Downstream sedimentary and geomorphic impacts of the Three Gorges Dam on the Yangtze River

Although large dams have been constructed and continue to be constructed on many rivers, the lack of long-term gauging data often makes it difficult to document either reservoir sedimentation or the dams' downstream impacts. More than 50 years of water and sediment data from 20 gauging stations within the Yangtze River's basin provide us a unique opportunity to delineate the impacts from the Three Gorges Dam (TGD), the world's largest dam. During the first decade after TGD completion in 2003, 1.8 Gt of sediments were trapped in the Three Gorges Reservoir (TGR). The TGR's sediment retention rate increased from ~ 65% during the first three years of operation to ~ 85% by 2008-2012, when the TGD was in normal operation; in the low-discharge drought years of 2006 and 2011, reservoir retention exceeded 90%. Sedimentation in the TGR has been discontinuous, the most prominent depocenters being at the broad section near the up-river entrance to the reservoir and just upstream of the dam, where sediment thickness locally exceeds 60 m. Median size of the sediments trapped in the TGR is 11 μm, whereas sediments discharged from the TGR are finer than 5 μm. As a result of sediment retention in the TGR, the river downstream has been eroded at a rate of 65 Mt/yr. Riverbed sediments have coarsened considerably in the first several hundred kilometers downstream of TGD. Sediment discharge into the Yangtze estuary, as measured at the Datong downstream gauging station, decreased by 130 Mt/yr relative to the normal water years of 2001-2002, nearly 90% of which can be attributed to the TGD. With planned construction of large upstream Cascade Reservoirs, the amount of sediment entering the TGR will decline dramatically, thus reducing sedimentation in the TGR and thereby extending its lifespan; by the end of the 21st century, the TGR should have retained more than 80% of its original storage capacity. Sediment outflow from the TGR will likely be less than 15 Mt/yr, compared to 50 Mt/yr at present. Even with downstream channel erosion, the long-term average sediment discharge into the Yangtze estuary in future decades most likely will decrease to ca. 110 Mt/yr, only 20% of its level in the 1960s, and further delta erosion is expected.