Dunes in the world’s big rivers are characterized by low-angle lee-side slopes and a complex shape


Dunes form critical agents of bedload transport in all of the world’s big rivers, and constitute appreciable sources of bed roughness and flow resistance. Dunes also generate stratification that is the most common depositional feature of ancient riverine sediments. However, current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments. For these dunes, the downstream lee-side is often assumed to be simple in shape and sloping at the angle of repose. Here we show, using a unique compilation of high-resolution bathymetry from a range of large rivers, that dunes are instead characterized predominantly by low-angle lee-side slopes (<10°), complex lee-side shapes with the steepest portion near the base of the lee-side slope and a height that is often only 10% of the local flow depth. This radically different shape of river dunes demands that such geometries are incorporated into predictions of flow resistance, water levels and flood risk and calls for rethinking of dune scaling relationships when reconstructing palaeoflow depths and a fundamental reappraisal of the character, and origin, of low-angle cross-stratification within interpretations of ancient alluvial sediments.

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Fig. 1: PDF plots for the mean and maximum dune lee-side angles in each river and all rivers combined.
Fig. 2: Maximum slope of the dune lee-side angle.
Fig. 3: Relation between flow depth and dune height.
Fig. 4: Hotspot graph of the potential for flow separation.

Data availability

Data plotted herein will be available through https://databank.illinois.edu/datasets/IDB-7525764. Bathymetric data of all rivers are available through the respective survey team that acquired the data. Requests should be made to the authors referenced in the Supplementary Information.

Code availability

The code for BAMBI is available from the corresponding author on request.


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J.C. is supported by a National Science Foundation Graduate Research Fellowship (NSF GRF). This material is based on work supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1746047. J.C. is also supported by the Department of Geology, University of Illinois, and the Jack and Richard C. Threet chair to J.B. The Huang He (Yellow) River single-beam echosounder data acquisition was supported by the National Natural Science Foundation of China (grant no. 51379087), the Department of Geology, University of Illinois, and the Jack and Richard C. Threet chair to J.B. We also thank the São Paulo Research Foundation (FAPESP) for Research Grant nos. 2014/16739-8 and 2017/06874-3, supporting the acquisition of the Amazon River Multibeam Echo Sounder data. J.B. would like to acknowledge many discussions with R. Kostaschuk, who first highlighted the importance of low-angle alluvial dunes. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Author information

J.C. and J.B. conceived the study and identified the potential datasets to be analysed. J.C. developed the BAMBI code from initial conceptual ideas on utilizing slope and aspect maps by J.B., and conducted the analysis and data plotting. All authors provided bathymetric data. J.C., J.B. and T.v.D. wrote the manuscript, which was then reviewed and edited by all authors.

Correspondence to Julia Cisneros or Jim Best.

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Peer review information Primary Handling Editors: Xujia Jiang, Melissa Plail.

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Extended data

Extended Data Fig. 1

Flow chart illustrating the BAMBI methodology.

Extended Data Fig. 2 River conditions and dune morphology statistics.

Statistics of flow discharge, Froude number (Fr, Eq. 1a), grain size, H/Y, dune height (H), dune wavelength (\(\it \lambda\)), H/\(\it \lambda\), superimposed dune height (Hs), Hs/\(\it {\lambda }_{{\rm{s}}}\), and Hs/Hmean. xEstimated mean discharge during survey. *Discharge range for multiple surveys. +N total is not same for superimposed dunes and large dunes. Mean values are found from first calculating the value of each individual dune then averaging.

Extended Data Fig. 3 Dune schematic showing the morphologic parameters measured in BAMBI.

Values measured are dune height (H), wavelength (\(\lambda\)), mean lee-side angle, maximum lee-side slope, height of the maximum lee-side slope (h), and flow depth (Y).

Extended Data Fig. 4 Lee-side angle statistics.

Statistics for mean and maximum lee-side angles in each river and for all rivers compiled. +N total is not same for superimposed dunes and large dunes.

Extended Data Fig. 5 Distribution of mean and maximum dune lee-side angles in the Huang He (Yellow) River.

Lee-side measurements were acquired using the BAMBI method from single echosounder lines.

Extended Data Fig. 6

Distribution of mean dune lee-side angles in the Jamuna (Brahmaputra) River (data from15).

Extended Data Fig. 7 Distribution of dune aspect ratio (\(\it \lambda\)/H) for all rivers and dunes with different lee-side angles.

Statistics given for the mean and standard deviation (std dev.) for all rivers and dunes with lee-side angles \(<1{0}^{\circ }\), \(10-2{4}^{\circ }\) and \(>2{4}^{\circ }\), which are related to zones of no flow separation, developing flow separation, and permanent, fully developed flow separation. N represents the number of data points that fall within each category.

Extended Data Fig. 8

Flow depth vs mean lee-side angle for all rivers.

Extended Data Fig. 9 Dune height vs lee-side angle.

a) mean and b) maximum lee-side angle for all rivers, and c) mean and d) maximum lee-side angle for the: i) Amazon, ii) Mekong, iii) Paraná, iv) Mississippi, v) Missouri, and vi) Waal rivers. As examples, grey arrows in part c), panels i) and iv) for the Amazon and Mississippi rivers, highlight the trends of increasing minimum mean angle (solid line) and decreasing maximum mean angle (dashed line) for dunes.

Supplementary information

Supplementary Information

Supplementary Fig.1.

Source data

Source Data Extended Data Fig. 1

Excel table of Extended Data Fig. 4

Source Data Extended Data Fig. 2

Excel table of Extended Data Fig. 2

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Cisneros, J., Best, J., van Dijk, T. et al. Dunes in the world’s big rivers are characterized by low-angle lee-side slopes and a complex shape. Nat. Geosci. 13, 156–162 (2020). https://doi.org/10.1038/s41561-019-0511-7

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