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

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.