Alluvial architecture of mid‐channel fluvial–tidal barforms: The mesotidal Lower Columbia River, Oregon/Washington, USA

Abstract

Barforms of mesotidal to macrotidal fluvial-tidal transitions, regardless of fluvial-discharge, are currently thought to display a sedimentary architecture dominated by tidal signatures. Due to the scarcity of observations from modern mesotidal fluvial-tidal transitions, especially those of multi-channelled large-rivers (mean annual discharge ≥7000 m 3 s À1 and peak discharges ≥15 000 m 3 s À1) with mid-channel bars, this concept remains unproven. The present study analyses data produced by a combination of high-resolution ground penetrating radar and coupled shallow vibracores (<5 m depth), collected from modern fluvial-tidal mid-channel bars of the mesotidal multichannelled Lower Columbia River, Washington/Oregon, USA, which can experience peak discharges ≥18 000 m 3 s À1. These data were used alongside time-sequenced aerial imagery to characterize the spatio-temporal sedimentological evolution of these barforms in singular flows or combined flows consisting of river, tidal and/or wind-wave oscillatory, current components operating in unique fluvial-tidal transition regimes. Results indicate that ca 75% of the Lower Columbia River fluvial-tidal transition produces braidbars with basal to bar-top sedimentological architectures that are indistinguishable from fluvial-only braid-bars recorded in the literature. Barform stratal characteristics within the fluvial-tidal transitions of mesotidal largerivers are therefore more likely to be dominated by downstream-oriented currents. Furthermore, a new style of low-angle (<5°) inclined heterolithic stratification found in bar-top accretion-sets within upper-mixed tidal-fluvial

The morphology of fluvial‐tidal dunes: Lower Columbia River, Oregon/Washington, USA

“…Thus, only the most‐seaward main channel of the fluvial‐tidal transition (maximum of ~14% of total) displays smaller‐scale, simple dunes with spatio‐temporally fluctuating orientations indicative of a ‘fluvial‐tidal to tidal’ signature (Figure 16A,B). Overall, this large‐scale morphological pattern is more comparable to a tidally‐influenced fluvio‐deltaic environment, rather than a tidally‐dominated estuary, which supports the findings of Prokocki et al (2015, 2020). Furthermore, the downstream to upstream trend of increasing dune size through its fluvial‐tidal transition at low‐river stage suggests an accompanying upstream‐directed increase in their form drag.…”

“…Yet, in contrast to this finding, many oceanographic and geologic studies describe the LCR as an estuary (e.g., Fox et al, 1984; Gelfenbaum, 1983; Hamilton, 1990; Hughes & Rattray, 1980; Jay, 1984; Jay et al, 1990; Jay & Smith, 1990; Peterson et al, 2013; Peterson et al, 2014; Sherwood & Creager, 1990; Simenstad et al, 2011). However, its geologic designation as an estuary has been re‐evaluated by Prokocki et al (2015, 2020), who suggest that the mid to late Holocene geomorphology of the LCR is that of an ‘entrenched' fluvio‐deltaic environment possessing a subaqueous deltaic top‐set extending to ~ rkm 50, thus supporting fluvial dominance, whilst providing a physical explanation for its geomorphic designation as an estuary.…”

Floodplain Wetland Channel Planform, Cross‐Sectional Morphology, and Sediment Characteristics Along an Estuarine to Tidal River Gradient

“…The most landward wetland we sampled at rkm 228 is presumably directly exposed to effects of the dam, but had a positive accretion rate consistent with its protected position on an inlet enclosed by an island. The potential role of winds in sediment transport and trapping efficiencies (Prokocki et al., 2020; van der Deijl et al., 2017) was not considered.…”

“…The floodplain hydrogeomorphic mosaic is highly patchy (Cannon, 2015). For example, fluvial and ebb‐tidal flows in the downstream direction as well as flood‐tidal flows in the upstream direction and oscillatory flows driven by wind waves affect sediment deposition, which controls the succession of tidal wetland vegetation from marshes to forests on braided barforms in Cathlamet Bay (Keefer et al., 2008; Peterson, Vanderburg, et al., 2014; Prokocki et al., 2015, 2020). From 16 ka to present, the decline in the rate of increase of accommodation space for the 120 km estuary and lower tidal river was spatially variable (Baker et al., 2010; Peterson et al., 2013).…”

“…Variation in the Columbia River estuarine circulation regime incorporates partial mixing, strain‐induced periodic stratification, and a time‐dependent salt wedge affecting depositional rates and processes (Geyer & MacCready, 2014; Jay & Smith, 1990; Yellen et al., 2017). Spatially variable wind‐wave currents are also important controls on sedimentary processes (Peterson, Vanderburgh, et al., 2014; Prokocki et al., 2020). Since 1942, both higher high water and lower low water levels have decreased at Astoria, OR (∼rkm 30; mean greater diurnal tidal range ∼2.6 m) and Vancouver, WA (rkm 175), but the tidal range increased (Jay, 2009; Jay et al., 2011).…”