The research of river morphology transition and sediment variation: Shule River, northwest of China

Sediment grain size and river morphology are essential controls on fluvial sedimentology. Research in this field is well documented, but few prior studies have analyzed the response of gravel refinement and river morphology. The downstream fining pattern observed in the Shule River is a classic example of a morphology transition in a large alluvial channel. However, important questions regarding the river morphology transition and sediment variation remain unanswered. Here, the research present observations of the downstream change in bed gravel grain size, river morphology, and sediment elemental content through the reach within the Yumen fan. These observations indicate that the gravel is coarse in the bed of the braided river belt and has a mean grain size of 36.09 mm, the elemental content 69.34% was SiO2, and 17.00% was Al2O3. The grain size in the river transition belt was smaller than that upstream, with a mean grain size of 22.40mm, the elemental content 65.01% was SiO2, and 19.41% was Al2O3. The mean grain size of gravel in the meandering river belt was 9.70 mm, the elemental content 63.55% was SiO2, and 25.96% was Al2O3. Through quantitative analysis of changes in river morphology, gravel grain size, and elemental content, the result show that the changes in river morphology are mainly affected by slope, the grain size of the gravel is mainly affected by hydrodynamic conditions. Elemental content changes are mainly affected by the depositional environment. This study also found that these three changes are interconnected. The Shule River is close to the apex, with a large slope and steep terrain. The fluvial is braided river, the hydrodynamic conditions are strong, and the sediments are primarily coarse gravel. Downstream the slope decreases, the terrain is gentle, the river morphology is mainly meandering, the hydrodynamic conditions are weak, and the sediments are primarily fine gravel and sand. Our research propose that a systematic study of the Shule River at multiple scales is a useful analogue for research of fluvial system sedimentology.