First Author: AN Chenge
Corresponding Author: FU Xudong
Journal: Geophysical Research Letters

Abstract

Mountain rivers are characterized by steep gradients and wide grain-size distributions, while alluvial plain rivers have gentler slopes and finer sediments. At the mountain–alluvial transition, dramatic variations in flow regime lead to the sorting and deposition of sediment mixtures. The strong selective sorting of wide-grading sediments results, on the one hand, in abrupt changes in riverbed material composition (the gravel-sand transition), accompanied by a deficiency of intermediate-size grains in the 1–5 mm range; and on the other hand, in sharp spatial variability and instability of channel morphology.

Traditional fluvial dynamics often treat mountain streams and alluvial rivers separately, with relatively few studies addressing sediment sorting and transport processes in the transition zone. This study focuses on the grain-size sorting processes in the mountain–alluvial transition and analyzes sediment transport dynamics, particularly the change in sediment transport mode (from traction to suspension) induced by grain-size fraction deficiency. Special attention is given to the role of bimodal sediment transport, where fine sand and coarse gravel jointly enhance the mobility of intermediate grains (the “magic sand” effect).

Findings reveal that even when the initial grain-size distribution of bed material is unimodal, under boundary conditions, the transport system evolves toward a bimodal distribution with a persistent deficiency of 1–5 mm grains. This self-organized deficiency phenomenon is caused by the enhanced mobility of medium grains when fine sand fills pore spaces between coarse gravel, thereby increasing entrainment probability.

Field evidence further shows that local flow acceleration in the mountain-alluvial transition is a key driver of gravel-sand transition formation. Increased transport of fine sediment accelerates the transition from bedload-dominated to suspended-load-dominated transport, and the abrupt shift in transport mode leads to the distinctive grain-size composition and abrupt textural break observed in gravel–sand transition reaches.

Keywords: Mountain–alluvial transition zone; Gravel-sand transition; Grain-size deficiency; Sediment transport mode; Sediment texture