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

E. W. Prokocki, J. L. Best, M. M. Perillo, P. J. Ashworth, D. R. Parsons, Greg Sambrook-Smith, A. P. Nicholas, C. J. Simpson

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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 at low-river stage. Measurements were derived from a Multibeam Echo Sounder dataset that captured bedform dimensions within the thalweg (≥ 9m depth; H/Hmax ≥ 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 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°). At low-river stage, where H/Hmax ≥ 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 H/Hmax ≥ 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 Lower Columbia River and may provide a distinct signature for the fluvial-tidal transition zone
Original languageEnglish
JournalEarth Surface Processes and Landforms
Early online date25 Mar 2022
Publication statusE-pub ahead of print - 25 Mar 2022


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