The morphology of fluvial-tidal dunes: Lower Columbia River, OR/WA, USA

Eric Prokocki, James L. Best, Marco Perillo, Philip Ashworth, D.R. Parsons, Gregory H. Sambrook Smith, Andrew Nicholas, Christopher J. Simpson

Research output: Contribution to journalArticlepeer-review


This paper quantifies changes in primary dune morphology of the mesotidal Lower Columbia River (LCR), USA, through ~ 90 river kilometres of its fluvial-tidal transition. Measurements were derived from a Multibeam Echo Sounder dataset that captured low-river stage bedform dimensions within the thalweg (≥ 9m depth) of the LCR main channel with respect to fluvial-tidal current interactions. Measurements revealed two categories of dunes: i) fine to medium sand ‘fluvial-tidal to tidal’ (upstream-oriented, simple, and 2D) low-angle dunes (heights ≈ 0.3-0.8m; wavelengths ≈ 10-25m; mean lee-angles ≈ 7-11°), and ii) medium to coarse sand ‘fluvial’ (downstream-oriented, compound, and 2.5-3D) low-angle dunes (heights ≈ 1.5-3m; wavelengths ≈ 60-110m; mean lee-angles ≈ 11-18°). Approximately 86% of the fluvial-tidal transition is populated by ‘fluvial’ dunes, whilst only ~ 14% possesses ‘fluvial-tidal to tidal’ dunes that form in the downstream-most reaches. River currents are thus the first-order control governing dune morphology, with tidal-currents exerting a second-order influence (especially in the downstream part of the transition zone). Two mechanisms are reasoned to dictate their low-angle character: (1) high-suspended sediment transport near peak tidal-currents that lowers the leeside-angles of ‘fluvial-tidal to tidal’ dunes, and (2) superimposed bedforms that erode the crests, leesides, and stoss-sides, of ‘fluvial’ dunes, which results in the shallowing of leeside-angles. Fluctuations in river discharge creates a ‘dynamic morphology reach’, spanning river kilometres 12-40, which displays the greatest variation in dune morphology. Similar channel reaches likely exist in fluvial-tidal transitions with similar physical characteristics as the LCR and may provide a distinct signature of their fluvial-tidal transition.
Original languageEnglish
Article numberESP5364
Number of pages67
JournalEarth Surface Processes and Landforms
Publication statusPublished - 25 Mar 2022


  • Lower Columbia River, fluvial-tidal bedforms, low-angle dunes, superimposed bedforms


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