka) sands are preserved within depositionally active bars/floodplains in the Lower Columbia River Valley (LCRV). These LCR sands thus bypassed through the LCRV, and were driven by channel “entrenchment” that was forced by rapid rates of bedload loss to the Columbia River Littoral Cell sediment dispersal system. The magnitude of LCR channel “entrenchment” then relaxed during the late-Holocene (ca. 2.0 ka-present) allowing bars located across the mixed tidal–fluvial, hydraulic regime (~ river kilometer (rkm) 20–56) to initiate vertical aggradation at identical times (ca. 2.2 ka-present). This contradicts the hypothesis that these bars represent the progradation of a late-Holocene “bay-head delta” into the LCRV. Instead, these bars represent the vertical building of the late-Holocene LCR “fluvial top-set.” The products of continued late-Holocene LCR channel “entrenchment” are: (a) the absence of a subaerial delta, (b) a shallow-water central bay within the LCRV, extending from its mouth to ca. rkm 37, whose upper bar sediments are exposed to intertidal processes, and (c) the present day occurrence of estuarine processes extending to ca. rkm 37. These conditions preserve a sedimentological signature in upper bar deposits composed of dirty and/or clean stacked successions (~ 0.5–1.5 m thick) of current ripple cross-laminae, which are the result of shallow flows over bars during tidal cycles and/or oscillatory currents derived from intrabasinal wind-waves modified by slackwater periods and activity of the estuarine turbidity maximum.