## 4 Results and Discussion
### Discussion
#### 4.5.2 Temporal Evolution of the Quaternary Coarse Sandy-Gravelly Braided River System  
The braided river in this study is a coarse sandy-gravelly channel in a typical continental monsoon climate. Due to seasonal precipitation, the energy of the river varies significantly in different seasons. Under the control of bank stability and flow strength, the active riverbed is restricted to a certain spatial range in which braided channels can expand and contract freely for a long time, which actually determines the complexity of the evolution of the braided riverbed [57; 58]. Meanwhile, the braided river architecture presents a certain regularity, including the form and scale characteristics of elements, mutual contact, time sequence, and span (length), and the deposition and preservation of internal sediments of different particle sizes are not random. However, it also controls the deposition and preservation of sediment in the braided river system, which makes the architectural structure of the braided river present a certain regularity. In the whole evolution process, the lower two ancient river channel belts maintained similar braiding degrees, with mid-channel downstream accreting bars, bank-attached laterally accreting bars, and fluvial channels as the main filling units, and the number of bars did not decrease significantly, which could easily imply a relatively stable flow fluctuation process. In addition, the lower sedimentary strata are dominated by large planar cross-bedded or trough cross-bedded coarse sandy-gravelly (pebble) sediments lacking parallel bedding and ripple bedding, which represent a sustained high-energy environment [56] and a steady supply of sediment and are consistent with the characteristics of perennial river systems with long flood waning periods [14; 56]. For high-energy braided rivers, mudstone deposition is common in the strata of this research area, which is obviously different from the viewpoint of a lack of fine-grain deposition emphasized in previous studies [24] and may be attributed to the abrupt seasonal discharge in the study area. With the significant decrease in precipitation after September, the relatively high concentration of suspended sediment carried by the water flow can settle when the hydrodynamic force decreases sharply [10]. Furthermore, the relatively low filling positions, such as the backwater side of the bar and the previously flowing river channel, promote the preservation of mudstone deposition [59]. However, the common mudstone deposits and mudstone clasts indicate the existence of a cohesive riverbank [14]. Different from the relatively high lateral migration of a sandy braided bar [14,24], the mid-channel downstream accreting bar in the study area does not show obvious lateral deviation in the vertical evolution process, which may be attributed to the stability of the coarse sandy-gravelly bar. The dip directions of large-scale inclined strata in bank-attached laterally accreting bars are perpendicular to the paleocurrent, and secondary helical circulation is lacking, which indicates the existence of low-sinusosity channels [60]. Well-preserved cross-bedding reflects the constant hydrodynamic force and perennial flow environment over a period of time [61], and the difference in the extents of laterally accreting bars at different periods reflects the strong hydrodynamic environment and the fluctuation of sediment supply duration. Erosionally based fluvial channel filling occurring in the lower sedimentary system is explained by the strong hydrodynamic forces caused by continuous heavy rain in a short period of time, which is consistent with the fact that 70-80% of the region's annual rainfall is concentrated in three days of the year [61], and its features of relatively deep downcutting of the channel while extending laterally also confirm the existence of a relatively stable riverbank [62].