River beds typically fine downstream gradually, yet worldwide they commonly show an abrupt gravel–sand transition (GST), where the bed surface drops from ~10 mm gravel to ~1 mm sand within a few channel widths, together with a widespread depletion of 1–5 mm (fine gravel) sediment on the bed — the grain-size gap. Competing mechanisms (size-selective deposition, particle attrition, onset of washload deposition, enhanced mobility of gap sizes) each have experimental or field support but have not been unified. The open problem is to construct a general, physically based mechanistic account — typically embodied in a quantitative morphodynamic model — that simultaneously explains the location of the grain-size gap in grain-size space, the downstream position of the GST, and its sharpness, across diverse river environments.

Background

Rivers transport and deposit sediment of mixed grain sizes, and the median grain size of the bed almost always decreases ("fines") in the downstream direction, typically in a gradual, approximately exponential way governed by size-selective deposition, sorting, and slow particle attrition (abrasion). Against this smooth backdrop, one feature stands out: in many rivers the bed surface changes abruptly from gravel (median size around 10 mm) to sand (around 1 mm) over a downstream distance of only a few channel widths. This is the gravel–sand transition (GST), and it is the only abrupt downstream reduction in grain size observed within fluvial systems. Closely associated is the grain-size gap: river-bed sediments are commonly depleted of material in the 1–5 mm range (fine gravel, sometimes called pea gravel), even though hillslope sediment supplied to rivers often contains such sizes, so it is puzzling why these particles are so rare on bed surfaces. A 2021 Earth-Science Reviews review by Dingle, Kusack and Venditti compiled worldwide evidence that both phenomena are pervasive and surveyed the candidate mechanisms, but concluded that no general mechanistic synthesis reconciles them. The main competing explanations are: (1) size-selective deposition from bimodal (gravel-plus-sand) sediment supply, in which the river deposits each fraction where transport capacity drops; (2) particle attrition, the progressive wearing and fragmentation of gravel during transport; (3) the washload-deposition hypothesis — washload being fine sediment carried in suspension that scarcely exchanges with the bed — under which sand abruptly falls out of suspension when the shear velocity (a measure of flow strength near the bed, related to the square root of bed shear stress divided by fluid density) drops below roughly 0.1 m/s, depositing a sand bed within a short distance; and (4) the enhanced relative mobility of gap-sized particles over sand-covered beds, a consequence of hydraulic smoothing in which deposited sand bridges the interstitial pockets of a fine-gravel bed surface. Subsequent work sharpened rather than settled the question. Flume experiments by Dingle and Venditti (2023) produced a stable GST when sand fed over a near-threshold gravel bed fell out of suspension at shear velocities of 0.09–0.10 m/s, supporting the washload hypothesis, and showed that gap material (1–5 mm) becomes preferentially mobile once sand deposits, explaining why pure gap-gravel beds are so rare. Church and Hassan (2023), however, showed that a grain-size gap can also develop under purely bedload, size-selective transport with negligible suspension, so several mechanisms can each independently produce part of the signal. Field study of the Yangtze River (He et al., 2023) revealed a 60-km-long, non-abrupt GST controlled by shear-stress drops associated with channel-width variability and distributaries plus dam operations, showing that GST sharpness and its controls are case-dependent in large rivers. On the theoretical side, Parker et al. (2024) used dimensionless arguments to show that within the 1–5 mm band, suspension becomes negligible at normal flood conditions on Earth while the relative mobility of such particles over finer beds is enhanced — a physical reason the gap sits precisely in that size range — and An et al. (2024) built the first one-dimensional morphodynamic model (a model coupling flow, sediment transport, and bed elevation change) with size-specific bedload and suspended-load transport that generates bimodal bed grain-size distributions and the gap autogenically even from a unimodal sediment feed; however, in their model an abrupt GST forms only when an abrupt increase in floodplain width is imposed externally, attrition is not included, and broad field validation is lacking. What remains missing is a general account that unifies these mechanisms and predicts, from specified sediment supply, flow regime, and channel geometry, both the grain-size gap and the position and sharpness of the GST across the range of rivers where they occur — including why some large rivers show extended, diffuse transitions.

Problem Statement

Provide a general, physically based mechanistic account of the fluvial gravel–sand transition and grain-size gap — typically embodied in a quantitative morphodynamic model — that, from specified sediment supply (grain-size distribution and feed rates), flow regime, and channel geometry, simultaneously predicts (i) the widespread depletion of 1–5 mm particles on river-bed surfaces (the location of the grain-size gap in grain-size space), (ii) the downstream position of the gravel–sand transition, where the bed surface fines from ~10 mm gravel to ~1 mm sand, and (iii) the sharpness of that transition, commonly only a few channel widths. The account must reconcile the documented candidate mechanisms — including size-selective deposition from bimodal supply, particle attrition, abrupt onset of washload deposition near shear velocities of ~0.1 m/s, and the enhanced relative mobility of gap-sized particles over sand-smoothed beds — indicating, where the evidence supports it, the conditions under which particular mechanisms dominate; it may legitimately conclude that a given candidate, such as abrasion, is secondary in a given setting, and it should remain open to other documented controls, such as grain-size patchiness and externally imposed changes of channel or floodplain width and dam operations, which are admissible physical determinants of transition position where justified. It must also explain the documented exceptions, such as the extended (~60 km), non-abrupt GST of the Yangtze River, and hold across diverse river environments — covering abrupt transitions and diffuse or extended ones alike — rather than rest on consistency with a single mechanism or a single setting.

References

  1. Dingle, E. H., Kusack, K. M., Venditti, J. G. (2021). The gravel-sand transition and grain size gap in river bed sediments. Earth-Science Reviews 222, 103838. doi:10.1016/j.earscirev.2021.103838, https://doi.org/10.1016/j.earscirev.2021.103838
  2. Dingle, E. H., Venditti, J. G. (2023). Experiments on Gravel-Sand Transitions: Examination of Washload Deposition. Journal of Geophysical Research: Earth Surface 128(7), e2023JF007116. doi:10.1029/2023JF007116, https://doi.org/10.1029/2023JF007116
  3. 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), e2023JF007117. doi:10.1029/2023JF007117, https://doi.org/10.1029/2023JF007117
  4. 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
  5. 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), e2022JF006984. doi:10.1029/2022JF006984, https://doi.org/10.1029/2022JF006984
  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. 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), e2024GL109109. doi:10.1029/2024GL109109, https://doi.org/10.1029/2024GL109109
  8. Reynolds, T. (2024). Grain size from source to sink – modern and ancient fining rates. Earth-Science Reviews 250, 104699. doi:10.1016/j.earscirev.2024.104699, https://doi.org/10.1016/j.earscirev.2024.104699