## 4 Discussion
### Effect of the Sediment Flux of the Hanjiang River on Erosion and Deposition in the Subaqueous Delta
In the 1990s, the sediment flux into the sea from the Hanjiang River began to decrease sharply, and the area near the estuary changed from deposition to erosion, resulting in the coarsening of the upper part of the columnar samples (\(\mathrm{ZZY51}\), \(\mathrm{ZZY52}\), and \(\mathrm{ZZY55}\)) near the estuary. The frequency curve for the particle size from columnar sample \(\mathrm{ZZY04}\) (below 84 cm) shows a bimodal distribution. Although the peaks of the finer components are dominant, the peaks representing the coarser components are also more significant (Figure 7). However, since the 1950s (above 84 cm), the frequency curve has generally shifted toward a unimodal distribution, with peaks representing finer components (Figure 7). The frequency curve of the \(\mathrm{ZZY03}\) particle size below 42 cm has a similar bimodal shape to that of \(\mathrm{ZZY04}\) (below 84 cm) (Figure 8). The frequency curve above 42 cm generally features a unimodal distribution, and the peaks represent finer components (Figure 8). The frequency curve of the particle size of \(\mathrm{ZZY07}\) below 12 cm shows a bimodal distribution (Figure 9), and the frequency curve above 12 cm generally features a unimodal distribution (Figure 9, Table 3). In 2003, the depositional location was 12 cm, and 2003 was also the time at which this location changed from deposition to an erosion-deposition balance. Figure 6: Vertical characteristics of the particle sizes of columnar samples from the Hanjiang underwater delta. At the end of the last century, numerous hydropower stations and reservoirs were constructed in the upper reaches of the Hanjiang River, and these structures began to intercept a large amount of sediment that otherwise would have entered the sea. Notably, a large amount of coarse-grained sediment has been retained in the reservoirs, and the sediment that is discharged into the ocean from the reservoirs is mainly fine-grained suspended sediment. Therefore, the sources of coarse-grained sediment in the underwater delta of the Hanjiang River have been reduced, resulting in the sources being at a relatively large distance from the estuary. The far columnar samples exhibit a pattern of fining upward (ZZY03, ZZY07, ZZY54, and ZZY04). [PERSON] et al. (2020) believed that a large amount of coarse-grained sediment remained in a reservoir after a dam was built, and the sediment that was discharged into the ocean after the reservoir was impounded was mainly fine-grained sediments, which has led to the presence of fine-grained sediments since the mid-1980s ([PERSON] et al., 2020). Figure 8: Particle size frequency curve of the ZZY03 columnar sample (**left**: 42–174 cm; **right**: 0–36 cm). Figure 7: Particle size frequency curve of the ZZY04 columnar sample (**left**: 84–198 cm; **right**: 0–78 cm). Figure 9: Particle size frequency curve of the ZZY07 columnar sample (**left**: 36–198 cm; **right**: 0–30 cm). In addition, although the significant reduction in sediment flux from the Hanjiang and Rongjiang Rivers into the sea resulted in an insufficient sediment supply and erosion in the underwater delta area near the estuary, the sediments produced by erosion became a new source, compensating for the sediment flux reduction to a certain extent. The insufficient sediment supply at the far estuary end makes the vertical variation in particle size less obvious than at the near estuary end [11; 24]. Thus, the particle size pattern of the columnar samples from the estuary has no significant change in the vertical direction. The spatial response pattern of the Hanjiang underwater delta to the sharp reduction in sediment flux into the sea is not quite the same as that of the Yangtze underwater delta and the Yellow River underwater delta. The sediments flowing into the sea from the Yangtze River are mainly deposited in the underwater delta of the Yangtze River and the muddy areas along the Zhejiang and Fujian coasts [25]. The sediment from the Yangtze River entering the sea is the main source of sediment for the Yangtze underwater delta.