The grain size gap and abrupt gravel‐sand transitions in rivers due to suspension fallout

Median grain sizes on riverbeds range from boulders in uplands to silt in lowlands; however, rivers with ~1–5 mm diameter bed sediment are rare. This grain size gap also marks an abrupt transition between gravel‐ and sand‐bedded reaches that is unlike any other part of the fluvial network. Abrupt gravel‐sand transitions have been attributed to rapid breakdown or rapid transport of fine gravel, or a bimodal sediment supply, but supporting evidence is lacking. Here we demonstrate that rivers dramatically lose the ability to transport sand as wash load where bed shear velocity drops below ~0.1 m/s, forcing an abrupt transition in bed‐material grain size. Using thresholds for wash load and initial motion, we show that the gap emerges only for median bed‐material grain sizes of ~1–5 mm due to Reynolds number dependencies in suspension transport. The grain size gap, therefore, is sensitive to material properties and gravity, with coarser gaps predicted on Mars and Titan.