## 4 Discussion
### Effect of the Sediment Flux of the Hanjiang River on Erosion and Deposition in the Subaqueous Delta
The lack of sand caused by the interception of sediment by the Three Gorges Dam has caused a change in the erosion and deposition state of the Yangtze underwater delta. The threshold of sediment flux into the sea from the Yangtze River is 270 Mt/a. When the sediment flux into the sea drops below 270 Mt/a, the underwater delta of the Yangtze River changes from a depositional state to an erosional state [18]. The sediment 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 a relatively stable depositional environment is maintained. However, with the further reduction in sediment in the underwater delta of the Yangtze River, the deposition rate of the muddy zone along the Zhejiang-Fujian coast will gradually decrease and eventually suffer from erosion [11]. This shows that the sharp reduction in the sediment flux from the Yangtze River into the sea affects the deposition of the Yangtze underwater delta in the short term, causing the erosion and deposition state transition of the Yangtze underwater delta. However, the impact on the muddy areas along the coasts of Zhejiang and Fujian has hysteresis. Due to the frequency of diversions from the Yellow River [26] and the sharp reduction in sediment flux into the sea, the deposition of the current estuary underwater delta has slowed [27], and the underwater delta of the abandoned estuary has undergone erosion [28]. The sediment flux (0.84 Gt/a) from the Yellow River into the sea from 1976 to 1981 was more than twice the sediment flux threshold (\(\sim\)0.35 Gt/a) needed to maintain the balance of erosion and deposition, which was conducive to the rapid deposition of the Yellow River underwater delta. From 1981 to 1996, engineering-induced increases in river runoff and median suspended sediment particle size resulted in a decrease in the sediment flux threshold to 0.29 Gt/a. Since the sediment flux of the Yellow River into the sea is 0.55 Gt/a, the underwater delta is still receiving sediments at a rate of 11.9 km\({}^{2}\)/a. From 1996 to 2002, the sediment flux threshold dropped further to 0.15 Gt/a, and the delta suffered from net erosion due to the insufficient sediment supply (0.11 Gt/a). From 2002 to 2013, the adjustment of the dam led to intense erosion of the downstream channel, which resulted in the delivery of coarser sediment to the sea, effectively reducing the sediment flux threshold to 0.06 Gt/a, which was much lower than the sediment flux into the sea (\(\sim\)0.16 Gt/a). The underwater delta of the Yellow River entered a slight deposition stage [27]. This result shows that both the current estuary underwater delta and the abandoned estuary underwater delta of the Yellow River exhibited a timely response to the sharp decrease in sediment flux into the sea. Compared with the underwater delta of the Yangtze River and the underwater delta of the Yellow River, the area near the estuary of the underwater delta of the Hanjiang River, such as the area where the columnar samples ZZY51, ZZY52, and ZZY55 were collected, changed from a deposition state to an erosion state due to the sharp reduction in the sediment flux into the sea. In the areas farther from the estuary, the coarse particles of the columnar sediments were reduced due to the interception of coarse-grained sediment by the reservoir, which may have resulted in a change from a deposition state to an erosion-deposition equilibrium state, such as for ZZY07. Erosion occurs in the underwater delta area near the estuary, and the sediment produced by the erosion becomes a new sediment source, which makes up for the low sediment supply in the far estuary to a certain extent, causing the sediments of the underwater delta far from the estuary (ZZY01, ZZY05, and ZZY57) to be very important. The response to the sharp decrease in sediment flux into the sea is not clear.