Riverbeds commonly fine downstream through a gravel-bedded reach, a relatively abrupt gravel–sand transition (GST), and a sand-bedded reach, and bed grain-size distributions are often bimodal with a paucity of sediment in the 1–5 mm range (the grain-size gap). A 2024 morphodynamic modeling study (An et al., GRL) showed that bimodality readily evolves autogenically even under a unimodal sediment feed, but in their model an abrupt GST formed only when a floodplain-width increase was externally imposed at a chosen location. The open problem is whether the grain-size gap and an abrupt GST can emerge entirely autogenically — from unimodal sediment supply, uniform boundary conditions, and no externally imposed change in floodplain width, base level, or supply. As of mid-2026 no published model or experiment has demonstrated a fully autogenic GST from unimodal supply, and the necessity of external width control remains untested.

Background

Along most rivers the bed material fines downstream, and this fining is not gradual: a gravel-bedded reach (median grain size of order 10 mm) typically gives way, over a distance of only a few channel widths, to a sand-bedded reach (median of order 1 mm). This gravel–sand transition (GST) is the only abrupt downstream reduction in grain size within fluvial systems, and it coincides with a striking statistical pattern: bed sediment in the size range roughly 1–5 mm ('fine gravel' or 'pea gravel') is systematically rare on riverbeds, a phenomenon called the grain-size gap, so that bed grain-size distributions are frequently bimodal (two modes, gravel and sand, with a deficit between them). Two families of explanation compete. In the suspension-falloff hypothesis (Lamb and Venditti, 2016), rivers abruptly lose the ability to carry sand as washload — sediment that travels in suspension without exchanging with the bed — when the bed shear velocity (a measure of near-bed turbulent stress) drops below about 0.1 m/s; Reynolds-number effects make settling of 1–5 mm particles especially inefficient at flood stages, so the gap and the GST are imposed by fluid physics wherever the flow decelerates enough. Flume experiments by Dingle and Venditti (2023) reproduced a stable GST by feeding sand over a gravel bed and confirmed the predicted shear-velocity threshold and washload fallout, and Church and Hassan (2023) confirmed that the gap can develop by purely hydraulic size-selective transport. However, these experiments impose effectively bimodal sediment feed (sand introduced over an existing gravel bed) under fixed flow, so they demonstrate the mechanics of an existing transition rather than its genesis. A complementary question is whether the gap and the GST can form autogenically — that is, through internal morphodynamic feedbacks (bed composition sorting, selective deposition and erosion) rather than external (allogenic) forcing such as a change in sediment supply, base level, tectonics, or floodplain width. An et al. (2024) built a one-dimensional morphodynamic model (a model that evolves channel bed elevation and bed-surface composition by balancing sediment transport of each grain-size class with supply) with size-specific bedload and suspended-load transport, and showed that bimodal bed grain-size distributions evolve readily from a unimodal feed with no external control — but in their simulations an abrupt GST arose only when a floodplain width that abruptly increases at some point was inserted into the domain, which enhances deposition of non-gap sand from suspension at that location. Dimensionless analysis by Parker et al. (2024) supports the special role of the 1–5 mm band: there, suspension becomes negligible at normal flood conditions while relative bedload mobility over finer beds is anomalously high. Field evidence from the Yangtze River (He et al., 2023) shows that width variability and distributaries can drive the shear-stress drops that produce a (there non-abrupt) GST. What remains missing is a demonstration, by model, experiment, or theory, of a GST and gap arising from unimodal supply with no externally imposed spatial change.

Problem Statement

Can the grain-size gap (the deficit of roughly 1–5 mm bed sediment) and an abrupt gravel–sand transition arise entirely autogenically in an alluvial river — that is, from a uniform unimodal sediment feed and steady boundary conditions, with no externally imposed change in floodplain width, base level, sediment supply, or tectonics — or is some external (allogenic) control, such as the abrupt floodplain-width increase imposed in the model of An et al. (2024), genuinely necessary? Here 'steady boundary conditions' prohibits externally imposed spatial or temporal changes such as a prescribed floodplain-width increase; it does not exclude a uniform, steady subsidence, which the model of An et al. (2024) includes while still describing the resulting grain-size distribution as autogenic. Concretely, determine, by morphodynamic modeling, laboratory experiment, field analysis, or theory, whether the width increase (or any other external control) is necessary, or whether the required width change — or an equivalent flow-deceleration mechanism, such as backwater dynamics (the gradual deceleration of flow where the water surface is set by a downstream water-level control) or self-formed channel widening driven by the evolving deposit itself — can emerge from the internal morphodynamics of a river fed unimodally. Answers must demonstrate that the sediment feed is genuinely unimodal and that no hidden external forcing produces the gap or the transition.

References

  1. An, C., Parker, G., Venditti, J. G., Lamb, M. P., Hassan, M. A., Miwa, H., Fu, X. (2024). Autogenic Formation of Bimodal Grain Size Distributions in Rivers and Its Contribution to Gravel‐Sand Transitions. Geophysical Research Letters 51(17). DOI: 10.1029/2024GL109109. https://doi.org/10.1029/2024GL109109
  2. Lamb, M. P., Venditti, J. G. (2016). The grain size gap and abrupt gravel‐sand transitions in rivers due to suspension fallout. Geophysical Research Letters 43(8), 3777–3785. DOI: 10.1002/2016GL068713. https://doi.org/10.1002/2016GL068713
  3. Dingle, E. H., Venditti, J. G. (2023). Experiments on Gravel‐Sand Transitions: Examination of Washload Deposition. Journal of Geophysical Research: Earth Surface 128(7). DOI: 10.1029/2023JF007116. https://doi.org/10.1029/2023JF007116
  4. Dingle, E. H., Venditti, J. G. (2023). Experiments on Gravel‐Sand Transitions: Behavior of the Grain Size Gap Material. Journal of Geophysical Research: Earth Surface 128(11). DOI: 10.1029/2023JF007117. https://doi.org/10.1029/2023JF007117
  5. Church, M., Hassan, M. A. (2023). The fluvial grain‐size gap: Experimental confirmation of hydraulic origin. Earth Surface Processes and Landforms 48(8), 1502–1511. DOI: 10.1002/esp.5562. https://doi.org/10.1002/esp.5562
  6. Parker, G., An, C., Lamb, M. P., Garcia, M. H., Dingle, E. H., Venditti, J. G. (2024). Dimensionless argument: a narrow grain size range near 2 mm plays a special role in river sediment transport and morphodynamics. Earth Surface Dynamics 12(1), 367–380. DOI: 10.5194/esurf-12-367-2024. https://doi.org/10.5194/esurf-12-367-2024
  7. He, Z., Sun, Z., Zhou, W., Li, Y., Hu, Y. (2023). Gravel‐Sand Transition of the Yangtze River: Human Disturbances, Migration Processes, and Controlling Factors. Journal of Geophysical Research: Earth Surface 128(6). DOI: 10.1029/2022JF006984. https://doi.org/10.1029/2022JF006984
  8. Hassan, M. A., Parker, G., Hassan, Y., An, C., Fu, X., Venditti, J. G. (2024). The roles of geometry and viscosity in the mobilization of coarse sediment by finer sediment. Proceedings of the National Academy of Sciences 121(38). DOI: 10.1073/pnas.2409436121. https://doi.org/10.1073/pnas.2409436121
  9. Zhang, Y., Gong, Z., Ma, H., Jing, L., Fu, X. (2026). Hump‐Shaped Enhancement of Coarse‐Grain Transport in Sediment Mixtures Induced by Fine Grains of Different Sizes. Geophysical Research Letters 53(8). DOI: 10.1029/2026GL122366. https://doi.org/10.1029/2026GL122366
  10. Feng, Z., Deng, A., Lu, Q., An, C., Lv, R., Tang, Y. (2025). Evaluation of bed load transport equations in gravel bed rivers: A case study of the lower Jinsha River (SW China). Journal of Hydrology: Regional Studies 60, 102566. DOI: 10.1016/j.ejrh.2025.102566. https://doi.org/10.1016/j.ejrh.2025.102566
  11. Stark, K., Cadol, D., Leary, K., Laronne, J. B. (2025). Persistently high bedload flux in ephemeral channels. Geomorphica 1(1). DOI: 10.59236/geomorphica.v1i1.38. https://doi.org/10.59236/geomorphica.v1i1.38