Late Holocene Geomorphology of the Columbia River Estuary, Oregon and Washington, USA

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 (0-2,50014 CyrBP) 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.

Mid to late Holocene geomorphological and sedimentological evolution of the fluvial–tidal zone

“…1.8 to 1.0 ka, and (c) 7-10 mmyr À1 average aggradation rates from ca. 1841 (see figures 13 and 14 in Peterson et al, 2014). If these bars represented the building and progradation of the LCR late-Holocene "bay-head delta" or braid delta, a significant temporal delay would be expected with respect to the onset of lower bar deposition and coupled high sediment aggradation rates at Taylor Sands and CB bar1 relative to Welch Island.…”

“…large earthquakes along the central Cascadia Subduction Zone margin (Peterson et al, 2014); (iv) the mid to late Holocene LCR sediment sink occurs within what is considered an infinitely large zone of accommodation (i.e., the OR/WA continental shelf ), and (v) in contrast to the relatively "open" lateral boundary conditions of the experimental setup ( Fig. The sea-level curve is defined by 14 C dated peat samples from Grays Harbor and Willapa Bay, WA, and the LCR, OR/WA.…”

“…The majority of past oceanographic and sedimentological studies on the middle to late Holocene lower Columbia River (LCR) describe it as an estuary (e.g., Baker et al, 2010;Fox et al, 1984;Gelfenbaum, 1983;Hamilton, 1990;Hughes and Rattray, 1980;Jay, 1984;Jay and Smith, 1990;Peterson et al, 2012Peterson et al, , 2013Peterson et al, , 2014Sherwood and Creager, 1990;Simenstad et al, 2011). This classification is based on the broad oceanographic definition proposed by Pritchard (1967), which defines an estuary as "a semi-enclosed coastal body of water in which the salinity is measurably diluted by fresh water derived from land drainage" (p. 3).…”

“…6.1B): (i) it lacks a distinct subaerial delta at its mouth, but does possess a subaqueous ebb-tidal delta seaward of its tidal inlet and an offshore ebbtidal bar (Gelfenbaum et al, 1999(Gelfenbaum et al, , 2001Kaminsky et al, 2010), (ii) the LCR possesses a relatively shallow, open-water, central bay that extends $30 km upriver FIGURE 6.1 (A) 2009 aerial image of the lower Columbia River (LCR) with location map of the Columbia River drainage basin (inset), denoting prominent mid-channel bars/islands as well as the sedimentologically active Cathlamet Bay region and the off-axis back-bay regions of Grays and Youngs Bays, when the water surface elevation was below the Mean Lower-Low Water (MLLW) mark. from its mouth that has not in filled with tidal marshes and/or laterally/vertically accreting floodplain deposits, and is exposed to intrabasinal shallow water wind-waves (Peterson et al, 2014), (iii) brackish water extends to $river kilometre (rkm) 37 during low fluvial discharge periods (Fox et al, 1984), and (iv) the LCR develops seasonal and/or spring-neap tide cycle estuarine circulation patterns (Hughes and Rattray, 1980;Jay, 1984;Jay and Smith, 1990). Note that all unvegetated mid-channel bars are fully submerged when the water surface height is equal to MSL.…”

Tidal Flat Depositional Response to Neotectonic Cyclic Uplift and Subsidence (1–2 m) as Superimposed on Latest-Holocene Net Sea Level Rise (1.0 m/ka) in a Large Shallow Mesotidal Wave-Dominated Estuary, Willapa Bay, Washington, USA

“…In the mid-1980s Willapa Bay was investigated for evidence of tidal marsh subsidence records that could confirm great earthquakes in the central part of the Cascadia subduction zone (Atwater, 1987). As many as seven episodes of abrupt marsh submergence are recorded by their burials by bay mud sequences in Willapa Bay (Atwater, 1997), and in adjacent estuaries (Figure 1) of the central Cascadia margin (Atwater et al, 2003;Peterson & Cruikshank, 2014). The tidal marsh responses to the cyclic interseismic uplift and coseismic subsidence (1-2 m) with recurrence intervals of several hundred years are well documented in Willapa Bay during latest-Holocene time (3.4 to 0.3 ka) (Atwater & Hemphill-Haley, 1997;Atwater et al, 2003).…”