Shape Matters: How particle morphology affects the location ofthe Gravel-Sand Transition.

In foreland basins adjacent to collisional mountain belts, rivers exhibit an abrupt gravel-sand transition (GST) at ~10-40 km downstream of mountain fronts, where surface median grain size reduces from ~10 mm to ~1 mm. This is the only abrupt downstream reduction in grain size in fluvial systems. Existing theories attribute GST formation to size-selective transport of bimodal sediment, rapid deposition of sand from the washload, and gravel exhaustion. These mechanisms predict that GST location should respond systematically to changes in hydraulic conditions (channel gradient, flow strength), sediment supply (gravel flux), and accommodation space (subsidence rate). However, observations from the Himalayan foreland basin reveal significant along-strike variability in GST locations despite similar gravel lithology, comparable subsidence rates, and uniform climatic forcing. This unexplained spatial variability indicates that additional controls on GST formation remain poorly understood.

Here, we hypothesize that particle shape—an intrinsic sediment property traditionally considered secondary to grain size—exerts first-order control on GST location through its influence on gravel mobility. To test this hypothesis, we developed a force-balance framework accounting for drag, lift, and rotational forces to model gravel transport as a function of particle shape. Experiments with varying bed matrix characteristics demonstrate that gravel mobility is strongly modulated by shape variations under identical hydraulic conditions. Field measurements of particle shape distributions from Himalayan foreland rivers reveal that GST locations coincide spatially with downstream increases in the proportion of low-mobility shapes (equant and platy forms). Progressive accumulation of these less mobile shapes reduces the bulk mobility of the gravel bedload, causing the gravel front to stall.

Our results demonstrate that particle shape exerts first-order control on GST formation and location, operating independently of climate and tectonic forcing. This intrinsic control has likely influenced sediment routing in both ancient and modern foreland basins worldwide. The findings suggest that GST positions in stratigraphic records reflect the evolution of particle shape rather than solely changes in external forcing. Understanding this shape-controlled mechanism is essential for interpreting sedimentary archives, predicting downstream sediment delivery, and refining landscape evolution models in mountain-foreland systems.