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 −1 and peak discharges ≥15 000 m 3 s −1) 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 multi-channelled Lower Columbia River, Washington/Oregon, USA, which can experience peak discharges ≥18 000 m 3 s −1. 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 braid-bars 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 large-rivers 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 regime braid-bars is observed. This common stratification is created by combined-flows characterized by intrabasinal wind-wave oscillatory-currents and bidirectional tidal-currents. This inclined heterolithic stratification marks the initial downstream fluvial–tidal crossover point from Lower Columbia River up-dip fully-fluvial braid-bar architectures, to those possessing bar-top facies produced by the hydraulic-sedimentation response of combined intrabasinal wind-wave and tidal influence. When preserved, this form of mid-channel bar inclined heterolithic stratification provides a unique sedimentological signature of multi-channelled fluvial–tidal transitions that possess an open-water lower basin with intrabasinal wind-waves.
Bibliographical noteThis is the peer reviewed version of the following article: Prokocki, E.W., Best, J.L., Ashworth, P.J., Sambrook Smith, G.H., Nicholas, A.P., Parsons, D.R. and Simpson, C.J. (2020), Alluvial architecture of mid‐channel fluvial–tidal barforms: The mesotidal Lower Columbia River, Oregon/Washington, USA. Sedimentology, which has been published in final form at https://onlinelibrary.wiley.com/doi/abs/10.1111/sed.12754. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
- Fluvial‐tidal bars
- inclined heterolithic stratification
- intrabasinal wind‐waves
- Lower Columbia River
- intrabasinal wind-waves
- Fluvial–tidal bars