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

Eric W. Prokocki, James L. Best, Maurico M. Perillo, Philip J. Ashworth, Daniel R. Parsons, Gregory H. Sambrook Smith, Andrew P. Nicholas, Christopher J. Simpson

Research output: Contribution to journalArticlepeer-review


This article quantifies changes in primary dune morphology of the mesotidal Lower Columbia River (LCR), USA, through ~90 river kilometres of its fluvial-tidal transition at low-river stage. Measurements were derived from a multibeam echo sounder dataset that captured bedform dimensions within the thalweg (≥ 9 m depth; (Formula presented.) ≥ 0.7) of the LCR main channel. Measurements revealed two categories of dunes: (i) fine to medium sand ‘fluvial-tidal to tidal’ (upstream-oriented, simple, and two-dimensional) low-angle dunes (heights ≈ 0.3–0.8 m; wavelengths ≈ 10–25 m; mean lee-angles ≈ 7°–11°), and (ii) medium to coarse sand ‘fluvial’ (downstream-oriented, compound, and 2.5-dimensional to three-dimensional) low-angle dunes (heights ≈ 1.5–3 m; wavelengths ≈ 60–110 m; mean lee-angles ≈ 11°–18°). At low-river stage, where (Formula presented.) ≥ 0.7, approximately 86% of the fluvial-tidal transition is populated by ‘fluvial’ dunes, whilst ~ 14% possesses ‘fluvial-tidal to tidal’ dunes that form in the downstream-most reaches. Thus, throughout the majority of the deepest channel segments of the fluvial-tidal transition, seaward-oriented river and ebb-tidal currents govern dune morphology, whilst strong bidirectional tidal-current influence is restricted to the downstream most reaches of the transition zone. Two mechanisms are reasoned to explain dune 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 reduction of leeside-angles. Fluctuations in river discharge create a ‘dynamic morphology reach’ at depths where (Formula presented.) ≥ 0.7, which spans river kilometres 12–40 and displays the greatest variation in dune morphology. Similar channel reaches likely exist in fluvial-tidal transitions with analogous physical characteristics as the LCR and may provide a distinct signature for the fluvial-tidal transition zone.

Original languageEnglish (US)
Pages (from-to)2079-2106
Number of pages28
JournalEarth Surface Processes and Landforms
Issue number8
StatePublished - Jun 30 2022


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

ASJC Scopus subject areas

  • Geography, Planning and Development
  • Earth-Surface Processes
  • Earth and Planetary Sciences (miscellaneous)


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