Downstream fining and rounding in sand-bed river and its significance: A case study from the Ganjiang River, China

Study region: The Ganjiang River in southern China. Study focus: The ubiquitous pattern and mechanisms of downstream fining and rounding of sediments in gravel-bed rivers is well studied. However, little is known about these processes in sand-bed rivers. In this study, 37 river sand, 25 beach sand, and 17 dune sands samples across a 625 km transect from source to sink were analyzed, by dynamic image technique, and geochemical elements, to investigate the downstream evolution of particle size and shape in different sand fractions in a sand-bed river. New hydrological insights for the region: Our results reveal that medium sand fractions and bulk samples gradually become rounder and finer with downstream distance, whereas fine sand fractions transported in suspension display no significant downstream trend. The medium sand fraction exhibits a much smaller diameter reduction rate, but a roughly equal shape improvement rate compared to the bulk samples. This indicates that abrasion dominates shape evolution and hydraulic sorting plays a key role in downstream fining. In addition, abrupt changes of the sediments in particle shape from source to sink demonstrate the occurrence of aeolian-fluvial interaction processes along the Poyang Lake, which significantly improved particle shape of beach sand. These findings have significant implications for predicting the downstream evolution of sediments, and utilizing particle size and shape to aid in paleoenvironment reconstruction and source area constraints.