Erosion of the Yangtze River and Its Delta

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50,000 dams later: Erosion of the Yangtze River and its delta

Article in Global and Planetary Change · November 2010 DOI: 10.1016/j.gloplacha.2010.09.006

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50,000 dams later: Erosion of the Yangtze River and its delta

S.L. Yang a,⁎, J.D. Milliman b,⁎,P.Lia,K.Xuc a State Key Lab of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, China b School of Marine Science, Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, VA 23062, USA c Department of Marine Science, Coastal Carolina University, P.O. Box 261954, Conway, SC 29528, USA article info abstract

Article history: Using 50 years of hydrologic and bathymetric data, we show that construction of ~50,000 dams throughout Received 13 July 2010 the Yangtze River watershed, particularly the 2003 closing of the Three Gorges Dam (TGD), has resulted in Accepted 27 September 2010 downstream channel erosion and coarsening of bottom sediment, and erosion of the Yangtze's subaqueous Available online 27 October 2010 delta. The downstream channel from TGD reverted from an accretion rate of ~90 Mt (1Mt = 1000 000 t)/yr between the mid-1950 s and mid-1980 s to an erosion rate of ~60 Mt/yr after closing of the TGD. The delta Keywords: front has devolved from ~125 Mm3 (1 Mm3 = 1000 000 m3)/yr of sediment accumulation in the 1960 s and Yangtze Delta 3 Three Gorges Dam 1970 s, when river sediment load exceeded 450 Mt/yr, to perhaps 100 Mm /yr of erosion in recent years. As of sediment load 2007 erosion seemed to have been primarily centered at 5–8 m water depths; shallower areas remained delta erosion relatively stable, perhaps in part due to sediment input from eroding deltaic islands. In the coming decades the Yangtze's sediment load will probably continue to decrease, and its middle-lower river channel and delta will continue to erode as new dams are built, and the South-to-North Water Diversion is begun. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As the largest (1,800,000 km2) and longest (6300 km) river in southern Asia (Fig. 1A), the Yangtze ranks 5th globally in terms of Over the past several millennia, accelerating human landuse led to water discharge (900 km3/yr) and, until recently, 4th in terms of increased terrestrial erosion and a corresponding increased sediment sediment load (470 Mt/yr) (e.g., Milliman and Farnsworth, 2010). Due discharge to the coastal ocean (Milliman et al., 1987; Syvitski et al., to its large sediment supply, since the mid-Holocene the Yangtze delta 2005), but over the past century river damming and irrigation, as well has prograded more than 200 km into its incised river valley (Hori et as improved landuse practices, have decreased sediment loads of al., 2001; Li et al., 2002). Vertical Holocene accretion in the river's many rivers, thereby triggering erosion of many deltas (Milliman, present-day subaqueous delta has locally exceeded 60 m (Hori et al., 1997; Syvitski et al., 2009), including those of the Nile (Fanos, 1995), 2002; Li et al., 2002; Liu et al., 2007). With a watershed population Colorado (Carriquiry et al., 2001), Mississippi (Blum and Roberts, approaching one half billion people, the Yangtze is also one of the 2009), Ebro (Sánchez–Arcilla et al., 1998), Godavari and Krishna (Rao most heavily impacted rivers in the world. To store water and to et al., 2010), and Yellow (Chu et al., 2006; Wang et al., 2007; Xu, 2008) minimize flooding, furnish hydroelectric power, and facilitate irriga- rivers. Inadequate monitoring of these and other rivers, unfortunately, tion, more than 50,000 dams, ranging in size from dams on farmers' has often limited the extent to which changes in discharge and their fields to dams more than 100 m high, have been built throughout the impacts could be identified, let alone adequately quantified. Yangtze's watershed since 1950 (Yang et al., 2005). As of 2000, the Unlike many other major rivers, the Yangtze River, the subject of Yangtze had 15 dams taller than 100 m in height, and another 20 or this paper, has been intensively monitored for water and sediment more scheduled to be constructed by 2015. Of particular interest has discharges since the mid 1950s, and, since 1987, for suspended been the extent to which the world's presently largest dam, the Three sediment particle size. In addition, a series of detailed bathymetric Gorges Dam (TGD), 185 m high, which was closed in 2003, would soundings between 1958 and 2007 allows us to delineate bathymetric impact the river and its delta (e.g., Li et al., 2004; Yang et al., 2006, and volumetric changes to the Yangtze's delta front, a topic that has 2007a; Xu et al., 2007; Chen et al., 2008). been inadequately addressed previously. Collectively, this extensive Thirty-five years after the accelerated dam construction and more database makes it possible to quantify changes in Yangtze and its than five years after the commissioning of the TGD, it is perhaps now subaqueous delta in greater detail than would be possible for most time to reflect on some of the impacts that these activities have had other impacted rivers. on the Yangtze and its delta. Any number of previous studies have focused on various water and sediment aspects of the Yangtze impact (e.g. Chen et al., 2005, 2008; Dai et al., 2008; Hu et al., 2009; Xu and ⁎ Corresponding authors. Tel.: +86 21 62233115. E-mail addresses: [email protected] (J.D. Milliman), [email protected] Milliman, 2009; Xu et al., 2010a; Yang et al., 2001, 2002, 2005, 2006; (S.L. Yang). Zhang and Wen, 2004; Zhang et al., 2006), but to the best of our

Fig. 1. A) Yangtze River watershed, showing locations of the Three Gorges Dam (TGD) and the Yichang, Hankou and Datong gauging stations. B) Subaqueous delta, showing Study Areas 1 and 2, the Twin-Jetties Groyne Complex (TJ-GC) and its associated dredged channel. knowledge, no previous work has shown the collective impact both soundings for the ﬁve surveys totaled 1350 (1958), 1580 (1977), 2220 on the middle and lower courses of the river as well as the impact on (2000), 1570 (2004) and 2590 (2007) km in length. All surveys were the Yangtze's subaqueous delta, the objective of the present study. carried out between early May and early June, prior to peak discharge. Tidal corrections used recorded tidal levels in and near the study area. 2. Materials and methods Maps were processed using ArcGIS software developed by ESRI (Environmental Systems Research Institute, USA). For this study we gathered monthly and annual water and Each set of depth soundings was interpolated to a grid with suspended sediment discharges between the years 1953 and 2008 30×30-m cells using Kriging interpolation technique. Digitized maps at the Yichang, Hankou and Datong gauging stations (Fig. 1) were used to calculate the vertical accretion/erosion rates and for maintained by the Yangtze Water Conservancy Committee, Ministry delineating accretion/erosion areas. At each grid, deduction of the of Water Conservancy of China. We also utilized the mean grain size later depth from the earlier depth gave the thickness of accretion data of suspended sediments at the Yichang and Hankou stations as (positive) or erosion (negative). The total volumes of accretion and well as the bottom sediment texture between Yichang and Hankou. erosion and the difference between them were then calculated. The To calculate temporal and spatial changes in the Yangtze subaqueous rate of annual accretion or erosion rate could then be calculated with delta, we used bathymetric maps at a scale of 1:75,000 obtained by the the net volume divided by area and time length (years). Maritime Survey Bureau of Shanghai, Ministry of Communications of The Kriging interpolation technique is widely used in GIS analysis. China. 2000, 2004 and 2007 data were collected using a DESO-17 echo- The error associated with the value of sediment volume based on sounder, precision 0.1 m; navigation was obtained using a GPS (Trimble bathymetry and the Kriging interpolation technique rests with the Co., USA) with a horizontal error of ±1 m 1958 and 1977 echo-sounder difference in depth between the neighboring bathymetric data points data had vertical precisions of ~0.1 m; navigation error was ~50 m in and the complexity of the seabed morphology. The greater the 1958 (using a sextant) and ~20 m in 1977 (using Loran S). Bathymetric difference in depth between the data points and the more complicated 16 S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20 the seabed morphology, the greater the error associated with the calculated sediment volume. If there are numerous data points in a study area, however, the overall error is reduced because of the counter- balancing between positive and negative errors. As a result, error estimation associated with sediment volume using the Kriging interpolation technique is customarily omitted. In the present study, there are numerous data points on each bathymetric map (as addressed above), and the seabed of the subaqueous delta is smooth, with gradients typically b1‰ (See Fig. 1b). So, errors associated with sediment volume estimation using Kriging interpolation is assumed to be very low (b1%, as we estimated).

3. Sedimentary changes along the middle and lower reaches of the Yangtze

Except for several extremely wet or dry years, particularly in 1954, Yangtze water discharge has remained relatively constant since the 1950s (Fig. 2), averaging about 900 km3/yr as measured at Datong, 600 km from the river mouth, just upstream of the tidal influence (Fig. 1A). The sediment discharge past Datong, by contrast, fell from ~490 Mt/yr in the 1950s and 1960s to ~150 Mt/yr after the closure of the TGD (Fig. 2). The decline in sediment discharge in the upper reaches of river, as measured at Yichang, 37 km downstream from the TGD and 1800 km from the river mouth (Fig. 1A), has been even greater, falling from ~530 Mt/yr in the 1950s and 1960s to ~60 Mt/yr after 2003 (Fig. 3A). Of particular interest has been the evolving sedimentary regime in Fig. 3. A) Down-river change in annual sediment load as delineated at Yichang (upper river), Hankou (middle river), and Datong (lower river), 1955–2008. The Three Gorges the middle reaches of the river, as measured at Hankou, ~700 km Dam (TGD) was closed in 2003. B) Deposition and erosion in the middle section of the downstream from Yichang and 500 km upstream from Datong (Fig. 1A). river as represented by the difference between calculated sediment supply (sediment Sediment reaching Hankou should reflect the sediment passing Yichang load at Yichang+sediment load from tributaries and net sediment exchange between plus sediment introduced from the Han River and ungauged tributaries the river and Lake Dongting) and sediment load at Hankou. (Yang et al., 2007a)aswellassedimentexchangewithDongtingLake. Between the mid-1950s and mid-1980s, the total amount of sediment river (Fig. 3B). Similarly, between 2003 and 2008 the annual sediment reaching Hankou from these various sources should have totaled discharge past Datong was 154 Mt/y, 29 Mt/yr more than the one ~530 Mt/yr, whereas only ~440 Mt/yr were measured at Hankou. This measured at Hankou, suggesting further erosion in the lower reaches ~90 Mt/yr sediment “loss” (Fig. 3) suggests deposition along the middle of the river. Using the approach by Yang et al. (2007a), the tributaries reaches of the river. The slightly higher sediment load at Datong during between Hankou and Datong supplied 18 Mt/yr in 2003−2008. That the same period, 470 Mt/yr, may reflect the sediment import from is, the combined erosion rate between Yichang and Datong was downstream tributaries (Xu and Milliman, 2009). Between the mid- 61 Mt/yr, ~40% of the amount of sediment trapped behind the TGD 1980s and late 1990s, sediment deposition in the middle reaches of the (Yang et al., 2007a,b); the erosion along the 700 km reach from river declined to ~60 Mt/yr, in large part the response to upstream Yichang to Hankou (0.071 Mt/yr/km) was three times higher than damming (Xu et al., 2007), and from 2000 to 2002, total sediment along the 500 km reach from Hankou to Datong (0.024 Mt/yr/km). It import between Yichang and Hankou and the amount reaching Hankou is necessary to indicate that here the erosion, based on the sediment were essentially equal, suggesting little or no net sediment gain or loss budget, reflects only the responses to decline in the sediment supply. in the middle reaches of the river (Fig. 3A). Sand extractions in the Yangtze (Chen et al., 2005) and other rivers in For the first six years (2003–2008) following the TGD closure, the China (e.g. the Pearl River, Lu et al., 2007) presumably also have sediment discharge past Hankou averaged 125 Mt/y, 50 Mt/yr more resulted in significant additional erosion. than the sediment input from Yichang and downstream sources Erosion along the middle reaches of the river is also suggested by the (Fig. 3A), suggesting sediment erosion in the middle reaches of the coarsening of the Yangtze channel sediment, the mean grain size of

Fig. 2. Annual water and sediment load for the Yangtze River as measured at Datong, the farthest downstream gauging station, 1953–2008. S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20 17 river-bottom samples taken at fixed locations between Yichang and extensions of Chongming, Hengsha and Jiuduansha define the Hankou increasing from 210 μmin2002to300μm in 2008. The closer western limits of the study area; 15 to 20-m depth contours define the sampling sites to the TGD, the greater the difference in grain size the eastern limits. Most of the study area, which we term Area 1 between 2008 and 2002. At Yichang, the grain size increased from (1280 km2, Fig. 1B), was surveyed all five years; Area 2 (545 km2), 0.36 mm in 2002 to 25 mm in 2008; at Chenglinji, 400 km downstream directly off the North Channel of the Yangtze's South Branch, was not from Yichang, grain size increased from 0.18 mm in 2002 to 0.19 mm in surveyed in 2007. Calculating bathymetric changes based on the less 2008 (Xu et al., 2010b). Another measure of channel erosion along the accurate 1958 and 1977 navigation was partly compensated for by the middle reaches of the river is seen in the change of the grain size of the longer time intervals between subsequent surveys (19 and 23 years, suspended sediment. Between 1987 and 2002, the mean grain size of respectively). the suspended sediment at Yichang and Hankou averaged ~8–10 μm, Delineating bathymetric change was somewhat complicated in whereas after 2003 the suspended sediment particle size at Yichang 1997 by construction of the Twin Jetty–Groyne Complex (TJ-GC; declined to ~3 μm in response to the settling of coarser sediments 103 km2 within Area 1) in the North Passage of the South Channel, to behind the TGD, but increased to 17–19 μmatHankou(Fig. 4), we facilitate navigation (Du and Yang, 2007). By 2004 the TJ-GC extended assume because of the aforementioned channel erosion. into the southwestern portion of the study area (Fig. 1B). In Decline in sediment load along the Yangtze also affected sedimen- conjunction with the TJ-GC, a 350-m-wide channel (8.5 km2 in tation in the linked lakes. A great deal of water and sediment flows area) has been dredged regularly to 10 m below lowest tide level, from the Yangtze into Lake Dongting via several inlets, although the most of the spoil being dumped beyond the 10-m depth contour. water and some of the sediment return to the Yangtze via an outlet Historically most of the Yangtze's annual sediment load was (Dai et al., 2005). The deposition rate within Lake Dongting decreased deposited on the subaqueous delta during flood season, much of it from 146 Mt/yr between 1956 and 1980 to 84 Mt/yr between 1981 eroded during winter storms (DeMaster et al., 1985; Milliman et al., and 2002 (Yang et al., 2007b). Since the closure of the TGD, the 1985) and transported southward to form an extensive nearshore mud sediment transported from the Yangtze into Lake Dongting has been wedge extending into Taiwan Strait (Liu et al., 2007). Isotopic data from drastically reduced, and the deposition rate within Lake Dongting has several cores suggest that sediment accumulation in the subaqueous decreased to b10 Mt/yr, in some years (2006 and 2008) showing a net delta in the 1950s–1980s averaged in the range of 3−5 cm/yr erosion of 2 and 1 Mt/yr, based on our calculated sediment budget. (DeMaster et al., 1985; Zhang et al., 2008). Although there is only one channel linking the Yangtze with Lake Between 1958 and 1977, when the Yangtze river (as measured at Poyang, the decline in the Yangtze's sediment load might have Datong) discharged an average of 470 Mt/yr of suspended sediment, somewhat impacted sedimentation in this lake because water and bathymetric data indicate that the 5-m contour east of Hengsha Island sediment occasionally flow from the river into the lake, particularly prograded as much as 5 km (Fig. 6). The 10-m isobath prograded during river floods and lake droughts. Similarly, the deposition rate of overall more than 12 km although retreating ~2 km in the north. The Lake Poyang decreased from 11 Mt/yr in 1956–1980 to 8 Mt/y 1981– 20-m contour, by contrast, showed little overall change (Fig. 6). 2002 (Yang et al., 2007b). Based on our sediment budget, the lake During this 19-year period, 55% of the area shoaled by N5 cm/yr, eroded at a rate of 7 Mt/yr in 2003–2008, which might be relevant with whereas 8% deepened by more than 5 cm/yr, mostly off the North the Yangtze's declining sediment load. Channel and the North Passage of the South Channel (Fig. 7A). Average shoaling within the study area was 6.8 cm/yr. 4. Morphologic changes in Yangtze Delta Between 1977 and 2000, most of the Yangtze subaqueous delta continued to prograde, but, presumably because of declining sediment It is still not clear how islands at the seaward edge of the Yangtze discharge (Fig. 2), shoaling was generally b5 cm/yr. Although some of estuary have responded to change in river flow, in part because the the north-central delta and areas off Jiuduansha shoaled by as much as islands' shorelines are reinforced by jetties and revetments. A 15 cm/yr, there was little net change to the south (Fig. 7A). Calculated comparison of transects taken in 1982 and 1990 show a 2-km net shoaling for the entire study area was 3.2 cm/yr. progradation of eastern Chongming Island. By contrast, between 2006 Between the 2000 and 2004 bathymetric surveys, Yangtze and 2010, salt marsh progradation was only ca.100 m and the sediment discharge at Datong fell from 340 (2000) to 205 Mt/yr mudflats vertically eroded as much as 30 cm (Fig. 5). (2003) (Fig. 2). The b5-m contour east of Hengsha Island continued to To delineate temporal trends in the Yangtze subaqueous delta prograde — locally as much as 2 km — but showed little change in bathymetry and morphology, we compared bathymetric data from position east of Chongming or Jiuduansha islands. The 10-m contour, 1958, 1977, 2000, 2004 and 2007 within an 1825-km2 area in the by contrast, retreated 0.5–1.5 km throughout much of the study area, river's South Branch through which more than 98% of the Yangtze particularly east of Hengsha (Fig. 6). Overall, between 2000 and 2004, water and sediment are discharged (Chen et al., 1985). Bathymetric 70% of the study area experienced erosion; the average vertical erosion rate for the study area was −3.8 cm/yr. The difference in vertical erosion between Areas 1 and 2 is noteworthy: −0.7 vs. −11 cm/yr, much of Area 2 deepening by as much as 25 cm/yr (Fig. 7A). Between 2004 and 2007, after the closing of the TGD, the 5-m contour off both Henhsha and Chongming islands continued to prograde, locally by as much as 1–2km(Fig. 6), perhaps in response to erosion from the eastern ends of the islands (see above). The 10-m isobath, in contrast, retreated on average ~1 km, and locally by nearly 2 km. Erosion seems to have been concentrated between 5 and 8 m depths, this depth range decreasing in Area 1 by 30% (~150 km2) (Fig. 8); the 8–11 m depth interval, by contrast, increased by 120 km2. Depths greater than 11 m showed no significant change in total area (Fig. 8), which in part may reflect dumping of the dredge spoil removed from the TJ-GC channel. Calculated net erosion rate for Area 1 was −4.5 cm/yr. Fig. 4. Mean grain size of suspended sediment at Yichang and Hankou, 1987–2008. Note Interestingly, since 1958 bathymetric changes within the study the abrupt grain size change after the 2003 TGD closure. area have shown a strikingly linear correlation with the Yangtze's 18 S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

Fig. 5. Changes in eastern Chongming Island topographic proﬁles — (A) 1982–1990 (after Yang et al., 2001) vs. (B) 2006–2010. The elevation of the low marsh edge is 2.8 m. Location of the transect is shown in Fig. 1B. sediment load as measured at Datong (Figs. 7B, 9). Between 1958 and 5. Conclusions 1977, when the average annual suspended sediment discharge was 470 Mt, the subaqueous delta shoaled by ~125 Mm3/yr. In contrast, The sediment discharge of the Yangtze River, as measured at between 2000 and 2004 an average discharge of 245 Mt/yr resulted in Datong, fell from 490 Mt/yr in the 1950s and 1960s to ~150 Mt/yr 70 Mm3/yr of delta front erosion (Fig. 7B). These data and trends after the closure of the TGD. In contrast, water discharge remained suggest that the Yangtze delta will continue to erode as long as the relatively stable, indicating that the decline in sediment load stemmed river annually discharges less than ~270 Mt/yr of sediment (See primarily from trapping behind the watershed's 50,000 dams, in Fig. 9). In 2007 and 2008, sediment discharge past Datong averaged particular the TGD. In response to this drastic decrease in sediment only 130–140 Mt/yr; extrapolating the linear trends shown in Fig. 9 supply, the river channel downstream from the TGD has changed from suggests that erosion in the subaqueous delta may have already one of net accretion to one of net erosion. Since 2003, the middle surpassed 100 Mm3/yr. Whether the erosion continues to be conﬁned stretches of the river have experienced on average ~50 Mt/yr of primarily to 5–8 m depths remains to be seen. erosion, compared with a deposition rate of ~90 Mt/yr between the

Fig. 6. Left: 5-, 10- and 20-m bathymetric contours on the Yangtze delta of the years 1958, 1977, 2000, 2004 and 2007. Right: changes in bathymetric proﬁles (A) and (B). S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20 19

Fig. 7. A) Accumulation/erosion in Study Areas 1 and 2, 1958–1977, 1977–2000, 2000–2004, 2004–2007. By 2004 the TJ-GC and dredged channel extended into the study area. B) Comparison of sediment load at Datong and sediment accumulation/erosion in Study Areas 1 and 2; sediment load converted to Mm3/yr assuming a bulk density of 1.25 t/m3. mid-1950s and mid-1980s. River channel erosion, which also is along the main river channel of the middle and lower reaches and in reﬂected by the coarsening of bottom sediments, has only compen- the delta front will continue in the coming decades. sated for about 20% of the river's decreased sediment discharge. As a result, the estuary has experience sediment starvation, and a Acknowledgements corresponding decrease in coastal salt marsh accretion and net erosion in subaqueous delta front. The erosion seems to have occurred We are grateful to the Editor and reviewers who provided valuable primarily between 5 and 8 m below the lowest tide. The conversion comments and suggestions. The study was supported by the Ministry from delta front accretion to erosion seems to have occurred when river's sediment load fell below 270 Mt/yr. Because of the TGD operation and the construction of new large dams and the South-to- North Water Diversion in the watershed, the sediment load of the Yangtze River most likely will continue to decline and thus the erosion

Fig. 9. Comparison of subaqueous delta accumulation/erosion (Area 1) between various bathymetric surveys and mean sediment load past Datong during the periods 1958–77, Fig. 8. Depth distributions in subaqueous delta area. Note the marked decrease in area 1977–2000, 2000–04, and 2004–07 (r: correlative coefficient; P: level of significance). represented by 5–8 m depths and increase in 8–11 m. Data also displayed in Fig. 7B. 20 S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20 of Science and Technology of China (2010CB951202, 2008DFB90240), Milliman, J.D., Shen, H.-T., Yang, Z.-S., Meade, R.H., 1985. Transport and deposition of river sediment in the Changjiang estuary and adjacent continental shelf. the Natural Science Foundation of China (40721004), and the State Continental Shelf Research 4, 37–45. Key Laboratory of Estuarine and Coastal Research of China (SKLEC- Milliman, J.D., Qin, Y.-S., Ren, M.-E., Saito, Y., 1987. 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