Climate and Human Induced Regime Shift of Suspended Sediment Concentration in the Yangtze Estuary

Daidu Fan, State Key Laboratory of Marine Geology, Tongji University, Shanghai, China and Binbin Jiang, Tongji University, China
The sediment route from the river to the sea has been greatly altered by human activities together with climate change. This is typically true in the Asian mega-river deltas. The average annual sediment discharge from the Changjiang (Yangtze) River has decreased from a maximum estimate of 497 Mt during 1951—1968 to 134 Mt during 2003—2018 after the completion of Three Gorges Dam in 2003. The direct response to sharp sediment discharge decrease is the significant decline of suspended sediment concentration (SSC) at the river mouth. We report here the spatiotemporal variation of SSC in different distributaries in the Yangtze using the remote sensing data.

The Yangtze River bifurcates thrice by the river-mouth bars/islands into the North Branch (NB) and South Branch (SB), the North Channel (NC) and South Channel (SC), and the North Passage (NP) and South Passage (SP). Satellite derived SSC data show a general decreasing SSC trend from 1986 to 2017 over the entire estuary with average annual SSC decreasing rates (ASDR) of 10.2 mg/l and 10.9 mg/l for the wet and dry seasons, respectively. The absolute ASDRs were higher at the inner estuarine sections than the outer sections, potentially resulting from higher direct influence of recently significant decrease in riverine sediment discharge at the inner sections. The relative ASDRs in comparison with the background SSC were still higher at the SB and SC, decreasing seaward to the NC, NP and SP. However, the relative ASDR was the lowest at the NB where the SSC background value reached up to 800 mg/L, significantly higher than 200-400 mg/l at the SB and SC. The NB has been gradually abandoned since the 1850s, and <3% of river runoff goes through the NB into the sea. Therefore, the SSC at the NB is less influenced by the change of the riverine sediment discharge, but high related to tidal activities and human disturbances. Long-term SSC variations in the wet season showed some periodic cycles. The six estuarine sections showed similar periodic variations before 2003. After the completeness of Three Gorges Dam in 2003, the inner estuarine sections demonstrated a different cycle with the outer sections including the NB, indicating the occurrence of regime shift due to increasing human interventions.