## 2 Regional Settings and Methods  
### Regional Settings  
(1) Yangtze River (Changjiang)  
As shown in Figure 1, The Yangtze River (24\({}^{\circ}\)-35\({}^{\circ}\) N, 90\({}^{\circ}\)-122\({}^{\circ}\) E) is the largest river in China, with a length of approximately 6300 km and a drainage area of 1.81 \(\times\) 10\({}^{6}\) km\({}^{2}\)[34]. The entire drainage area is controlled by temperate and subtropical climate types, with an annual average precipitation of approximately 1100 mm. The geological background in the drainage area is complex. The Yangtze River spans the South China Orogenic Belt, Yangtze Platform, Qinling-Dabie Orogenic Belt, Sanjiang Paleotethys and other tectonic units. The lithology in the drainage area is varied, comprising clastic sedimentary, metamorphic and igneous rocks. Research has demonstrated that the annual average sediment discharge reaches 0.5 \(\times\) 10\({}^{9}\) t due to the high sediment content upstream and north of the drainage area [34]. Approximately 50% of the sediments are deposited near the estuary [34]. In addition, seaward sediments originating from the Yangtze River are mainly towards the southeast, with a smaller northward component. Sediment is exchanged with the coast of the Subei littoral plain area. (2) Ancient Yellow River  
The ancient Yellow River (Figure 1), also referred to as the waste Yellow River, is located in the northern Huaihe River drainage area. This river is an old course of the Yellow River and flows from the north of Lankao city in Henan Province toward the southeast, passing through Minquan County, north of Shangqiu city in Dangshan County in Anhui Province, north of Xuzhou city in Jiangsu Province, south of Suqian city and north of  
Figure 1: Study area and sampling sites. GPS point information can be found in Appendix A.  
Huai'an city; then, the river turns northeast, flowing to the south of Lianshui County and north of Binhai County, and entering the Yellow Sea at Taowan village, where the ancient Yellow River Estuary is located. Its downstream channel is also the sea channel of the ancient Huaihe River. According to historical records, the Yellow River has changed its course 11 times throughout history. During the Pleistocene and more recent historical periods, the Yellow River entered the sea in northern Jiangsu many times [35; 36]. Research has indicated that the Yellow River' annual average sediment discharge in the estuary reaches 8.39 \(\times\) 10\({}^{8}\) t [37] since the main provenance area is widely covered with loess [38]. Thus, the Yellow River is well known worldwide for its high concentration of fine clay and sand. As the Yellow River migrated and captured the Huaihe River, a large amount of sediments was entrained and transported. Fine-grained detritus was rapidly dominated by silty clay deposits. This change caused the land and underwater deltas of the ancient Yellow River to rapidly prograde. In particular, the coastline rapidly prograde. After 1855, the Yellow River merged into the Bohai Sea in the north. Research has revealed that approximately 90% of the detritus entering the sea by way of the Yellow River originates from the Loess Plateau in the midstream region, which is the main source of surface sediments along the northern Jiangsu coast [39]. Subei littoral plain area is located between the Yangtze River Delta and ancient Yellow River Delta (Figure 1). It is a wide plain formed by the development and accumulation of ancient bays under the interaction of rivers and the sea during the Quaternary Period [40]. It is generally accepted that the Subei littoral plain area was formed under the combined action of tides and seagoing rivers, such as the Yangtze River and the ancient Yellow River [40]. As the main controlling factors, the detritus carried by these two rivers are the main material sources of the Subei littoral plain area.