[1] On the basis of estimates of sediment accumulation in reservoirs, the impact of 50,000 dams on sediment supply and intertidal wetland response in the Yangtze River catchment is examined. The total storage capacity of reservoirs is 200 Â 10 9 m 3 , or 22% of the Yangtze annual runoff. The sediment accumulation rate in reservoirs has increased from $0 in 1950 to >850 Â 10 6 t/yr in 2003. Although sediment yield has increased with broader soil erosion in the river basin, the total riverine sediment discharge rate shows a strong decreasing trend from the late 1960s to 2003, likely due to dam construction. Consequently, the total growth rate of intertidal wetlands at the delta front has decreased dramatically. A significant relationship exists between intertidal wetland growth rate and riverine sediment supply that suggests riverine sediment supply is a governing factor in the interannual to interdecadal evolution of delta wetlands. Regression analysis of intertidal wetland growth rate and sediment supply shows that intertidal wetlands at the delta front degrades when the riverine sediment discharge rate reaches a threshold level of <263 Â 10 6 t/yr. Owing to the construction of the Three Gorges Dam and other new dams, the sediment discharge rate of the Yangtze River will most likely decrease to below 150 Â 10 6 t/yr in the coming decades. Therefore unless current management policies are adjusted, drastic recession of Yangtze River delta intertidal wetlands can be expected to occur.Citation: Yang, S. L., J. Zhang, J. Zhu, J. P. Smith, S. B. Dai, A. Gao, and P. Li (2005), Impact of dams on Yangtze River sediment supply to the sea and delta intertidal wetland response,
The surface erosion area in the Yangtze River basin increased from 364×10 3 km 2 in the 1950s to 707×10 3 km 2 in 2001 due to a great increase in population. Based on the regression relationship between surface erosion area and population, the surface erosion area was predicted to be about 280×103 km 2 at the beginning of the 20 th century. The sediment yield, which increased by about 30% during the first six decades of the 20 th century, was closely related to the surface erosion area in this river basin. The Yangtze annual suspended sediment flux into the estuary was about 395×106 t a -1 at the beginning of the century, and this gradually increased to an average of 509×10 6 t a -1 in the 1960s. The increase in the suspended sediment flux into the estuary was accelerated in the 1950s and the 1960s due to the rapid increase in population and land use immediately after the Second World War and the Liberation War. After the riverine suspended sediment flux reached its maximum in the 1960s, it decreased to <206×10 6 t a -1 in 2003. Construction of dams was found to be the principal cause for this decreasing trend because, during the same period, (a) the riverine water discharge did not show a decreasing trend, (b) water diversion was not influential and (c) sedimentation in lakes and canals of the middle and lower reaches did not increase. The total storage capacity of reservoirs has increased dramatically over the past half century. The amount of sediment trapped in reservoirs has increased to more than half a billion t a -1 . As a result, the suspended sediment flux into the estuary dramatically decreased, even though the sediment yield from many areas of the basin increased in recent decades. Human activities gradually increased the suspended sediment flux into the estuary before the 1960s and then rapidly decreased it. The last century was a period when the Yangtze suspended sediment flux into the estuary was dramatically affected by human activities.
Lake Dongting, the second largest freshwater lake in China and located in the middle reaches of the River Yangtze catchment, was formed at the beginning of the Holocene period by sea level rise and has varied in size with changes in local weather patterns. The sedimentation rate in Lake Dongting during the Holocene is about 50×10 6 m 3 yr 1 , or 80×10 6 t yr 1 (a sand bulk density of 1.6×10 3 kg m -3 ), given the sediment deposition rate as 10 mm yr 1 and the average lake size as 5000 km 2 . By comparing the sediment import and export, it is estimated that the sediment deposition rate of Lake Dongting was 110.6×10 6 t yr 1 from 1956 to 2003. Siltation and raised embankments reduced the size of the lake and its capacity to accommodate floods. The sediment delivery ratio (SDR) of the middle and lower Yangtze is about 0.92 (total sediment output divided by total sediment input) given that the total sediment supply into the middle and lower Yangtze is 455.1×10 6 t yr 1 and the total sediment discharge into the sea is 419×10 6 t yr 1 . Therefore, if it were not for Lake Dongting, the sediment flux at Datong would be 73.6×10 6 t yr 1 (80 ×10 6 t yr 1 ×0.92) more, an increase of 27% during the Holocene and an increase of 26% to 101.75×10 6 t yr 1 from 1956 to 2003. Historically, Lake Dongting had a considerable influence in regulating the sediment budget of the Yangtze. However, afforestation and the construction of large dams, such as the Three Gorges Dam, reduced significantly the sediment deposition in Lake Dongting. In 2003, the completion of the Three Gorges Dam and the subsequent impoundment of water reduced the sediment input from the Yangtze and net deposition in Lake Dongting dropped to 25% and 18% of the mean values of the historic records (19562003). During the same period, the amount of sediment deposited in Lake Dongting was only 10% of the sediment discharge at Datong. The influence of the sediment deposited in Lake Dongting on the sediment flux to the sea from the Yangtze has fallen since the completion of the Three Gorges Dam and will be further reduced in future. The evolution of the relationship between Lake Dongting and the Yangtze is a compound result of human impacts coupled with natural self-adjusting processes in the river system.
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