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.
Bibliographical noteFunding Information:
This research was funded primarily by ExxonMobil, but was also supported by UK Natural Environment Research Council (NERC) grant awards NE/H007954/1, NE/H006524/1, NE/H007261/1, NE/H00582X/1, and the Jack and Richard Threet Chair in Sedimentary Geology (JLB). The authors thank Mike Blum and Howard Feldman for their encouragement during this research, and the Clatsop Community College and Environmental Research and Training Station for providing additional resources and support. The authors graciously thank Michael Wilkin, Katie Rathmell and António Baptista, for their advice and expertise regarding the Lower Columbia River, as well as Hans Moritz from the Portland District Office of the US Army Corp of Engineers (USACE) for providing information regarding the timing, boundaries, and effects, of federal navigation channel dredging operations from 2005 to 2010. The authors also thank Pat Killion, captain of the Tansey Point, whose years of experience on the Lower Columbia River were paramount towards enabling this research. Lastly, the authors especially thank Bob Dalrymple for his extremely insightful and stimulating review, as well as the thorough reviews from one anonymous referee and editor that greatly improved the article.
© 2022 John Wiley & Sons, Ltd.
- Lower Columbia River
- fluvial-tidal bedforms
- low-angle dunes
- superimposed bedforms
- tidal bedforms