The bimodal distribution of sediments in the river beds has been widely reported. The most exciting work belongs to
Shaw and Kellerhalls (1982) The Composition of Recent Alluvial Gravels in Alberta River Beds, Bulletin 41, Alberta Research Council, Canada.
They have studied the bimodal distribution of gravel and sand in the 12 mountainous rivers of Alberta, spanning 7,500 km in length. That has brought out the presence of three distinct zones:
1) A mountain zone where the grain size increases with distance.
2) The central reach where the grain size decreases exponentially with distance (Stenberg's relationship).
3) The lower reach is the sand bedded river.
Other works report the bimodal distribution of the sediments in the sand and silt fraction, and references are available in the above publication.
They have proposed several inherent mechanisms that may account for the origin of bimodal distribution of the grain size in the river sediments. Though I am not contradicting their interpretations, I suppose that the very basis of such a mechanism lies in Boltzmann distribution that describes the energy function of the sediments across their size spectrum. Day by day, I am finding more pieces of evidence of the involvement of Boltzman distribution in natural processes, including adsorption - the most common phenomenon observed in the interacting sediment particles with trace metals dissolved in solution (natural water). For example, please refer to our publication:
Anandaraj, B., Eswaramoorthi, S., Rajesh, T.P., Aravind, J., Suresh Babu, P., 2018. Chromium(VI) adsorption by Codium tomentosum: evidence for adsorption by porous media from sigmoidal dose-response curve. Int. J. Environ. Sci. Technol. 15, 2595–2606.
The geochemical implications of the size sorting of the sediments that form three different mountain zones are monumental. They have implications in understanding sediment provenance, weathering intensity, source apportionment (quantitative provenance analysis), reservoir capacity, and aquifer characteristics worldwide.
I firmly believe that Shaw and Kellerhalls (1982) findings are extensible to global rivers.
Further reading:
Bolton, A.J., Maltman, A.J., Fisher, Q., 2000. Anisotropic permeability and bimodal pore-size distributions of fine-grained marine sediments. Marine and Petroleum Geology 17, 657–672. https://doi.org/10.1016/S0264-8172(00)00019-2
Colombini, M., Carbonari, C., 2020. Sorting and bed waves in unidirectional shallow-water flows. J. Fluid Mech. 885, A46. https://doi.org/10.1017/jfm.2019.1039
Houssais, M., Lajeunesse, E., 2012. Bedload transport of a bimodal sediment bed. J. Geophys. Res. 117, https://doi.org/10.1029/2012JF002490
Lee, B.J., Toorman, E., Molz, F.J., Wang, J., 2011. A two-class population balance equation yielding bimodal flocculation of marine or estuarine sediments. Water Research 45, 2131–2145. https://doi.org/10.1016/j.watres.2010.12.028
Sambrook Smith, G.H., 1996. Bimodal fluvial bed sediments: origin, spatial extent and processes. Progress in Physical Geography: Earth and Environment 20, 402–417. https://doi.org/10.1177/030913339602000402
Smith, G.H.S., Nicholas, A.P., Ferguson, R.I., 1997. Measuring and defining bimodal sediments: Problems and implications. Water Resour. Res. 33, 1179–1185. https://doi.org/10.1029/97WR00365
Sonu, C.J., 1972. Bimodal composition and cyclic characteristics of beach sediment in continuously changing profiles. Journal of Sedimentary Research 42, 852–857. https://doi.org/10.1306/74D72653-2B21-11D7-8648000102C1865D
Wathen, S.J., Ferguson, R.I., Hoey, T.B., Werritty, A., 1995. Unequal mobility of gravel and sand in weakly bimodal river sediments. Water Resour. Res. 31, 2087–2096. https://doi.org/10.1029/95WR01229
Wolcott, J., 1988. Nonfluvial control of bimodal grain-size distributions in river-bed gravels. Journal of Sedimentary Research 58, 979–984. https://doi.org/10.1306/212F8ED6-2B24-11D7-8648000102C1865D
Wu, W., Li, W., 2017. Porosity of bimodal sediment mixture with particle filling. International Journal of Sediment Research 32, 253–259. https://doi.org/10.1016/j.ijsrc.2017.03.005
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