The gravel-sand transition and grain size gap in river bed sediments

River bed sediments typically fine downstream, where fining of median grain sizes are often described as exponential, except gravel abruptly transitions to sand. Across the gravel-sand transition, can reduce by more than 10 mm (>90%) over a distance only few channel widths. There several viable theories for why transition occurs, but they remain matter ongoing discussion in literature. Here, we present review known morphological characteristics associated with and existing their development (e.g., abrasion, size selective transport, washload deposition). This is combined global database published across range climatic, tectonic geographic settings. We identify an absence universal transition. However, position relatively predictable, occurring either small downstream mountain ranges or at characteristic backwater upstream base-level control. supports previous findings sensitive long-term changes runout through supply, basin subsidence rate) and/or base level sea rise). Both effects exhaustion supply generate distinct abrupt change water surface slope between sand reaches, suggesting this control on location which develop. The nature then considered terms two that seem most able explain phenomenon granular scale. also focus apparent river beds diameters ~1–10 mm, within ‘grain gap’, better understand how relates transitions. may be reflection statistics, these actually never dominate material. Alternatively, absent from hillslope sediment sources. Finally, consider particle dynamics prevent formation stable gap Even if produced hillslopes, particles raft disperse. Research into material generated, transported deposited systems should future priority.