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Mud in rivers transported as flocculated and suspended bed material

Abstract

Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s−1, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory.

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Fig. 1: Sediment suspension profiles.
Fig. 2: Mud transport data compared to theory.
Fig. 3: Floc properties.
Fig. 4: Suspended bed-material entrainment.

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Data availability

All data used in the study are previously published (Extended Data Table 1) and available in de Leeuw et al.20.

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Acknowledgements

This research was sponsored by a National Science Foundation grant (number EAR 1427262) to M.P.L., J.A.N. and G.P., and a Caltech Discovery grant to M.P.L. and W.W.F.

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Authors

Contributions

M.P.L., W.W.F. and G.P. designed the study. M.P.L. and J.d.L. analysed data. A.J.M., J.G.V., J.A.N. and D.H. contributed data. M.P.L. led the writing of the manuscript with contributions from all authors.

Corresponding author

Correspondence to Michael P. Lamb.

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The authors declare no competing interests.

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Peer review information Primary Handling Editor: Tamara Goldin.

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Extended data

Extended Data Fig. 1 Mud abundances in sand-bedded rivers.

Fraction of mud measured in the bed material and that measured from the total suspended sediment in the water column when depth averaged from the field data in our database (Table S1). Except for the Loup River, mud typically constitutes > 70% of the suspended sediment, but generally < 10% of the bed material (with the exception of the Yellow River).

Extended Data Table 1 Data sources.

Suspended sediment concentration profiles used in our analyses38,39,40,41,42,43,44,45.

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Lamb, M.P., de Leeuw, J., Fischer, W.W. et al. Mud in rivers transported as flocculated and suspended bed material. Nat. Geosci. 13, 566–570 (2020). https://doi.org/10.1038/s41561-020-0602-5

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