## 4 Discussion
### Effect of the Sediment Flux of the Hanjiang River on Erosion and Deposition in the Subaqueous Delta
The sediment flux from the Yangtze River into the sea decreased from 470 million t/a during 1958-1977 to 390 million t/a during 1977-2000 and continued to drop to 210 million t/a during 2000-2007. The average erosion and deposition rate changed from 6.8 cm/a during 1958-1977 to 3.2 cm/a during 1977-2000 to \(-\)2.3 cm/a during 2000-2007 [18]. The sediment flux of the Yellow River into the sea has decreased. The erosion and deposition evolution of the current estuary can be divided into four stages: 1996-2002 medium-speed deposition stage, 2002-2007 rapid deposition stage, 2007-2015 slow deposition stage, and 2015-2016 rapid erosion stage [19]. In the 1980s, the sediment flux from the Pearl River into the sea was 2754.5 kg/s and decreased to 2386.8 kg/s in the 1990s and to 1143.8 kg/s from 2000 to 2010. From 1974 to 1985, the deposition volume of West Beach in the Lingdingyang Estuary Delta was 3.63 \(\times\) 10\({}^{6}\) m\({}^{3}\); from 1985 to 1999, the erosion volume of West Beach was 425.9 \(\times\) 10\({}^{6}\) m\({}^{3}\); and from 1999 to 2011, the erosion volume of West Beach was 122.11 \(\times\) 10\({}^{6}\) m\({}^{3}\) (Table 7) [20]. The researchers above believe that the significant reduction in sediment flux into the sea has an important impact on the evolution of underwater delta topography, such as the transition from the deposition state to the erosion-deposition equilibrium state or the reduction in the deposition rate of the underwater delta, which is consistent with the results of this paper. The research results of the sediment flux into the sea from the Hanjiang River and the evolution of the erosion and deposition of the Hanjiang underwater delta are consistent. The Effect of the Sharp Decrease in the Sediment Flux from the Hanjiang River into the Sea on the Deposition Rate  
The significant reduction in the sediment flux into the sea had a significant impact on the deposition rates at the \(ZZY51\), \(ZZY55\), and \(ZZY04\) sites but had a lesser effect on the deposition rates at the \(ZZY01\) site (Table 4). The significant reduction in the sediment flux into the sea from the Hanjiang and Rongjiang Rivers generally reduced the deposition rate of the underwater delta. In the area close to the estuary, the changes in the deposition rates of the columnar samples \(ZZY51\), \(ZZY55\), and \(ZZY04\) were more pronounced than those farther from the estuary (\(ZZY01\)) (Figure 5). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Erosion and** & & & & **Pearl River** \\
**Deposition Situation** & **Mississippi River** & **Yangtze River** & **Yellow River** & **Lingdingyang Estuary** \\
**in Different Stages** & **Underwater Delta** & **Underwater Delta** & **Underwater Delta** & **West Beach** \\ \hline Rapid deposition stage & 317 \(\pm\) 54 Mt/a & 6.8 cm/a & 0.93 \(\times\) 10\({}^{8}\) m\({}^{3}\)/a & 3.63 \(\times\) 10\({}^{6}\) m\({}^{3}\) \\  & (1874–1940) & (1958–1977) & (2002–2007) & (1978–1985) \\ \hline Decreased deposition rate & 145 \(\pm\) 25 Mt/a & & & \\
1940–1979); & 3.2 cm/a (1977–2020) & 0.57 \(\times\) 10\({}^{8}\) m\({}^{3}\)/a & 425.9 \(\times\) 10\({}^{6}\) m\({}^{3}\) \\  & 87 \(\pm\) 15 Mt/a & & (2007–2015) & (1985–1999) \\  & (1979–2005) & & & \\ \hline Erosion stage & - & \(-\)2.3 cm/a (2000–2007) & \(-\)0.53 \(\times\) 10\({}^{8}\) m\({}^{3}\)/a & \(-\)122.11 \(\times\) 10\({}^{6}\) m\({}^{3}\) \\  & & & (2015–2016) & (1999–2011) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Erosion and deposition in multiple underwater deltas at different times. In other areas, previous studies have performed similar analyses and found that the change in sediment flux into the sea has a certain influence on the deposition rate in the lower delta. The average deposition rate of the columnar samples collected in the Yangtze underwater delta during 2014-2015 ranged from 0.38 to 1.34 cm/a, which was significantly lower than the average deposition rate of the columnar samples collected in 2003 in this area (3-5 cm/a).