Abstract
Abundant river sediment supply and an open-water central bay area characterize the geomorphology of the large Columbia River estuary (~ 100 km in length). Lateral floodplains and marsh islands do constrict the uppermost reaches of the estuary, but the central axes of the lower estuary are dominated by shallow sand shoals (0–4 m water depth). A total of 58 vibracores are used to document the grain size and age (0–2,500 14CyrBP) of late Holocene deposits in the estuary. Sedimentation rates in stable floodplains (1.1 m ka-1) reflect rates of relative sea level rise (0.75 m ka-1). Sedimentation rates of muddy sand accretionary banks and prehistoric sand shoals (1.5–7 m ka-1) greatly exceed coeval rates of sea level rise, so they must represent short–term rates of vertical accretion resulting from channel lateral migration and associated cut and fill processes. The apparent paradox of unfilled accommodation space in the estuary is resolved by 1) winter wind–wave erosion of sand shoals to -3 m NAVD88 elevation and 2) asymmetric fluvial-tidal advection that results in net seaward transport of bed load in shallow tidal channels (> – 10 m NAVD88) and shallow subtidal shoals (> – 4 m NAVD88) during spring river flooding.
Highlights
The extensive Columbia River estuary (~ 100 km in length) provides a geomorphic paradox
The paradox in the Columbia River estuary is resolved by analyzing prehistoric core deposits, early historic bathymetry and riverine-tidal discharges, and modern wind-wave forcing conditions
Low sedimentation rates for peaty mud deposits in semi-stable floodplain and marsh island settings were controlled by low rates of net sea level rise in the estuary
Summary
The extensive Columbia River estuary (~ 100 km in length) provides a geomorphic paradox. Latest Holocene infilling of the central bay did not reach completion, as demonstrated by 1) the lack of substantial lateral floodplain development and 2) the presence of an open-water central bay that extends 30 km upriver of the tidal inlet. Historic bathymetric change maps were compiled by CREDDP (1983) to establish potential erosion or accretion following 1) wide–scale impoundments of the Columbia River and its tributaries (1937–present), 2) long-term impacts of repeated jetty construction in the Columbia River mouth (1905-present), and 3) extensive deepening and channel bank stabilization of the Columbia River ship channel (1903–present) (Simenstad et al, 1990; Sherwood et al, 1990; Byrnes & Li, 1998). Bathymetric and hydrographic surveys near the mouth of the Columbia River (Hickson & Rudolf, 1951) demonstrate the presence of a shallow central bay since earliest–historic time in 1792 (Figure 2). The narrow south channel (~ 500 m width) was artificially deepened during the 1900s to enable the passage of large ships to upriver ports at the Port of Portland, OR (Figure 1)
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