Experiments on the grain size gap across gravel-sand transitions

An abrupt transition in river bed grain size occurs from gravel to sand over a short downstream distance, often only a few channel widths, and is termed the gravel-sand transition. At this point, the bed structure also changes from framework- to matrix-supported. Whether the gravel-sand transition is externally imposed, a result of internal dynamics (sediment sorting, abrasion, suspension deposition) or due to some other emergent property is unclear. Interestingly, there is a general absence of rivers beds with median surface grain sizes between ~1 and 5 mm. Here we present a new global compilation of gravel-sand transition characteristics across a diverse range of settings. We identify commonalities in the location of gravel-sand transitions, finding they occur at upstream extents of externally imposed backwater effects, where the gravel supply is exhausted (i.e. downstream of mountain ranges), or where both effects are coincident. A series of laboratory channel experiments, examining changes in fluid and sediment dynamics across a gravel-sand transition, show systematic changes in near bed turbulence that control sand deposition patterns. Gravel coarser than ~10 mm prevents sand deposition at the bed surface. We also find that gravel-sand transitions cannot form where river beds contain substantial amounts of ~1 to 5 mm particles, because these grain sizes enhance the mobility of coarser gravel, preventing a shift to a sand bed.