## 2 Geological Setting  
The Songliao Basin is one of the largest typical continental sedimentary basins in the world, located in Northeast China, covering an area of approximately 260,000 square kilometers and spanning Heilongjiang, Jilin, Liaoning, and Inner Mongolia autonomous regions. In the middle and upper sedimentary strata with a thickness of up to 10,000 m, braided river facies developed from the Lower Cretaceous Denglouku Formation to the Quaternary sediments, but the proportions are small, and the outcrops are very limited. The object of this study is the Quaternary sedimentary strata of the Second Songhua River located in the middle of the basin. It is a coarse sand/gravel braided river deposit formed by the development of multistage floods, and most of the strata are not diagenetic. The study area was unaffected by human activities such as dam-building during the sedimentary period. The outcrops are exposed west of Denglouku village, Songyuan city, Jilin Province, adjacent to the Second Songhua River (Figure 1). Excellent exposures of the formations provide conditions for the direct observation of the internal structure and spatial characteristics of the sand body. A total of eight well-exposed sections (up to 12 m in thickness and between 150 and 400 m in length) were selected from four areas for detailed observation, measurement, and dissection.  
The Second Songhua River, as the southern source of the Songhua River, originates from the main peak of Changbai Mountain in the Baitou Mountains and flows to the northwest through Songyuan city in Jilin Province. Before entering the Songhua River, it joins the Nen River. The total length of the river is 790 km, and the drainage area is approximately 7.3 \(\times\) 10\({}^{4}\) km\({}^{2}\), which accounts for 38.2% of the total area of Jilin Province. The river valley is dominated by a series of Holocene accumulative formations. Upper Holocene formations in the Songhua River valley are composed of light-yellow sand and clay, and the thickness of the formations gradually decreases in the downstream direction. The lower Holocene formations of the Songhua River valley are siltstone, and the grain size gradually decreases in a downstream direction from pebbly to medium- to coarse-grained sandstone or pebbly coarse sandstone. This area is located in the north temperate monsoon climate zone with obvious continental climate characteristics. The average annual precipitation is approximately 500 mm, and 6-9 precipitation events in the flood season account for 60-80% of the total yearly amount. The Second Songhua River Fuyuhydroological monitoring station provides flow and sediment transport data from 1955 to 2005. The annual runoff has been 14.65 billion cubic meters, the annual sediment transport has been 5.23 million tons, and the annual sediment content has been 0.141 kg/cubic meter. The law of water and sand conforms to the characteristics of \"much water-much sand, stable water-stable sand, and little water-little sand\", which also indicates that the Second Songhua River basin has good vegetation protection and slight soil erosion.  
Figure 1: (**A**) Enlarged map showing the location of the Songliao Basin in Northeast China; (**B**) geological characteristics map of the Songliao Basin; (**C**) location numbers of outcrops in the study area: I, II, III, and IV—four observation areas; V—enlarged image of site II; VI—site II rotated 180\({}^{\circ}\) to explain the illusion of four regions raised by visual differences, which are essentially sunken pits.