There is increasing interest in the changing sediment loads of the world's rivers as a result of human impact [e.g., Walling and Fang, 2003; Syvitski et al., 2005; Walling, 2006]. River damming is found to produce adverse impact on downstream environments by retaining sediment and nutrients [Fanos, 1995; Humborg et al., 1997; Milliman, 1997; Sánchez-Arcilla et al., 1998; Carriquiry et al., 2001; Vorosmarty et al., 2003; Nilsson et al., 2005; Syvitski et al., 2005]. In many cases, dam-induced decrease in sediment load can not be well seen by downstream observations because of other variations. For example, the decease in sediment supply from the Nile to the sea is attributed to both the Aswan High Dam and water diversion [Wiegel, 1996]. Nor is dam-induced decrease in sediment load reflected by sediment retained in the reservoir, due to downstream erosion of river bed. It is necessary to develop an approach to estimate the dam-induced decrease in sediment load by filtering other influences.
 The world's largest dam, the Three Gorges Dam (TGD), has recently been constructed on the Yangtze River. After the TGD began operation, the sediment load at Datong, the tidal limit of the Yangtze, decreased to 206, 147, and 216 mt in 2003, 2004 and 2005, respectively, compared with an average of 320 mt/yr in 1993–2002. Although several studies have reported annual depositions in the Three Gorges Reservoir (TGR) and sediment loads at downstream gauging stations, they were based on data up to 2003 [Yang et al., 2005; Chu et al., 2006] or 2004 [Xu et al., 2006; Yang et al., 2006]; and they failed to quantify the TGD-induced decrease in sediment load along the downstream reaches. The fluctuation of sediment load at Datong from 2003 to 2005 suggests that the influence of TGD is more complex than expected. To quantify the influence of the TGD, variations in water and sediment supplies from tributaries and anthropogenic impacts (e.g. downstream water extraction and sand mining) must be considered. This quantification is important for determining the role of TGD in reducing the riverine sediment load and for forecasting future sediment supply to the sea. The present study calculates the TGD-induced decrease in downstream sediment load from 2003 to 2005 by comparing gauged data with values predicted for a non-TGD case, and forecasts the magnitude of sediment load for the coming decades.
2. Materials and Methods
 Dam data were provided by the Construction Organization of Three Gorges Project (COTGP). Hydrological data at gauging stations and riverbed accretion/erosion were collected from the Yangtze River Water Conservancy Committee (YRWCC). Cuntan, Yichang, Hankou, and Datong are four stations on the main river. The stations of Beibei, Wulong, and Huangzhuang are on the lower reaches of three tributaries, namely Jialingjiang, Wujiang, and Hanjiang, respectively. Chenglingji and Hukou are two stations located where the Dongting and Poyang lakes flow into the main river. There is a gauging station on each of the four tributaries of Dongting. Additionally, there is a gauging station located on each of the five channels through which the main river flows into Lake Dongting. Cuntan is at the upstream limit of the TGR (620 km in length). Yichang is 40 km downstream from TGD. Hankou is 660 km downstream from Yichang and 500 km upstream from Datong (Figure 1). Downstream from Yichang, the bed load of the river is less than 2% of the total sediment flux and, since bed load data are commonly absent [Yang et al., 2002], the suspended load is used as a surrogate for the total sediment load in the following analysis.
 We established sediment budgets [Walling et al., 2002] for four sections: the TGR reach, the reach from Yichang to Hankou, Lake Dongting linked with the main river between Yichang and Hankou, and the reach from Hankou to Datong. In establishing sediment budgets, the amounts of sediment from the ungauged areas (defined as the regions around the main river that the gauging stations of the tributaries fail to cover) were calculated using sediment yields collected from the neighboring regions [Yang et al., 2007]. Relevant equations used for the sediment budgets are provided in detail in the auxiliary material.
3. Results and Discussion
3.1. Sediment and Water Supplies From the Ungauged Areas
 Total sediment (water) supply from the ungauged areas was 68.9 mt/yr (118 km3/yr) in 1956–2002 and 19.9 mt/yr (115 km3/yr) in 2003–2005, representing 12.6% and 7.9% (16.3% and 16.5%) of total sediment (water) supply from the gauged areas for the same periods (Table S1 of the auxiliary material). For each of the sections (e.g., TGR in Table 1), the absolute difference between water export and import including gauged and ungauged sources is ≤2% of the export, which suggests that the estimates for water supply from the ungauged areas are correct. Although there could be greater errors in the sediment data, the estimates for sediment supply from the ungauged areas are still usable. The ∼10% ratio of ungauged to gauged sediment means the ungauged areas are an important source of sediment and need to be considered in sediment budgets.
3.2. Sediment Trapped in TGR
 Sediment budgets show that 144, 133, and 177 mt of sediment were trapped in TGR in 2003, 2004, and 2005, respectively (Table 2). These numbers are 20% higher, on average, than those reported by YRWCC (124, 102, and 151 mt in 2003, 2004, and 2005). The average deposition ratio (the ratio of deposition to sediment import) of the present study was 64% (Table 2), which is also higher than the 60% reported by YRWCC. These differences are mainly because sediment from ungauged areas around TGR and erosion between TGD (no gauging station) and the downstream gauging station were taken into account in the present study. In contrast to strong deposition in the post-TGD period (2003–2005), slight erosion occurred in the Three Gorges riverbed in 1956–2002 (Table 2).
3.3. Sediment Loads at Major Stations Predicted in the Absence of TGD in 2003–2005
 If the TGD were not in operation in 2003–2005, a deposition rate of −5.1 and 4.1 mt/yr as in the pre-TGD case (Table 2) would be expected for the sections of TGR and TGD-Yichang, respectively. In that case, the sediment load at Yichang would be 232, 191, and 288 mt in 2003, 2004, and 2005 (Table 3), and the mean suspended sediment concentration (SSC) at Yichang would be 0.566, 0.462, and 0.626 g/l in 2003, 2004, and 2005, respectively, given water discharges at Yichang for the same period (Table 1). The mean SSC from the main river into Lake Dongting (auxiliary equation 7 of the auxiliary material) through the five outlets would be 0.708, 0.595, and 0.773 g/l in 2003, 2004, and 2005, and the sediment load from the main river into Lake Dongting would be 40.3, 31.2, and 49.7 mt in 2003, 2004, and 2005, respectively. In combination with sediment loads from the ungauged areas and the four tributaries (Tables S1 and S2), the sediment import to Lake Dongting (auxiliary equation 8) would be 60.7, 41.6,and 57.0 mt in 2003, 2004, and 2005, respectively. Given water import of Lake Dongting for the same period (Table S3), the deposition within the lake (auxiliary equation 9) would be 39.2, 23.6, and 36.1 mt in 2003, 2004, and 2005, and therefore the sediment load from Lake Dongting into the main river (auxiliary equation 10) would be 21.5, 18.0, and 20.9 mt in 2003, 2004, and 2005, respectively.
 For the main river between Yichang and Hankou, given sediment supplies from upstream at Yichang and the ungauged areas and the Hanjiang (Tables S1 and S2), the net sediment import (auxiliary equation 11) would be 235, 186, and 283 mt in 2003, 2004, and 2005. The sediment load at Hankou (auxiliary equation 12) would be 255, 234, and 279 mt in 2003, 2004, and 2005. For the main river between Hankou and Datong, given sediment supplies from the upstream at Hankou and the ungauged areas and Lake Poyang (Tables S1 and S2), the sediment import (auxiliary equation 13) would be 277, 250, and 300 mt in 2003, 2004, and 2005. The sediment load at Datong (auxiliary equation 14) would be 280, 238, and 306 mt in 2003, 2004, and 2005.
3.4. Role of TGD in Reducing Sediment Load of the Yangtze
 In response to the deposition of 151 mt/yr (64%) in the TGR, the sediment loads at Yichang, Hankou, and Datong decreased by 147 (62%), 98 (38%), and 85 (31%) mt/yr, in the 2003 to 2005 period (Table 3), compared with the non-TGD case. From 1963–1972 to 2003–2005, the sediment loads at Yichang, Hankou, and Datong decreased by 448, 304, and 303 mt/yr (Figure 2). In other words, TGD was responsible for 33%, 32%, and 28% of the cumulative decrease at Yichang, Hankou, and Datong from 1963–1972 to 2003–2005. Although less than half of the decrease in sediment load can be attributed to TGD [Xu et al., 2006; Yang et al., 2006], TGD currently plays the most important role in retaining sediment because deposition in any other reservoir in the Yangtze basin is <50 mt/yr [Yang et al., 2005].
3.5. TGD-Induced Downstream Erosion
 As found in many other rivers [Shields et al., 2000; Phillips et al., 2004; Gilvear, 2003], severe erosion has occurred in the river channels downstream of TGD. As suggested in Table 2, the mean erosion rate between TGD and Yichang (40 km in length) in 2003–2005 was 0.15 mt/km/yr. Meanwhile, given that the increase in sediment load between Yichang and Hankou was 67.5 mt/yr (Table 3) and the net sediment supply from the intervening tributaries was 7.7 mt/yr, then the erosion between Yichang and Hankou was 59.8 mt/yr, i.e. 0.091 mt/km/yr. Similarly, the erosion rate between Hankou and Datong was calculated to be 0.047 mt/km/yr in 2003–2005. In contrast to this, strong deposition occurred in the main river between TGD and Datong prior to TGD operation [Yang et al., 2007]. Therefore, the TGD likely has changed deposition into erosion in these reaches for the fist time in the history, especially for the several hundreds of kilometers immediately downstream of the TGD. We predict that maximum erosion will extend downstream from TGD and will achieve a depth of 0.5–5 m. Sediment entrainment will be exhausted over a relevant timescale. This timescale is estimated to be 50 years at Hankou and 60 years at Datong [Lu, 2002].
 The TGD-induced downstream erosion in 2003–2005 was much weaker than expected. For example, COTGP predicted that, for the first decade after TGD operation, the erosion between Yichang and Datong would be 183 mt/yr. As shown in Table 3, in 2003–2005, the sediment load at Datong was 99.5 mt/yr higher than at Yichang. Deducting the net sediment supply from the tributaries between Yichang and Datong, the erosion rate between Yichang and Datong was 83 mt/yr, 45% of the value predicted by COTGP. Furthermore, the erosion rate of 183 mt/yr predicted by COTGP was for a case that sediment load reached 168 mt/yr at Yichang. According to sediment budget of the present study, if the sediment load at Yichang were 168 mt/yr in 2003–2005, the erosion rate between Yichang and Datong would be 53 mt/yr, only 29% of the value predicted by COTGP. The much weaker erosion that occurred in the middle and lower reaches of the river undoubtedly led to more severe environmental problems than expected in the delta because of the considerable reduction in sediment supply.
3.6. Delta Response to TGD Operation
 Prior to TGD operation, the subaqueous delta of the Yangtze experienced accretion [Yang et al., 2003]. Since 2003, however, net erosion has been observed [Li and Yang, 2007]. The sediment load at Datong decreased from 320 mt/yr in 1993–2002 (pre-TGD) to 190 mt/yr in 2003–2005 (post-TGD) (Figure 2). Repeated surveys of intertidal wetlands at the delta front have revealed recession since 2003 (S. L. Yang, personal unpublished data, 2007). Therefore, we can conclude that TGD is inducing irreversible erosion at the delta front. This delta erosion was not predicted by COTGP. Because of a high predicted post-TGD sediment load of 360–385 mt/yr at Datong, COTGP predicted that the delta impact of the TGD would be negligible.
3.7. Expectation for Future Sediment Load in the Yangtze Under the Impacts of TGD and New Dams
 The water level of the TGR will be raised from 135 to 175 m above the sea in 2008. For the first three decades after the full operation of TGD, 75% of the sediment from the upstream will be trapped in the TGR [Yang et al., 2002]. According to sediment budget, if all the other conditions maintained the situation experienced in 2003–2005, sediment load would be 65 mt/yr at Yichang and 183 mt/yr at Datong in the first two decades after 2008. In fact, besides TGD, other factors will affect the riverine sediment load. The most important aspect is the construction of four large dams on Jinshajiang River (Figure 1). The total storage capacity of these reservoirs will be 1.1 times higher than that of the TGD.
 The world's third and fourth largest hydropower stations, namely Xiluodu and Xiangjiaba, are now under construction and will be put into operation in 2013 and 2012. Two other dams, Wudongde and Baihetan, are also planned for construction in the next decade. For the first six decades of operation, the Xiluodu and Xiangjiaba dams will trap 85% of the Jinshajiang sediment [Lv and Luo, 2004]. Together with the other two dams, they will probably trap more than 95% of the Jishajiang sediment. In 2001–2005, 81% of the sediment supply of the TGR was provided by the Jishajiang River. Therefore, in the first six decades after 2013, the sediment supply to TGR will most probably decrease to ca.70 mt/yr, and the sediment loads at Yichang and Datong will be ca. 25 and 160 mt/yr, if sediment supplies from all the tributaries maintain the present levels. Considering influences of other human activities such as the South to North Water Diversion, afforestation, sand mining, and new dams in the tributaries [Yang et al., 2002; Xiqing et al., 2005], the sediment load at Datong will probably decrease to <150 mt/yr in the coming six decades, which is lower than the values of > 210 mt/yr predicted by other studies [e.g., Yang et al., 2006]. Because erosion will achieve a maximum depth in the main river from TGD to Datong within 60 years from TGD operation [Lu, 2002], sediment availability in the river downstream of TGD will be a limiting factor, and this low sediment load (<150 mt/yr) at Datong will be prolonged.
3.8. Life-Span of TGD With Regard to Trapping Sediment
 56% of the storage capacity of TGR is for regulation of water, with 44% (17.15 km3) of it for sediment deposition. Considering a bulk density of 1.3 g/cm3 [Yang et al., 2002], the TGR can store 22 300 mt of sediment. If the present deposition rate of 151 mt/yr is employed, we calculate it will take ca.150 years for the TGR to be filled. However, as shown above, four giant dams are being constructed or will soon be constructed. These new dams will prolong the life-span of TGR by reducing the sediment input into TGR. Because the total storage capacity of the four dams will be larger than that of TGR, the life-span of TGD with regard to trapping sediment will be prolonged to at least 300 years, without taking into account other dams. In other words, TGD will govern the downstream delivery of sediment for a long time.
Three Gorges Dam, dam on the Yangtze River (Chang Jiang) just west of the city of Yichang in Hubei province, China. A straight-crested concrete gravity structure, the Three Gorges Dam is 2,335 metres (7,660 feet) long with a maximum height of 185 metres (607 feet). It incorporates 28 million cubic metres (37 million cubic yards) of concrete and 463,000 metric tons of steel into its design. When construction of the dam officially began in 1994, it was the largest engineering project in China. At the time of its completion in 2006, it was the largest dam structure in the world. Submerging large areas of the Qutang, Wu, and Xiling gorges for some 600 km (375 miles) upstream, the dam has created an immense deepwater reservoir allowing oceangoing freighters to navigate 2,250 km (1,400 miles) inland from Shanghai on the East China Sea to the inland city of Chongqing. Limited hydroelectric power production began in 2003 and gradually increased as additional turbine generators came online over the years until 2012, when all of the dam’s 32 turbine generator units were operating; those units, along with 2 additional generators, gave the dam the capacity to generate 22,500 megawatts of electricity. The dam also was intended to protect millions of people from the periodic flooding that plagues the Yangtze basin, although just how effective it has been in this regard has been debated.
First discussed in the 1920s by Chinese Nationalist Party leaders, the idea for the Three Gorges Dam was given new impetus in 1953 when Chinese leader Mao Zedong ordered feasibility studies of a number of sites. Detailed planning for the project began in 1955. Its proponents insisted it would control disastrous flooding along the Yangtze, facilitate inland trade, and provide much-needed power for central China, but the dam was not without its detractors. Criticisms of the Three Gorges project began as soon as the plans were proposed and continued through its construction. Key problems included the danger of dam collapse, the displacement of some 1.3 million people (critics insisted the figure was actually 1.9 million) living in more than 1,500 cities, towns, and villages along the river, and the destruction of magnificent scenery and countless rare architectural and archaeological sites. There were also fears—some of which were borne out—that human and industrial waste from cities would pollute the reservoir and even that the huge amount of water impounded in the reservoir could trigger earthquakes and landslides. Some Chinese and foreign engineers argued that a number of smaller and far-cheaper and less-problematic dams on the Yangtze tributaries could generate as much power as the Three Gorges Dam and control flooding equally well. Construction of those dams, they maintained, would enable the government to meet its main priorities without the risks.
Because of these problems, work on the Three Gorges Dam was delayed for nearly 40 years as the Chinese government struggled to reach a decision to carry through with plans for the project. In 1992 Premier Li Peng, who had himself trained as an engineer, was finally able to persuade the National People’s Congress to ratify the decision to build the dam, though almost a third of its members abstained or voted against the project—an unprecedented sign of resistance from a normally acquiescent body. Pres. Jiang Zemin did not accompany Li to the official inauguration of the dam in 1994, and the World Bank refused to advance China funds to help with the project, citing major environmental and other concerns.
Nevertheless, the Three Gorges project moved ahead. In 1993 work started on access roads and electricity to the site. Workers blocked and diverted the river in 1997, bringing to a close the first phase of construction. In 2003 the reservoir began to fill, the ship locks—which allowed vessels of up to 10,000 tons to navigate past the dam—were put into preliminary operation, and the first of the dam’s generators was connected to the grid, completing the second phase of construction. (Following completion of this second phase, some 1,200 sites of historical and archaeological importance that once lined the middle reaches of the Yangtze River vanished as floodwaters rose.) Construction of the main wall of the dam was completed in 2006. The remainder of the dam’s generators were operational by mid-2012, and a ship lift, which allowed vessels of up to 3,000 tons to bypass the ship locks and more quickly navigate past the dam, began operating in late 2015.