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

Abstract

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