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

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 thatca75% 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 degrees) 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.