## 4 Discussion
### Effect of the Sediment Flux of the Hanjiang River on Erosion and Deposition in the Subaqueous Delta
This means that the Yangtze underwater delta has experienced a transition from rapid deposition to erosion. Compared with the sedimentation rate of the columnar samples collected in the area before 2003, the sedimentation rate of the columnar samples collected in the Zhejiang-Fujian coastal muddy area from 2014 to 2015 was only slightly lower, indicating that after 2003, the deposition rate in the area decreased slightly and the depositional environment remained stable. The material comprising the underwater delta of the Yangtze River mainly comes from the river itself. The low sand supply caused by the interception of sediment by the Three Gorges Dam has changed the erosion and deposition state of the underwater delta of the Yangtze River. The sediment  
Figure 5: Distribution map of sedimentation rate variation in Hanjiang underwater delta columnar samples. eroded from the underwater delta of the Yangtze River partially makes up for the low sediment supply in the muddy areas along the coasts of Zhejiang and Fujian, so the delta still exhibits a relatively stable depositional environment. With further sediment reduction in the underwater delta of the Yangtze River, the deposition rate of the muddy areas along the coasts of Zhejiang and Fujian will gradually decrease and eventually suffer from erosion [11]. During the 1960s-1980s, the sediment flux into the sea from the Minjiang River Basin increased significantly, and the average deposition rate of the Minjiang underwater predelta increased from 0.67 cm/a during 1954-1963 to 1.04 cm/a during 1963-1986. Afterward, the sediment flux into the sea from the Minjiang River Basin dropped sharply. Since 2001, the sediment flux into the sea from the Minjiang River has been only 34% of that before the 1980s, resulting in an average deposition rate of 0.46 cm/a in this area, which was only 44% during 1963-1986. The sedimentation rates of underwater deltas are positively correlated with the change in sediment flux into the sea, and the sedimentation pattern of underwater deltas responds promptly to the change in sediment flux into the sea [21]. Previous studies have also found that there are spatial differences in the impact of sharp reductions in the sediment flux into the sea on the deposition rate of underwater deltas [11; 21]. The Effect of the Sharp Decrease in the Sediment Flux from the Hanjiang River into the Sea on the Sediment Particle Size  
According to the particle size analysis results from the columnar samples, the upper part of the columnar samples \(\mathrm{ZZY51}\), \(\mathrm{ZZY52}\), and \(\mathrm{ZZY55}\) near the estuary of the Hanjiang River became coarser (Table 6), the upper part of the columnar samples \(\mathrm{ZZY03}\), \(\mathrm{ZZY07}\), \(\mathrm{ZZY54}\), and \(\mathrm{ZZY04}\) far from the Hanjiang estuary became finer (Table 6), and the particle sizes of the columnar samples \(\mathrm{ZZY01}\), \(\mathrm{ZZY05}\), and \(\mathrm{ZZY57}\) at the far estuary end exhibited no significant change in the vertical direction (Figure 6). In other areas, previous studies have investigated the causes of sediment coarsening. The average median particle size of sediment surface samples from the Yangtze underwater delta coarsened from 8.0 \(\mathrm{\SIUnitSymbolMicro m}\) in 1982 to 15.4 \(\mathrm{\SIUnitSymbolMicro m}\) in 2012. The main reason was that dam construction led to a sharp decrease in the sediment flux into the sea, and the underwater delta shifted from deposition to erosion [22]. Sediment coarsening of active delta lobes in the Yellow River Delta began in the 1990s and became more pronounced (from 17.7 \(\mathrm{\SIUnitSymbolMicro m}\) to 22.9 \(\mathrm{\SIUnitSymbolMicro m}\)) in the early 2000s. Reservoir construction and water and soil conservation measures in the watershed increased the proportion of mud delivered by the Yellow River to the sea [23].