Sediment load change in the Yangtze River (Changjiang): A review

GEOMOR-04386; No of Pages 14 Geomorphology xxx (2013) xxx–xxx

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Sediment load change in the Yangtze River (Changjiang): A review S.B. Dai a,⁎, X.X. Lu b,c a b c

College of Geographic Information and Tourism, Chuzhou University, Fengle Road 1528, Chuzhou City, Anhui Province 239012, China Department of Geography, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Global Change and Watershed Management Center, Yunnan University of Finance and Economics, Kunming, China

a r t i c l e

i n f o

Article history: Received 8 July 2012 Received in revised form 28 March 2013 Accepted 28 May 2013 Available online xxxx Keywords: Sediment Sediment load change Three Gorges Dam Yangtze River Changjiang

a b s t r a c t Extensive research into the changing sediment load throughout the Yangtze River (Changjiang) basin has been completed over recent years, and it provides an ongoing example of how to evaluate the consequences of natural and anthropogenic impacts on sediment processing in a very large fluvial system. This paper reviews these recent studies and critically assesses their findings regarding changes in sediment yield, load (both spatial and temporal variations), grain size, and rating curves, as well as the morphodynamic response of the channel and delta. We also discuss the factors driving these changes, including climate change, soil and water conservation measures, dam construction, and sand extraction, and consider the likely future trends in sediment load. Based on a consideration of the major outcomes of, and discrepancies between, recent studies, we conclude that sediment supply, transport, mobilization, and deposition in this large river system are complicated by the heterogeneous nature of its morphology and climate, as well as the progressive intensification of human activities. Therefore, the identification and interpretation of hydrological and sedimentological changes in the Yangtze basin can be difficult, and an in-depth study of the causal mechanisms of variations in sediment load and the impacts on the Yangtze River system is urgently required. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The large rivers of the world, such as the Yangtze River (Changjiang), have received increasing amounts of attention because of their proximity to large centers of population and exposure to unsustainable overuse in the context of global change (Nilsson et al., 2005). Riverine sediment is becoming a worldwide concern because of its great importance in fluvial geomorphology, biogeochemistry, and engineering, as well as land– ocean interactions (Walling and Fang, 2003; Dai et al., 2009). Over recent decades, many rivers have experienced decreasing sediment loads, triggering erosion of their deltas (Yang et al., 2011). The mechanism and impacts of this change have been discussed in the context of global change (Walling, 2006). Among the world's large rivers, the Yangtze River provides a valuable opportunity to evaluate the response of a river system to natural and anthropogenic impacts thanks to the systematic collection of water discharge and sediment load data from numerous gauging stations for more than 50 years. As the longest (6300 km) river in Asia, the Yangtze is ranked 5th globally in terms of water discharge (900 km3/yr), and until recently 4th in terms of sediment load (470 Mt/yr) (Yang et al., 2011). The enormous amounts of sediment and nutrients carried by the Yangtze River sustain coastal ecosystems that have relatively high levels of primary productivity (Wu et al., 2007). Thus, any change to the sediment load of the Yangtze River has a fundamental environmental and

⁎ Corresponding author. Tel.: +86 550 3519108.

geomorphological impact on the coastal regions of the western Pacific (K. Xu et al., 2009). The sediment load of the Yangtze began to decline in the 1970s, and detailed studies of these spatial and temporal changes have been completed (Yang et al., 2002, 2004, 2005a; S.L. Yang et al., 2006; Xu et al., 2006; Z.S. Yang, 2006; Yang et al., 2007a; Chen et al., 2008a; Dai et al., 2008; Xu and Milliman, 2009; Yang et al., 2011), and the impacts of natural and anthropogenic factors such as variations in precipitation amounts, dam construction, deforestation, soil conservation, water diversion, and sand extraction have been evaluated (e.g., Yang et al., 2002; Chen et al., 2005; S.L. Yang et al., 2006; Z.S. Yang, 2006; Dai et al., 2008; Yang et al., 2011). These discussions were intensified when the Three Gorges Dam (TGD) began to impound water in 2003. There are a variety of opinions regarding the nature of the changes in the sediment load of the Yangtze, and also of the impacts of the potential driving mechanisms. Shortly after the initial impoundment of the TGD in 2003, some researchers found that the sediment load of the Yangtze River was quite different to the official predictions (Dai et al., 2005; Yang et al., 2005b). However, the results of their studies were quite different or even contradictory (Dai et al., 2005; Yang et al., 2005b; Xu et al., 2006). For example, Zhou (2005) argued that the proportion of coarse sediment used in the series of feasibility studies was an underestimate, and that this may have led the numerical model to forecast less deposition in the upper backwater reaches of the reservoir. However, Han (2006) offered a different viewpoint, and indicated that the proportion of coarse sediment in the feasibility study was actually higher than the measured value. Obviously, there are discrepancies among the various

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Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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studies. Hence, a comprehensive analysis and summary of the state of our current knowledge is required if we are to accurately evaluate the nature and extent of changes in sediment load and their consequences. A review of these previous studies may prove helpful in this respect. This paper will review recent research regarding sediment processing in the Yangtze River and provide a summary of the major concerns. As the Yangtze River is rapidly developing into a case study of how best to examine the response of a river system to global change, this review may also be able to provide some broader insights that will be helpful in the management of other rivers.

2. Sediment processing in the Yangtze River Basin The Yangtze River originates on the Qinghai–Tibet Plateau at an elevation of 5100 m and runs for 6300 km eastwards to the East China Sea (Fig. 1). The catchment covers a total area of 1.81 × 106 km2 and, at present, is home to a population of more than 400 million people (Yang et al., 2005a). The uppermost 3300 km of the river is named the Jinshajiang, and this stretch extends to the confluence with the Minjiang River. The Minjiang, Jialingjiang, and Hanjiang rivers are major tributaries that join the main river from the north, while the Wujiang River, Lake Dongting, and Lake Poyang join the main river from the south (Fig. 1). The upper reaches of the Yangtze River end at Yichang, 40 km downstream from the TGD. Three key hydrological monitoring stations (Yichang, Hankou, and Datong) are located in the upper, middle, and lower reaches of the trunk river, respectively. Systematic hydrological measurements across this vast drainage basin began in the 1950s (Table 1), although the earliest station was established in 1865 at Wuhan (Hankou). The data from Hankou station provide the opportunity to extend the time series of water and sediment discharge data from the lowest station (i.e., Datong) and have been used to examine the hydrological variation of the river over a longer period (Yang et al., 2005a; Wang et al., 2008; Yang et al., 2010). Mean annual precipitation across the basin is approximately 1090 mm, but there is

Table 1 Comparison of water and sediment discharge at the main stations before and after completion of the TGD (data source: CWRC, 2000–2010).

Station

Main River Pinshan Zhutuo Cuntan Yichang Hankou Datong Tributaries Minjiang Jialingjiang Wujiang Hanjiang Dongting Lake Poyang Lake

Pre-TGD (1950–2002)

Post-TGD (2003–2010)

Water discharge (109 m3)

Sediment discharge (Mt)

Water discharge (109 m3)

Sediment discharge (Mt)

143.7 266.4 345.4 434.1 704.6 891.6 86.2 65.6 49.8 46.5 168.3

253.6 309.2 423.4 493.0 402.2 423.6 47.6 115.8 27.3 46.2 28.9

143.9 254.8 325.8 395.6 662.8 812.2 78.1 61.6 430 45.5 151.7

153.6 180.6 193.4 57.2 119.3 147.5 32.0 23.6 7.0 8.1 9.0

147.6

9.4

124.7

12.3

a high degree of spatial and temporal variation (Zhang et al., 2005). The average water discharge at Yichang (1950–2010), Hankou (1954–2010), and Datong stations (1950–2010) was 432, 707, and 896 km3/yr, and the average sediment loads were 434, 359, and 390 Mt/yr, respectively (Fig. 2). Before 2000, the sediment load of the stem river increased downstream from 22 Mt/yr at Shigu (2125 km away from the headwater, Fig. 1) to a maximum of 493 Mt/yr at Yichang (which is the end of the upper reaches and 4463 km away from the headwater, Figs. 1,2), and significant deposition occurred in the middle and lower reaches (Wang et al., 2007; Yang et al., 2007a,b). However, the deposition center shifted to the upper Yichang following the impoundment of the TGD in 2003 (Fig. 2). Along the middle and lower reaches of the river, numerous lakes are linked to the main channel.

Fig. 1. Sketch map of the Yangtze River. WDD: Wudongde Dam, BHT: Baihetan Dam, XLD: Xiluodu Dam, XJB: Xiangjiaba Dam, TGD: Three Gorges Dam, GZD: Gezhouba Dam, DJK: Danjiangkou Dam.

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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Fig. 2. Longitudinal variation of sediment load along the Yangtze River reaches (modified from CWRC, Changjiang Water Resource Committee, 2000–2010).

As a transitional depositional area, this lake–river system plays an important role in regulating water discharge rates and sediment loads (Dai et al., 2005). The Datong station is the final station on the river, and the water and sediment discharge rates recorded here are considered to be the amount that enters the sea (Dai and Lu, 2010), although it is over 600 km upstream from the mouth of the river. 3. Sediment sources and supply The main source of sediment in the Yangtze River is the lower section of the upper reach (especially in the lower Jinshajiang), and the upper Jialingjiang (Higgitt and Lu, 1996; Lu and Higgitt, 1998, 2001; Lu et al., 2003a,b; Du et al., 2010) (Fig. 3). Consequently, sediment from the Jinshajiang River (Pinshan station) and the Jialingjiang River (Beibei station) contributes 51% and 23% of the sediment discharge at Yichang station (1950–2000), respectively. Several studies have examined the extent of soil erosion and sediment yield in the upper reaches. Sediment yields from 62 long-term gauging stations had been analyzed, and it was found that sediment yield generally increases with precipitation, runoff, and population density, but decreases with elevation (Lu and Higgitt, 1999). An evident scale dependency within the basin was also identified (Lu et al., 2003a,b).

Using sediment loads recorded at 110 independent hydrological stations in the upper Yangtze River, Du et al. (2010) studied the spatial distribution of sediment yields across the river basin, and the contribution from soil, terrain, rainfall, land use, and lithological factors. They found that rainfall was the main factor controlling sediment yield, although the nature of the soil and terrain also had a significant impact. They also found that sediment yields were significantly correlated with population density and the cultivated area (J. Du et al., 2011). Moreover, their study showed that a critical value of population density (70–80 people/km2) exists in relation to sediment yield (Du et al., 2010); i.e., when population density exceeded 70–80 people/km2, sediment yields decreased as the population increased, but when population density was lower than 70–80 people/km2, sediment yields increased with an increase in population. This is probably because population distribution is closely related to the nature of the terrain. Eroded soil from the source region is not necessarily transported downstream (Walling, 1983), and sediment yield at the basin outlet is determined by both soil erosion and sediment delivery processes. The sediment delivery ratio (SDR) and scale effects are two key terms that are used to explain the relationship between soil erosion and sediment yield. The issue of the SDR in the upper Yangtze basin has yet to be fully addressed. Based on investigations covering 10 small rivers in the upper

Fig. 3. Soil erosion in the upper Yangtze River basin (modified from Du et al., 2010).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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Yangtze basin, Liu and Zhang (1995) found that the SDR of the studied rivers in the upper reach (above Yichang station) was only 0.34. Jing (2002) argued that in the higher mountain areas the SDR was close to 1 (excluding areas suffering from gravitational soil erosion and debris flow), but that it was likely to be less than 0.5 at lower elevations in the hill and wider valley areas. Though differences existed among the researchers, a value of 0.3 was accepted as reasonable (Jing, 2002). Between 1950 and 2000, the average sediment load at Yichang station was 493 Mt/yr. Applying an SDR of 0.3 gives a total eroded soil mass of 1643 Mt/yr. Scale effects also attracted considerable research attention. Based on data from 268 gauging stations, Shi (2008) studied the effects of scale on sediment yield in the upper Yangtze River and found a generally negative power relationship between sediment yield and drainage area. Shi (2008) also reported that for drainage areas between 1 × 104 and 1.58 × 105 km2 in size, there existed an abrupt decline in sediment yield with an increase in drainage area. A study of scale effects in the first order tributary basins was also conducted by Shi (2008), revealing that each tributary had its own characteristics that could be explained by the complexity of the natural environment, and the difference in the extent of human activity, dam construction, and land exploitation. Recently, Yan et al. (2011) studied the scale effects of sediment yields in each sub-basin of the Yangtze River based on a larger dataset (287 stations). They identified three types of trend: a decreasing trend, flat trend, and increasing trend in relation to the drainage area. They attributed the differences of the scale effects to the complex geology and land use in the sub-basins. 4. Temporal changes in sediment regime 4.1. Sediment load Variations in the sediment load of the Yangtze River have attracted considerable attention since the 1990s (Lu and Higgitt, 1998). Chen (1996) concluded that although the Changjiang drainage basin had experienced an unprecedented intensity of human activities in the past 40 years, the sediment discharge from either the upper basin or at the river mouth did not show any significant increase due to its small sediment delivery ratios. In another paper, Chen (1998) reported that a decreasing trend existed, though statistically not significant, in the time series of sediment discharge at Datong station for the period 1965–1987. Later, Chen et al. (2001) used a dataset that covered the period up to 1980 and argued that sediment load at Datong was quite stable. In another paper, Chen and Zhao (2001) stated that there existed an obvious decreasing trend in annual sediment load recorded in Datong station, and pointed out that it was a very interesting signal for the future study on the sediment budget of the delta coast They also noted that the data regarding the decreasing trend were obtained via personal communications, and gave no detailed information regarding their conclusions. However, Shen (2001) showed that a statistically significant decreasing trend exists in the time series for the period 1953–2000 in a monograph published in Chinese. In fact, the results of trend analysis depend on the time series data used. For example, based on the annual sediment load at Datong between 1951 and 1987, Yang et al. (2001) found a decreasing trend, which was not statistically significant (R2 = 0.053; P = 0.12). Later, using the dataset for 1951–2000 from the same gauging station, Yang et al. (2002) found a statistically significant decreasing trend in annual sediment load (R2 = 0.42). Specifically, they found that the sediment load at Datong increased by 10% from the 1950s to the 1960s, and then decreased by 34% from the 1960s to the 1990s (Yang et al., 2002), leading the authors to state that, “These changes are found to be governed by the balance between two aspects of human activities: deforestation and dam construction”. In other words, the positive effect of deforestation on sediment load exceeded the negative effect of dam construction

from the early 1950s to the late 1960s when the Danjiangkou Dam was completed (in 1968), and the opposite has been the case since the late 1960s (Yang et al., 2002). They suggested that the 1960s probably saw the highest sediment loads in recent history passing from the Yangtze River to the sea (Yang et al., 2002). To test this hypothesis, Yang et al. (2004) reconstructed the sediment load at Datong for the first decades of the 20th century, based on water discharge at Datong and using a regression relationship between the discharge at Datong and Hankou, and assuming that the sediment load was enhanced by increasing deforestation. They found that the sediment load increased from b400 Mt/yr in the 1900s, to N500 Mt/yr in the 1960s, then decreased rapidly to 192 Mt/yr in the 2000s (Fig. 4b). Considering that the sediment load of the Yangtze River before 2000 yr BP, based on delta progradation, was approximately 200 Mt/yr (Hori et al., 2001; Saito et al., 2001; Li et al., 2003; Wang et al., 2011), the conclusions of Yang et al. (2002) seem reasonable. Since the early 2000s, many research papers in international and Chinese journals have reported an ongoing decrease in the sediment load of the Yangtze River and examined relevant issues, including: What are the main causes of this trend? When did the decreasing trend begin, and what were the main impacts? In which areas has the sediment decline mainly occurred? As the upper reach is the main source of sediment in the Yangtze River, it is reasonable to examine changes to the sediment load in this part of the river. Shi and Du (2009), and K. Xu et al. (2009) examined variations in the sediment load in the upper reach using data from the periods 1950–2006 and 1950–2005, respectively. Though differences exist in their studies (Table 2), some consistent conclusions were reached. For example, apart from the main stem of the upper river (Jingshajiang), the sediment load decreased in all tributaries. At Yichang station, the sediment load began to decrease around 1990–1992, and Shi and Du (2009) indicated that it had decreased by around 28%, from an average of 526 Mt/yr between 1950 and 1992, to 377 Mt/yr between 1993 and 2006. This decrease would reduce the sediment load in the lower reaches, and so cause sediment discharge to decrease at Datong station. In fact, a decrease in the sediment load at Datong station occurred before that at Yichang due to the construction of the Danjiangkou Dam in 1968. Yang et al. (2002) found that the decadal sediment load at Datong began to decrease in the 1970s, mainly due to the closure of the dam in 1968. Z.S. Yang et al. (2006) indicated that sediment discharge at Datong decreased in three distinct phases: (1) from 1969, due to the closure of the Danjiangkou Dam on the Hanjiang River; (2) from 1986, mainly due to dam construction and soil preservation in the Jialingjiang River basin; and (3) from 2003, when the TGD began operation (Z.S. Yang et al., 2006). Unfortunately, the authors did not carry out a significance test of the data (Fig. 4a). Zhang et al. (2006) showed that the sediment load at Datong began to decrease in 1970, and that the decrease after ca. 1990 was significant at a confidence level of N 95%. After the Danjiangkou Dam was constructed (see DJK in Fig. 1), 93% of the Hanjiang River (the largest tributary in the middle and lower reaches of the Yangtze River) sediment was trapped behind the dam, with 50 Mt/yr of sediment deposited in the reservoir, and this was responsible for approximately 80% of the decrease in sediment load at Datong from the 1950–1960s to the 1970–1980s (Yang et al., 2002). Later, Z.S. Yang et al. (2006) found that after completion of the Danjiangkou reservoir, almost 99% of the sediment entering the reservoir was trapped behind the dam, which caused a net reduction in sediment transport of 41 Mt/yr from the Hanjiang to the main channel of the Yangtze, and explained 82% of the sediment reduction at Datong between the periods 1950–1968 and 1969–1985. There exists a significant relationship between the sediment loads at Hankou and Datong (Wang et al., 2008). Hence, it is not surprising that the sediment load at Datong was sensitive to a decrease in sediment load from the Hanjiang River (Hankou is immediately downstream from the confluence with the Hanjiang; see Fig. 1).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

S.B. Dai, X.X. Lu / Geomorphology xxx (2013) xxx–xxx

Datong

400

Predicted

2010

2007

2004

2001

1995

1992

1989

1983

1986

1977

600

1980

1974

1971

1968

TGD 1965

1962

1959

1956

1953

1950

DJK

1998

200

Observed

500 400 300 200

2000s

1990s

1980s

1970s

1960s

1950s

1940s

1930s

0

1920s

100 1910s

Sediment load (Mt/yr)

Hankou

600

0

b

Yichang

800

1900s

Sediment load (Mt/yr)

a

5

Fig. 4. Sediment load variation at the major stations on the trunk river. (a) Yearly variation at Yichang, Hank, and Datong: 1950–2010. (b) Decadal sediment load at Datong from the 1900s to the 2000s (modified from Yang et al., 2004). DJK: Closure of Danjiangkou Dam; TGD: Closure of Three Gorges Dam.

Fig. 4a shows the variations in sediment load at the three major stations on the main Yangtze River, and helps to illustrate the above discussion. The figure shows an overall decline in sediment load at all of these major stations. Detailed studies also showed that apart from the uppermost stem river (i.e., the Jinshajiang), all of the main tributaries of the Yangtze River have experienced a decrease in sediment load (Fu et al., 2003; Dai et al., 2008), which contributed to the sharp decline in sediment load at Datong. Various authors have produced different estimates of sediment discharge at Datong in the next decades (Table 3), and the latest studies show that it will probably be around 100 Mt/yr (Chen et al., 2008a,b; Hu et al., 2009). However, most of these studies were based on the empirical relationship between stations established over a very short period of time (i.e., in the 10 years since the TGD began operations), and the impact of alterations to the hydrological regime on sediment dynamics has been given little attention. As indicated by Yang et al. (2007a), erosion in the middle and lower reaches of the Yangtze River will probably be a key factor controlling the sediment load at Datong over coming decades. For the first six years of TGD operation (2003–2008), riverbed erosion from Yichang to Datong was found to be 61 Mt/yr, whist the

sediment load at Yichang was also 61 Mt/yr, and the tributaries in the middle and lower reaches together supplied 32 Mt/yr (Yang et al., 2011). From 2012 onwards, the four gigantic reservoirs on the lower Jinshajiang River are scheduled to come into operation. In total, they will probably trap 95% of the Jinshajiang sediment load, and the sediment load at Yichang will probably decrease to less than 25 Mt/yr (Yang et al., 2007a). However, a further decrease in sediment load at Yichang would result in severe erosion of the middle and lower reaches of the Yangtze River, unless riverbed erodibility is limited (Yang et al., 2007b). If riverbed erodibility and the sediment supply from the tributaries remain unchanged, which could be the case in the first few decades following the completion of the four dams on the lower Jinshajiang River, the sediment load would be unlikely to be less than 100 Mt/yr, except in drought years such as 2006. 4.2. Sediment rating curves Sediment rating curves empirically describe the relationship between suspended sediment concentration (SSC) and water discharge (Q) for a specific location, and have been widely used for a variety of

Table 2 Comparison of the break points (when significant change happen) in the annual sediment load at five stations in the upper reach of the Yangtze River, as proposed by Shi and Du (2009) (data before the ‘/’) and K. Xu et al. (2009) (after the ‘/’), and also the study of Zhang et al. (2005). First break points

Jingshajiang Minjiang Jialingjiang Wujiang Yichang a

Second break points

Results of Zhang et al. (2005)

Year

Direction

Significance

Year

Direction

Significance

–/2001 1967/1969 1985/1984 1984/1983 1992/1990

–/decrease Decrease/decrease Decrease/decrease Decrease/decrease Decrease/decrease

–/0.05 0.05/significanta 0.01/significanta 0.01/significanta 0.01/significanta

–/1997 1994/1993 1994/1993 1967/1966 –/2002

–/increase Decrease/decrease Decrease/decrease Increase/increase –/decrease

–/0.05 0.05/significanta 0.01/significanta 0.01/significanta –/significanta

Increasing since 1985, significant at 95% after 1995 Decreasing since 1970, significant at 95% after 1990 Decreasing during 1960–1970 and 1990–2000; Decreasing since 1975 (not significant at 95%), jump time lies 1986–1988

The authors did not indicate the significance level.

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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Table 3 Predictions of sediment discharge at Datong station during the post-TGD period. Reference

Future discharge (Mt/yr)

Duration

Yang et al., 2002 Yang et al., 2003 Z.S. Yang et al., 2006 Yang et al., 2007a Chen et al., 2008a,b Hu et al., 2009

150 160 210 b150 112–132 90

– 50 years after 2003 First 20 years since 2003 Coming 60 years – Coming decades

scientific and engineering purposes (Asselman, 2000). The general relationship between Q and SSC is expressed as: LogðSSC Þ ¼ logðaÞ þ b logðQ Þ where a, b are rating parameters. The Gezhouba Dam, which was the first dam built in the main stream of the river in 1981, changed the seasonal water and sediment regimes. In recent years, monthly sediment rating curves for the Yangtze River have been studied for various purposes; e.g., to reveal river morphology (G.F. Yang et al., 2007), reconstruct sediment loads (Wang et al., 2008), and evaluate the impact of dams (Xu and Milliman, 2009) and other human activities (Hu et al., 2009). G.F. Yang et al. (2007) identified the parameters that influence river channel morphology in selected reaches. High b values (N1.6) and low log (a) values (b − 4.0) occurred in the upper course of the steep rock-confined river, characterizing high unit stream power. Low b values (b 0.9) and high log (a) values (N − 1.0) occurred in the middle and lower Yangtze River, associated with the meandering reaches over low gradients, and implying aggradation in these reaches with low stream power. Moderate b values (0.9 to 1.6) and log (a) values (− 4.0 to − 1.0) characterized the reaches between Yichang and Xinchang, immediately below the TGD. Wang et al. (2008) used water discharge between 1865 and 2005, and the sediment load between the 1950s and 2005 at Hankou, to reconstruct the sediment load over the period from 1865 to the 1940s at the same station, and then used the correlation between sediment loads at Hankou and Datong between the 1950s and 2005 to reconstruct the missing sediment load at Datong over the period 1865–1950. Rating curves were established between stream discharge and suspended sediment concentration for datasets covering the past 50 years. The estimated mean annual sediment flux to the sea between 1865 and 1968 was approximately 488 Mt/yr, which was comparable to previously reported results (Milliman and Syvitski, 1992). Unlike the study by Yang et al. (2004), this estimate did not consider the possible change in SSC before and after the 1950s. Wang et al. (2008) used the relationship between SSC and water discharge for the period 1954–1968 to reconstruct the SSC from 1865 to 1953. In fact, the period 1954–1968 probably saw the highest SSC ever, because deforestation rates had reached their highest level, and dam impacts were insignificant at this time. The population of the Yangtze River basin nearly doubled from the 1860s to the 1960s (Yang et al., 2005a). Before efforts to control deforestation in the Yangtze River basin began in the late 1980s, the area suffering from surface erosion seemed to increase with population size. For example, the area of surface erosion in the Yangtze River basin was 364 × 103 km2 in the 1950s (Shi, 1999; Zhang and Zhu, 2001), but had reached 707 × 103 km2 by 2001 (CRWCCWCMC, 2002), whereas the population in the basin was around 220 × 106 in the 1950s and 420 × 106 by 2001 (Yang et al., 2005a). As sediment yield is closely correlated with the surface erosion area (Yang et al., 2004), the human-induced increase in erosion must be incorporated into the estimation of the SSC before the 1950s. The report of Milliman and Syvitski (1992) regarding sediment load in the Yangtze River was

based on gauging data at Datong from the 1950s to the 1970s, but did not cover the period before the 1950s. Xu and Milliman (2009) examined changes to sediment rating curves based on monthly water discharge and SSC data, and found that the hysteresis of rating curves downstream from the TGD had changed. At Datong station, all of the curves since 1991 fell below the 1950–1990 line, although they remained counter-clockwise (Fig. 5). Human activities have had a substantial impact on sediment rating parameters in the Yangtze River (Hu et al., 2009). For example, before the completion of the TGD, the sediment transport regime in the upper reaches of the Changjiang (Yichang) differed from that in the middle and lower reaches (Hankou and Datong); i.e., high water discharge at the Yichang station played a more important role in suspended sediment transport than at Hankou and Datong. Although a ca. 30% decrease in sediment load occurred at the three key stations along the Changjiang over the entire pre-TGD period (1955–2002), the sediment transport regimes appear to have remained unaltered. Since 2003, sediment transport patterns at the three stations have evolved into a similar regime, which is characterized by the decreased importance of high discharge rates in suspended sediment transport. 4.3. Sediment grain size Changes to sediment grain size have long been used to interpret environmental change in drainage basins. For example, Wang et al. (2009) completed a longitudinal study of the grain size of bed material (from Yichang to Datong), which revealed that the riverbed sediments became finer downstream from Yichang to the river mouth. A series of alternating sediment zones (with different patterns of size distribution) reflected the general character of the river channel. Such sediment zonation weakened in the Jingjiang reaches (from Yichang to Chenglingji), primarily due to the effects of dam construction over the last half century (Wang et al., 2009). Luo et al. (2012a) completed a more extensive longitudinal study, over a total length of 2100 km of the river, which examined sediment grain size changes downstream of the TGD and along the major sediment dispersal pathway into the East China Sea. They found that before the construction of the TGD, the relationship between median grain size and distance along the sandy bed of the middle and lower Yangtze showed an exponential downstream fining trend. However, after construction of the TGD, erosion caused an abrupt gravel–sand transition to develop in the section immediately downstream of the TGD (Luo et al., 2012a). They expect that the impact of the TGD on the grain size of

Fig. 5. Rating curves of monthly water discharge versus sediment concentration (modified from Xu and Milliman, 2009).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

S.B. Dai, X.X. Lu / Geomorphology xxx (2013) xxx–xxx

7

in sediment load from recent publications: dam trapping is ranked first, followed by climate change, sand extraction, and soil conservation. The most contentious factor relates to the impact of bank protection or flood plain deposition, with some authors (e.g., Yang et al., 2002; Z.S. Yang, 2006) identifying this as a positive factor, while others (e.g., Yang and He, 2005; Dai et al., 2008) consider that it has a negative effect. In fact, the relative importance of the influencing factors differs in time and space. For example, prior to the end of the 1960s, deforestation was the dominant factor driving the trend of an increasing sediment load, while dam construction had the opposite effect (Yang et al., 2002). From the end of the 1960s, dam construction was the dominant factor, and soil conservation a secondary, but still important cause, of the decrease in sediment load at Datong, whilst deforestation and bank protection were assumed to offset this decreasing trend to some extent (Yang et al., 2005b; Z.S. Yang et al., 2006; Dai et al., 2008). Climate change is the dominant factor controlling fluctuations in the sediment load time series over timescales of years to decades, but made only a minor contribution to recent trends in sediment load (Yang et al., 2004; Dai et al., 2008).

bed sediments in the Yangtze River and East China Sea will continue for some time, and the change in the grain size of bed sediments there will become more pronounced. Yang et al. (2011) also indicated that an abrupt shift in grain size occurred after the closure of the TGD in 2003. He et al. (2010) showed that the majority of the bed sediment at Datong was sand (95%). The median diameter of bed sediment tended to increase gradually from 1977 to 2006 (Fig. 6), which agrees with an earlier report by Chen et al. (2008b). He et al. (2010) concluded that the decline in SSC was the main reason for the coarsening of the riverbed sediment between 1977 and 2003. Since 2003, the grain size of bed sediment has tended to increase due to the TGD. Detailed studies of the impacts of the TGD on grain size were also carried out by Chu et al. (2006) and Luo et al. (2006). Chu et al. (2006) found that the annual mean and median grain sizes of suspended sediment in the Yangtze River at Yichang, Hankou, and Datong between 2003 and 2005 were finer than those in 2002, and also finer than the multi-year averages. However, grain size had markedly increased at Shashi (located 173 km downstream from Yichang), which indicates that the coarser suspended sediments, especially coarser particulates, had settled in the TGD, and implies that the river channel between Yichang and Shashi was badly eroded by clean water from the TGD. The coarser eroded sediment gradually settled along the channel between Shashi and Hankou due to the decrease in slope and current. A coarsening trend was also found in the Luoshan–Hankou reach after 2003 by Luo et al. (2006). As indicated by He et al. (2010) and Luo et al. (2006), the complexity of the boundary conditions in the middle and lower Yangtze River, and the limitations of the observational data, mean that the impact of the TGD on particle size requires further quantitative study.

5.1. Climate change Although it is thought that erosion rates, and the sediment loads transported by the world's rivers, provide an important and sensitive indicator of changes in the operation of the earth system in the context of global change (Walling, 2008), little is known of climatic impacts on riverine sediment loads (Zhu et al., 2008; Dai et al., 2011; Wu et al., 2012). The difficulty of disentangling the anthropogenic impact from climate change may be the main cause. In a small tributary of the upper Yangtze River, Zhu et al. (2008) studied the sensitivity of suspended sediment flux to climate change, using artificial neural networks (ANNs). They found that temperature, together with rainfall, determined the change in sediment flux. Their results indicate that, when temperature remains unchanged, an increase in rainfall leads to a rise in sediment flux, and when rainfall remains unchanged, an increase in temperature is likely to result in a decrease in sediment flux. The same percentage changes in rainfall and temperature are likely to trigger higher responses in wetter months than in drier months. However, it is the combination of change in temperature and rainfall that determines the change in sediment flux in a river. A wetter and warmer climate is likely to produce a greater sediment flux, because an increased transport capacity is accompanied by increased rates of erosion.

5. Driving forces behind the decline in sediment load In addition to research into the pattern of variation in sediment load across the Yangtze basin, the causes of this variability have also been considered. Sediment load is sensitive to both climate change and human activities (Walling and Fang, 2003; Lu et al., 2013). Whereas climate change is believed to be the dominant factor over longer periods (i.e., geological timescales), human activity is more important when examining variations over shorter periods (e.g., the decadal scale), especially in the modern era. Human activity has been recognized as the dominant factor in the sharp decrease in the sediment load of the Yangtze River in recent years (Yang et al., 2002; Dai et al., 2008; Wei et al., 2011). Table 4 collates opinions regarding the causes of the decline

Median size of bed sediment at Datong

16

0.19

14 12

0.18

10 8

0.17

6 4

0.16

2009

2007

2005

2003

2001

1999

1997

1995

1993

1991

1989

1987

1985

1983

1981

1979

0

1977

2

Medium size of bed sediment (mm)

0.2 Mean size of suspended load at Yichang Mean size of suspended load at Hankou

18

1975

Mean size of suspended load (µm)

20

0.15

Fig. 6. Changes in grain size since the 1970s (modified from He et al., 2010; Yang et al., 2011), showing the mean grain size of suspended sediment at (a) Yichang and (b) Hankou; and the median grain size of the bed load at Datong (c).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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Table 4 Classification and evaluation of the relative importance of the various factors that contributed to the decline in sediment supply. The factors have been classified into five groups; i.e., dominant (D), significant (S), moderate (Mo), minor or none (Mi), and negative (N), according to their impact. Resources

Dam trapping

Soil conservation vs. deforestation

Climate change

Sand mining

Bank protection or flood plain deposition

Chen et al., 2001 Yang et al., 2002 Chen et al., 2005 Yang et al., 2005b Z.S. Yang et al., 2006 Zhang et al., 2006 Dai et al., 2008 Chen et al., 2008a

Mo D S D D S D S

– Mo – N S – S –

D – Mo – – S Mo –

– Mo S – – – Mi S

– Mo – N Mo S N S

Note: 1) ‘–’ indicates that the authors either did not mention this factor at all, or did not evaluate its impact. 2) The various impact levels quoted in the original papers have been collated and standardized to produce this table.

Rainfall amounts over the Yangtze basin have been spatially variable in recent decades, increasing over most of the basin, but declining in the upper reaches (i.e., the upper Jialingjiang) (Su et al., 2004). However, studies of the impact of climate change have been very limited, partly due to a lack of quantitative tools. A quantitative study was carried out by Dai et al. (2008) based on assumptions regarding the relationship between water discharge and sediment load in the Yangtze River basin. Their assumptions included: (1) that the trend in water discharge, whether decreasing or increasing, was solely caused by climate change; and (2) that the change in water discharge caused a proportional change in sediment discharge. Their study showed that climate change contributed to a 3% increase in sediment discharge averaged across the entire Yangtze River between the 1950s and 1990s (Dai et al., 2008). However, Xiong et al. (2009) suggested that the reduction in precipitation contributed to ~33% of the decline in sediment load in the upper Yangtze basin between 1954–1990 and 1991–2005.

5.2. Soil conservation In 1988, to expedite soil and water conservation in the upper reaches of the Yangtze River and ensure the safe operation of the TGD, the State Council of China identified the upper reaches of the Yangtze River as a key region in the national soil and water conservation plan. A series of projects (together named the Changzhi Project) have been carried out in the lower reaches of the Jinshajiang, the middle and lower reaches of the Jialingjiang, and the TGD region since 1989. After 1994, the project expanded gradually to the middle reaches, and since 2008, the Changzhi project has covered 10 provinces and 214 counties (cities) (Liao et al., 2012). It is widely accepted in the Chinese literature that these soil conservation measures have substantially reduced soil erosion since the Changzhi Project began in 1989 (http://www.forestpest.org/senfang/ News/lyxw/2011-06-28/Article_3169.shtml; Chu et al., 2009). However, quantitative studies show that the soil conservation measures made a limited contribution to the decreasing sediment load of the Yangtze River. For example, in the Jialingjiang, the contribution of soil conservation measures to the declining sediment load was between 40% and 60% in the upper reaches, but only 15% to 20% in the lower reaches (Yang and He, 2005). Based on in situ observations in a small drainage basin, Zhang et al. (2004) found that the conservation measures effectively reduced the water yield after seven years, but had no effect on the sediment yield. Xu and Sun (2007) found that the sediment reduction effect of the soil conservation measures is dependent on river runoff in the upper Yangtze River basin. In years when the water discharge rate at Yichang station is more than 480 km3/yr, the conservation projects would fail to reduce soil erosion (Xu and Sun, 2007). As it is difficult to distinguish between the effects of soil conservation measures and dam construction in reducing the riverine sediment load, most previous studies failed to provide a quantitative estimate of

the contribution of soil conservation to the decrease in sediment load (Dai et al., 2008). 5.3. Three Gorges Dam Since the 1950s, almost 50,000 dams have been built in the Yangtze River basin. The total annual sediment deposition rate in reservoirs has increased from almost zero in 1950 to more than 850 Mt/yr in 2003 (Yang et al., 2005b). These dams have significantly influenced the sediment budget over several different scales (Li et al., 2011; Yang et al., 2011). We discussed the impact of the dams on sediment load changes in detail in Section 4.1; in this section, we concentrate on studies focusing on the TGD. The world's largest dam, the TGD, was built at the outlet of the Three Gorges. The reservoir of the TGD is 600 km long, with a surface area of 1804 km2 and a storage capacity of 39.3 × 109 m3 (Hu et al., 2009). The TGD began to impound water in June 2003, and has profoundly influenced sediment transport processes in the Yangtze River. After impoundment of the TGD, the Changjiang Water Resource Committee (CWRC) began to publish annual monitoring data regarding sedimentation in the TGD. These estimates by CWRC were based on sediment inflow at an upstream gauging station and sediment outflow at a gauging station 12 km downstream of the TGD. However, this method underestimates reservoir sedimentation because it does not take into account sediment supply from the local drainage basin between the upstream inflow and downstream outflow gauging stations (Yang et al., 2007a).

Table 5 Previously reported sedimentation rates in the TGD. Sedimentation rate (Mt/yr)

Time scale

Reference

Method of Estimationa

125 114 126 118 128 138 139 110 125 118 122 151 172 148

2003 2003–2004 2003–2005 2003–2006 2003–2007 2003–2008 2003–2009 2003–2004 2003 2003–2006 2003–2004 2003–2005 2003–2008 2003–2007

CWRC, 2003 CWRC, 2003–2004 CWRC, 2003–2005 CWRC, 2003–2006 CWRC, 2003–2007 CWRC, 2003–2008 CWRC, 2003–2009 Z.S. Yang et al., 2006 Chu and Zhai, 2008 Xu and Milliman, 2009 Dai et al., 2006 Yang et al., 2007a Hu et al., 2009 Zhang et al., 2010

CWRC CWRC CWRC CWRC CWRC CWRC CWRC Following CWRC Following CWRC Following CWRC Following CWRC Modified CWRC Modified CWRC Modified CWRC

a The estimation methods can be classified into three types. The CWRC method uses the difference between sediment inflow at an upstream station and sediment outflow 12 km downstream of the TGD. The Following CWRC method uses the same approach as the CWRC estimates. The Modified CWRC method adds the sediment supply from the local drainage basin, and downstream riverbed erosion between the TGD and the downstream gauging station, to the CWRC estimation method.

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

S.B. Dai, X.X. Lu / Geomorphology xxx (2013) xxx–xxx

Yang et al. (2007a) and later Hu et al. (2009), took the following into account: a) sediment supply from the local drainage basin of the reservoir; b) sediment erosion between the TGD site and the downstream gauging station; and c) possible sedimentation beyond the backwater region caused by water impoundment. By considering these factors, their calculated sedimentation rates in the TGD were much higher than those reported by the CWRC (Table 5). For example, reservoir sedimentation estimated for the period 2003–2005 by Yang et al. (2007a) was 20% higher than the CWRC estimate for the same period. Similarly, reservoir sedimentation estimated for the period 2003–2008 by Hu et al. (2009) was 25% higher than the comparable CWRC estimate. Due to the sharp decrease in sediment supply from the upper reaches, the actual sedimentation rate was lower than the rate predicted (3.0 and 1.68 Mt/yr based on the referenced time series of 1961–1970 and 1991–2000, respectively) by the CWRC before the construction of the TGD (Huang et al., 2008), though the average trapping efficiency (80%) was higher than predicted (67%). As the Jinshajiang and Jialingjiang rivers are the main sediment sources in the upper reaches of the Yangtze, future sedimentation in the reservoir will be largely determined by variations in sediment supply from these two rivers. Fu et al. (2006) showed that after the construction of the Xiluodu and Xiangjiaba dams (XLD and XJB, respectively; see Fig. 1) on the Jinshajiang River, sediment discharge rates downstream of the cascaded dams would sharply reduce to 15 Mt/yr, and sediment discharge into the reservoir would reduce to 120 Mt/yr. Yang et al. (2007a) indicated that, when the cascade reservoirs (i.e., Wudongde, Baihetan, Xiluodu, and Xiangjiaba) were all put into full operation, they would probably trap 95% of the Jinshajiang's sediment load (91% according to Hu et al., 2009). In the first six decades after 2013, the sediment supply to the reservoir will most probably decrease to ca. 70 Mt/yr (Yang et al., 2007a), compared with 492 Mt/yr prior to the TGD (1950–2002). Chen et al. (2009) calculated sediment discharge from the cascade reservoirs on the Jinshajiang River using a 1D model, and found that the proportion of sediment discharged from these dams would be only 13% to 18% in the first 60 years after water impoundment. Sediment discharge from the Jianlingjiang would continue to decrease because of dam trapping, and because of soil and water conservation measures across the tributary. Thus, sediment discharge into the TGD will be further reduced in the future. The impact of the TGD on downstream sediment delivery is of wide interest. However, due to the complicating effect of other factors, it is difficult to quantify the TGD's contribution to the post-TGD decrease in sediment load at a downstream gauging station some distance from the dam. In an attempt to overcome this difficulty, Yang et al. (2007a) established a series of regression relationships (all statistically significant) between sediment outflow and water/sediment inflow for downstream sections

9

Table 6 Downstream channel erosion calculated from a bathymetric map. Time interval

River section

Erosion intensity (million m3/km/yr)

Reference

2002–2005 2001–2005 2001–2005 2002–2008

Yichang-Chenglingji Chenglingji-Hankou Hankou-Jiujiang Yichang-Shashi

0.234 0.057 0.032 0.250

Lu et al., 2006 Lu et al., 2006 Lu et al., 2006 Xiong et al., 2010

(e.g., the river channel between Yichang and Hankou, Lake Dongting, and the river channel between Hankou and Datong). Based on the assumption that no sedimentation would have occurred in the Three Gorges Reservoir (TGR) region since 2003 if the TGD had not been constructed, they predicted the annual sediment load at Yichang, Five Routes (gauging stations for water and sediment discharge from the main channel of the Yangtze River into Lake Dongting), Chenglingji (a gauging station at the confluence with Lake Dongting), Hankou, and Datong for the non-TGD case in 2003–2005 (Yang et al., 2007a). Using this method, they successfully separated the impact of the TGD from other impacts on the downstream sediment load. They found that 151 Mt/yr of sediment had been retained behind the TGD between 2003 and 2005. In response to this reservoir sedimentation, significant erosion of the downstream riverbed occurred. This erosion did not offset the sediment trapped in the reservoir, and the sediment flow into the sea (at Datong) decreased by 85 Mt/yr (31%), which accounts for 28% of the cumulative decrease recorded between the periods 1963–1972 and 2003–2005 (Yang et al., 2007a). As a result of its success in quantifying the impact of the dam on the fluvial sediment load, the study of Yang et al. (2007a) was chosen as an AGU Journal Highlight (Kumar, 2007). Following the method of Yang et al. (2007a), Zhang (2011) found that 45% of the decrease in sediment load at Datong between 1996 and 2002 (7 years) and 2003–2009 (7 years) could be attributed to the TGD. 5.4. Sand extraction Sand extraction from the mid-lower Yangtze increased rapidly from 26 Mt/yr in the 1980s to 40–80 Mt/yr by the end of 1990s (Chen et al., 2005, 2006). After the ban on sand extraction from the stem river, operations switched to Lake Poyang (Leeuw et al., 2010). Leeuw et al. (2010) used pairs of near-infrared Aster satellite images to estimate the number of vessels leaving the lake, and calculated a rate of sand extraction of 236 M m3/yr between 2005 and 2006. Their study suggested that sand extraction may have had a significant impact on the sediment balance of the lower Yangtze River, as it is believed that it could reduce riverine sediment discharge (Chen et al.,

Channel deposition(+)/erosion(-)

350

250 200 150 100 50

2010

2005

2000

1995

1990

1985

1980

1975

1970

1965

-50

1960

0 1955

deposition(+)/erosion(-)(Mt)

300

-100 -150 Fig. 7. Channel deposition/erosion in the Yichang–Datong reach between 1956 and 2009 (modified from Dai et al., 2006; Yang et al., 2011).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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Table 7 Downstream erosion calculated using the sediment budget method. Time interval

River section

Erosion intensity (Mt/km/yr)

Reference

2003–2004 2003–2004 2003–2005 2003–2005 2003–2005 2003–2006 2003–2006

Yichang-Hankou Hankou-Datong TGD-Yichang Yichang-Hankou Hankou-Datong Yichang-Hankou Hankou-Datong

0.169 0.027 0.15 0.091 0.024 0.083 0.029

Dai et al., 2006 Dai et al., 2006 Yang et al., 2007a Yang et al., 2007a Yang et al., 2007a Xu and Milliman, 2009 Xu and Milliman, 2009

2006; Zhang et al., 2006; Yang et al., 2007a; Zhang et al., 2009). Chen et al. (2006) argued that sand extraction was second only to reservoir interception in reducing sediment discharge from the Yangtze River. But the in situ study on the infilling of the excavation pit indicated that the infilling was mainly the result of erosion from nearby sites (Mao, 2004; Mao and Huang, 2004). For example, an experimental study by Mao and Huang (2004) showed that the development of the excavation pit was affected by the interaction between flow in the vicinity of the pit and its boundary. The numerical simulation by Mao (2004) indicated that the in-stream flow scoured the upstream edge of the excavation pit, and the secondary current eroded the transverse area of the cross section. More work is required to evaluate the impact of sand extraction on the fluvial sediment regime, and related issues may include the mechanism and the amount of sediment trapped in the sandpits caused by the extraction process. As sand extraction occurs worldwide (Kondolf, 1997), further study of the above issues may help to clarify the nature of sediment processing in modern rivers affected by such operations. 6. Morphological impacts of the decrease in sediment load Changes in riverine sediment load have significant implications for downstream morphology, and many studies have examined the morphological impacts in the middle and lower reaches of the Yangtze River. 6.1. Impacts on the downstream channel Before construction of the TGD, a transition point in the erosion/ accretion balance existed in the Yichang-Hankou reach of the Yangtze River. When the sediment load at Yichang station exceeded 450 Mt/yr, accretion occurred; while if the sediment load there was less than 300 Mt/yr, erosion occurred (Dai et al., 2006). Below the Hankou station, the river channel was less sensitive to the decrease in sediment load (Dai et al., 2006; Yang et al., 2007a; Dai and Lu, 2010). Following completion of the TGD, the sediment load at Yichang decreased sharply from around 530 Mt/yr in the 1950s and 1960s to b60 Mt/yr after 2003, and downstream channel erosion was extensive (Yang et al., 2011) (Fig. 7). Riverbank collapse was aggravated, and riverbed incision was accelerated (Xu and Milliman, 2009) as a result of the channel erosion. Field surveys confirmed that serious bank collapse had occurred in the middle and lower reaches in recent Table 8 Channel erosion rates reported previously. Time interval

River section

Actual erosion volume

Predicted erosion volume

Reference

2003–2005 2003–2005

Yichang-Jiujiang Yichang-Datong

165 Mm3/yr 83 Mt/yr

87 Mm3/yr 183 Mt/yra

2003–2008 2003–2008

Yichang-Hankou 50 Mt/yr Hankou-Datong 11 Mt/yr

Lu et al., 2006 Yang et al., 2007a Yang et al., 2011 Yang et al., 2011

a

Data source: Construction Organization of Three Gorges Project (COTGP).

years (Liu, 2010). As a result of this serious erosion associated with the TGD, the government has implemented several new projects to prevent further negative impacts. Since 2003, the CWRC has published annual calculations, based on bathymetric surveys, regarding changes to the channel. However, they only provide results from specific sections of the channel, rather than the entire middle and lower stretches of the river. Consequently, a detailed and quantitative analysis of channel change in the middle and lower reaches was not publicly available. From the reports published by the CWRC, we compiled Table 6; and using the sediment budget method, other workers have estimated the sediment erosion/ accretion status downstream from the dam (Table 7). When comparing these results with the predictions, the differences are significant. Lu et al. (2006) indicated that the actual level of erosion was more severe than the predictions, but other researchers (Dai et al., 2006; Yang et al., 2007b) concluded that it was much less than expected (Table 8). Nevertheless, erosion did increase below the dam. We believe that the main cause of the significant differences between the results of Lu et al. (2006) and other authors (Tables 6–8) was the different approaches employed: Lu et al. (2006) was based on a bathymetric survey, which included both erosion and sand extraction, whereas the other studies were based on sediment inflow and outflow data, which excluded the effects of sand extraction. 6.2. Impacts on the river–lake linkage There are many large lakes (e.g., Dongting Lake and Poyang Lake) linked to the main river in the middle and lower reaches of the Yangtze (Fig. 1). These lakes have played an important role in regulating sediment transport, as well as accommodating flood waters (Dai et al., 2005; Yang et al., 2007b; Y. Du et al., 2011). Recent studies show that these large lakes were affected by the decrease in the sediment load of the main river. For example, because of the decrease in sediment load and incision of the main riverbed, the net input of sediment from the main river to Dongting Lake decreased from 86 Mt/yr between 1950 and 2000, to 16 Mt in 2002 (Xu and Milliman, 2009). After the opening of the TGD in 2003, sediment supply from the main river to the lake essentially stopped in 2004 (0 Mt) (Xu and Milliman, 2009). During the extreme drought of 2006, sediment was flushed out of the lake and into the main channel (14 Mt) (Xu and Milliman, 2009). Due to the drastic decrease in sediment inflow, the deposition rate in Lake Dongting (Fig. 8) was reduced from 146 Mt/yr (1956–1980) to 84 Mt/yr (1981–2002), and then to 20 Mt/yr in 2003 and 2004 (Yang et al., 2007b). Although this new lake-to-main-channel transport pattern is believed to be helpful in alleviating the shrinking of the lake and facilitating the management of freshwater resources in the middle reaches (Xu and Milliman, 2009), it has completely altered the conventional river–lake relationship, causing severe drought problems (Lu et al., 2011) and damaging the lake's ecological system. A similar situation has developed in Poyang Lake. Sediment input to the lake gradually decreased over the past 20 years, from 1.86 Mt/yr (1976–1985) to 0.41 Mt/yr (1996–2005), while the sediment flushed out of the lake increased significantly from 9.38 Mt/yr (1956–2000) to 14.01 Mt/yr (2001–2007) (Min et al., 2011). Human activities were identified as the main cause of these changes. Construction of the reservoirs is believed to have reduced the sediment load of the Poyang Lake basin (Zhang et al., 2011). The increase in the amount of sediment flushed from the lake has also been attributed to engineering work on the waterway and to sand extraction in the lake (Min et al., 2011). 6.3. Impacts on the Yangtze delta and coast Previous studies of other rivers have shown that deltas are sensitive to a decrease in fluvial sediment discharge; e.g., the Nile (Fanos, 1995), Colorado (Carriquiry and Sanchez, 1999), Ebro River (Mikhailova, 2003), and the Yellow River (Yang et al., 1998). The response of the Yangtze

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

S.B. Dai, X.X. Lu / Geomorphology xxx (2013) xxx–xxx

11

250 200 150 100

2010

2005

2000

1995

1990

1985

1980

1975

1970

-50

1965

0

1960

50

1955

Deposition(+)/erosion(-)(Mt/yr)

300

Fig. 8. Time series of deposition in Lake Dongting from 1955 to 2010 (modified from Dai et al., 2005).

River delta to variations in its sediment supply from the river basin has also attracted research attention in recent years. Geomorphological studies show that the sediment discharge from the Yangtze River is now below the critical level required to maintain the delta, and so the delta has begun to erode (Gao, 2010; Yang et al., 2011). Based on calculations using bathymetric data from an area of 6000 km2 at the delta front, Yang et al. (2003) found that the net accretion rate over their study area decreased from 38 mm/yr (1958–1978)

to 8 mm/yr (1978–1997), in response to a decline in sediment supply from 466 Mt/yr to 394 Mt/yr over the same period. The outer subaqueous delta is more sensitive to a decline in fluvial sediment supply than the inner subaqueous delta. They also found that the accretion rate decreased more rapidly than the rate of decline in sediment supply from the river (Yang et al., 2003). Yang et al. (2011) found that since 1958, bathymetric changes in the subaqueous delta front have shown a strikingly linear correlation with the Yangtze's sediment discharge into the

Fig. 9. Response of the Yangtze delta to decreasing fluvial sediment supply (modified from Yang et al., 2011). On lower diagram main columns show sediment load supplied to the delta by the Yangtze River; subsidiary columns show net sediment addition/loss from areas I and II (locations above).

Please cite this article as: Dai, S.B., Lu, X.X., Sediment load change in the Yangtze River (Changjiang): A review, Geomorphology (2013), http:// dx.doi.org/10.1016/j.geomorph.2013.05.027

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river mouth (Fig. 9). They showed that between 1958 and 1977, when the average annual suspended sediment discharge was 470 Mt/yr, the subaqueous delta shoaled by ca. 125 Mm3/yr. In contrast, between 2000 and 2004, an average discharge of 245 Mt/yr resulted in 70 Mm3/yr of delta front erosion. Yang et al. (2011) suggested that the Yangtze delta will continue to erode while the sediment discharge remains less than ca. 270 Mt/yr. Li et al. (2007) and Li (2010) also report the transformation from accretion to erosion of the Yangtze delta and surrounding coastline in response to the reduction in sediment supply. A coarsening trend on the seabed was identified in samples of surface sediment from cores collected in the area of recent erosion at the river mouth (Luo et al., 2012b). As sediment discharge from the Yangtze River is expected to decrease further (Fig. 3), a more detailed study of the response of the delta to the changing sediment supply regime is urgently required. 7. Discussion and perspective In the context of global change, the response of large river basins has attracted increasing attention. The Yangtze River provides a valuable opportunity to evaluate natural and anthropogenic impacts on a river basin environment, and in recent years hundreds of papers have been published on the sedimentology of the Yangtze. Although discrepancies, mainly due to the sources of data used, exist among the researchers, some general conclusions can be drawn from these extensive studies. A stepwise decrease in the sediment load at most stations has been recorded since the 1970s, and dam construction is the dominant factor driving this trend. Due to the decrease in sediment discharge from the upper reaches, sedimentation in the TGD is much lower than previously predicted. The lifespan of the TGR will be substantially prolonged by a further drastic decrease in sediment supply when the cascade dams on the upper reach begin operations after 2013. The rates of channel erosion and hydrodynamic change have increased since the TGD began operating. The decrease in sediment discharge into the river mouth has already led to erosion of the delta. A shift in sediment particle size distribution and textural parameters has accompanied this decrease in the sediment load. As illustrated by previous studies, sediment related issues across this vast river basin are complicated by the heterogeneous nature of its morphology and climate. In recent years, intensified human activity has made such problems even more complex. In view of this, it is reasonable to suggest that the identification and interpretation of hydrological changes in the Yangtze basin will become increasingly difficult. A detailed study of the causal mechanisms of variations in sediment load, and of the factors having the most significant impact on the Yangtze River, is urgently required. Since completion of the world's largest dam in 2003, the TGD has heavily regulated the middle and lower reaches of the Yangtze River (Fu et al., 2010). The TGD has fundamentally changed sedimentary processes along entire reaches, and sedimentological issues related to the TGD will inevitably be one of the main concerns for any study of the Yangtze River. After completing our review of the literature, we suggest that the following topics require further attention. 1) Sediment supply from the upper reaches, as well as from the area surrounding the reservoir. Synthesis studies should be carried out to determine future trends in sediment discharge into the TGR, as variations in sediment discharge into the TGR will not only impact upon the reservoir itself, but also on the reaches downstream of the dam, as well as the estuary and delta. The construction of new dams (many large dam cascades are either under construction or planned in the upper basin), water conservation measures, and climate change should be further addressed in view of their importance to sediment processing.

2) Sedimentation in the TGR. A uniform calculation method should be developed to illustrate the true nature of sedimentation in the TGR over various time intervals. In view of this, intensified measurements of the sediment load in the region near the reservoir, sampling of sedimentation in the reservoir, and channel bathymetric studies both above and below the reservoir should be carried out. 3) Changes in the operation of the TGD. The original design of the TGD was intended to store clean water and sluice silty water to minimize sedimentation in the reservoir. With a significant reduction in sediment supply from upstream, the reservoir operation may be altered to maximize its benefits, for example, by starting to store water from July or August, instead of from September. This may increase power generation, but may further aggravate sediment starvation downstream. 4) Impacts on the lakes. The large lakes such as Dongting and Poyang used to be sediment sinks and flood buffer areas. However, they are becoming sediment sources, and their buffering capacity during floods and droughts has been seriously affected by incision of the river channel. There is an urgent need to study the lake– river relationship, especially sediment exchange between the two. 5) Impacts on the downstream channel and delta evolution under the post-TGD circumstances. Although downstream erosion is becoming evident, it has not been treated seriously; the same is true of the impacts at the river mouth. While several studies have already indicated that the decrease in sediment supply may cause significant negative environmental impacts in the estuary, few have considered the potential impacts of a further decline in sediment supply.

Acknowledgments This study is part of the project funded by National University of Singapore (Grant: MOE2011-T2-1-101), the National Science Foundation of China (41130856, 41001301), the State Key Laboratory of Lake Science and Environment (2010SKL002) and the Talented Scholar Fund of Chuzhou University (2010qd04). Prof. S.L. Yang and an anonymous reviewer are thanked for their valuable comments and suggestions. We are also grateful to the Yangtze River Commission for access to the hydrological data.

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