## 4 Results and Discussion
### Interpretation
#### 4.2.3 Bank-Attached Laterally Accreting Bar Description  
In this study area, bank-attached laterally accreting bars are exposed frequently. Usually, the thickness of each period is between 0.6 and 3.2 m, and the thickness of multiple periods can be up to 12 m. The transverse extent is 30-120 m, often exceeding the exposed length. Each period is bounded by either a slightly raised wedge-shaped surface (Figure 3) or a flat surface (Figure 4). This configuration unit is mainly composed of trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), and planar cross-bedded conglomerate (Gp), with a small amount of parallel bedded sandstone/mudstone, and the grain size becomes smaller along the front stratification direction as a whole. In the whole section, the bottom of the bar is dominated by small cross-parallel bedded sandstone, and two large-scale planar and trough cross-bedded coarse sand-conglomerate sets (Sp, St, and Gp) are developed upward, with uniform dip angles of 16-28\({}^{\circ}\), accounting for 60%. The strata at the top of the bar are mainly composed of small-scale (20-40 cm thick) planar and trough cross-bedded sets with slightly larger internal bedding angles (12-20\({}^{\circ}\)), bounded by a low-angle inclined surface (less than 8\({}^{\circ}\)) and covered by a laminated fine- to medium-grained sandstone 30 cm thick. Interpretation  
Based on the transverse relationship between the dip direction of frosets indicated by multistage large-scale planar and trough cross-bedding and the local paleoflow, the lateral accretion bar can be easily identified. Flume experiments by [PERSON] show that lateral accreting bars in gravelly braided rivers develop sigmoidal cross-bedding (12-20\({}^{\circ}\) dip) perpendicular to flow direction, matching the outcrop observations [28]. Modern bank-attached bars in the study area also display similar grain size trends (coarser near banks, finer distally), verifying the lateral accretion process [49]. Small cross-parallel bedded sandstones at the bottom were gradually deposited, resulting in a relatively higher position near the river bank, providing a steep sliding surface for the development of the bar and causing flow dispersion at the head of the bar. Flows converged at the tail along the outer edge of the bar, and the sediments migrated vertically to the riverbank and the local paleoflow [49], which corresponds to the period of low water level when the flow rate  
Figure 6: Outcrop profile photographs (**A**) and interpretation map (**B**) showing the longitudinal profile depositional characteristics of the fluvial channel and mid-channel downstream accreting bar (site III, profile 5). Enlarged image (**C**) and interpretation chart (**C1**) from profile A show the changes in the internal structure of simple large plate-shaped cross-bedding to S-shaped cross-bedding downstream and small trough cross-bedding and parallel bedding upward. The enlarged images (**D,E**) from profile A show the silt-mudstone deposition in the downstream accretion process of the bar. (**F**) is a typical vertical log of the lithofacies association in the combination of a downstream accretion bar and channel. decreases [9; 14]. However, the existence of conglomerates in foresets indicates relatively high flow conditions, which represents the water flow fluctuations during the sedimentary period. It is believed that this configuration imitates a sinuous channel but actually has no link with channel migration itself [14]. There is little evidence of secondary helical circulation, which also confirms the difference from the formation of a sinuous channel. The two main types of erosion interfaces in the bar, \"wedge or flat\", may be related to the extent of the lateral aggradation strata in the study area. The erosion boundary with obvious undulations is attached to a channel bank, while the relatively flat erosion boundary is far away from the channel bank [14; 45].