## 4 Results and Discussion
### Interpretation
#### 4.2.2 Mid-Channel Downstream Accreting Bar
set (mSh) extending up to 40 m in the direction of water flow, with a centimeter-thick horizontal laminated silly mudstone layer (zMh). Interpretation  
The deposits at location 3 are dominated by large-scale high-angle trough cross-bedded and planar cross-bedded coarse sand-conglomerate and are marked by the presence of a mudstone silted layer, which is a typical sediment feature of bars. Modern braided reaches of the Second Songhua River provide direct analogs: downstream accreting bars here exhibit identical planar-sigmoidal cross-bedding (15-30\({}^{\circ}\) dip) and fining-upward trends, formed by dune migration during peak discharges [13]. This is further validated by sediment transport data (5.23 million tons/year) from Fuyu Station, confirming a sufficient sediment supply for downstream accretion [44]. Furthermore, a variety of characteristics synthetically confirm the status of the mid-channel downstream accretion bar, including the dip direction of large-scale foresets in line with the direction of paleotransport, the transverse section of the internal bedding with good symmetry, and the relatively flat erosion interface between different periods [45]. During the formation of the downstream accretion bar, dunes at the optimal location for the growth of the bar gradually evolve or merge, providing opportunities for the development of the bar, especially downstream accretion [5]. Usually, bar forms are constantly controlled by the surrounding channel flow. In this study area, the conglomerate layer at the base of the bar is well preserved, and the  
Figure 5: Outcrop profile photograph (**A**) and interpretation map (**B**) showing the three-dimensional characteristics of mid-channel downstream accretion bars (site IV, profile 9). (**C**) Close-up of (**A**) showing the conglomerate, mudstone deposition, and symmetrical bedding characteristics of the core of the bar. sedimentary range is relatively concentrated, which represents lag deposition due to a decrease in local hydrodynamic conditions after the erosion of the channel belt during the peak flood period [1]. Therefore, its formation provides the initial power for the growth of bars. The bar continues to grow through several sedimentary process combinations of downstream, lateral, and upstream accretion, corresponding to different flood stages [9; 14]. The upstream accretion is now in the early stage of the weakening flood period [47], which requires a relatively high hydrological water level and adequate sediment supply along the head of the bar, and its preservation requires a long-term dynamic balance between sediment supply and discharge. This stringent condition also explains the rare upstream accretion in the study area and the severe erosion of the existing upstream accretion. Upstream accretion is usually accompanied by downstream accretion [14], and simple large-scale cross-bedded foresets are supposed to be the result of downstream migration of bars with well-developed slip faces. Foresets with changes in planar-sigmoidal cross-bedding reflect changes in local flow intensity or sediment supply, resulting in accumulation at the top of the bar and cross-stratification sets at different speeds [3; 24]. However, the frequent occurrence of cross-bedded fine-grained foresets downstream indicates that the downstream position across the top of the bar may be the key location for fine-grain deposition [9], which also marks the beginning of a period of low flow [43]. When the hydrodynamic conditions are gradually weakened and the water level falls, the flows expand from the bar head to the two sides continuously, and the bar is widened by lateral accretion, which increases the sinuosity of the channel [48]. However, the sinuosity does not reach that of meandering rivers, which is consistent with the limited development of lateral accretion in this study area.