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50,000damslater:ErosionoftheYangtzeRiveranditsdelta

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Global and Planetary Change 75 (2011) 14–20

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Global and Planetary Change

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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. Li a, K. Xu c

a State Key Lab of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, Chinab School of Marine Science, Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, VA 23062, USAc Department of Marine Science, Coastal Carolina University, P.O. Box 261954, Conway, SC 29528, USA

⁎ Corresponding authors. Tel.: +86 21 62233115.E-mail addresses: milliman@vims.edu (J.D. Milliman

(S.L. Yang).

0921-8181/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.gloplacha.2010.09.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 July 2010Accepted 27 September 2010Available online 27 October 2010

Keywords:Yangtze DeltaThree Gorges Damsediment loaddelta erosion

Using 50 years of hydrologic and bathymetric data, we show that construction of ~50,000 dams throughoutthe Yangtze River watershed, particularly the 2003 closing of the Three Gorges Dam (TGD), has resulted indownstream channel erosion and coarsening of bottom sediment, and erosion of the Yangtze's subaqueousdelta. The downstream channel from TGD reverted from an accretion rate of ~90 Mt (1Mt = 1000 000 t)/yrbetween the mid-1950 s and mid-1980 s to an erosion rate of ~60 Mt/yr after closing of the TGD. The deltafront has devolved from ~125 Mm3 (1 Mm3 = 1000 000 m3)/yr of sediment accumulation in the 1960 s and1970 s, when river sediment load exceeded 450 Mt/yr, to perhaps 100 Mm3/yr of erosion in recent years. As of2007 erosion seemed to have been primarily centered at 5–8 m water depths; shallower areas remainedrelatively stable, perhaps in part due to sediment input from eroding deltaic islands. In the coming decadesthe Yangtze's sediment load will probably continue to decrease, and its middle-lower river channel and deltawill continue to erode as new dams are built, and the South-to-North Water Diversion is begun.

), slyang@sklec.ecnu.edu.cn

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Over the past several millennia, accelerating human landuse led toincreased terrestrial erosion and a corresponding increased sedimentdischarge to the coastal ocean (Milliman et al., 1987; Syvitski et al.,2005), but over the past century river damming and irrigation, as wellas improved landuse practices, have decreased sediment loads ofmany rivers, thereby triggering erosion of many deltas (Milliman,1997; Syvitski et al., 2009), including those of the Nile (Fanos, 1995),Colorado (Carriquiry et al., 2001), Mississippi (Blum and Roberts,2009), Ebro (Sánchez–Arcilla et al., 1998), Godavari and Krishna (Raoet al., 2010), and Yellow (Chu et al., 2006;Wang et al., 2007; Xu, 2008)rivers. Inadequatemonitoring of these and other rivers, unfortunately,has often limited the extent to which changes in discharge and theirimpacts could be identified, let alone adequately quantified.

Unlike many other major rivers, the Yangtze River, the subject ofthis paper, has been intensively monitored for water and sedimentdischarges since the mid 1950s, and, since 1987, for suspendedsediment particle size. In addition, a series of detailed bathymetricsoundings between 1958 and 2007 allows us to delineate bathymetricand volumetric changes to the Yangtze's delta front, a topic that hasbeen inadequately addressed previously. Collectively, this extensivedatabase makes it possible to quantify changes in Yangtze and itssubaqueous delta in greater detail than would be possible for mostother impacted rivers.

As the largest (1,800,000 km2) and longest (6300 km) river insouthern Asia (Fig. 1A), the Yangtze ranks 5th globally in terms ofwater discharge (900 km3/yr) and, until recently, 4th in terms ofsediment load (470 Mt/yr) (e.g., Milliman and Farnsworth, 2010). Dueto its large sediment supply, since themid-Holocene the Yangtze deltahas prograded more than 200 km into its incised river valley (Hori etal., 2001; Li et al., 2002). Vertical Holocene accretion in the river'spresent-day subaqueous delta has locally exceeded 60 m (Hori et al.,2002; Li et al., 2002; Liu et al., 2007). With a watershed populationapproaching one half billion people, the Yangtze is also one of themost heavily impacted rivers in the world. To store water and tominimize flooding, furnish hydroelectric power, and facilitate irriga-tion, more than 50,000 dams, ranging in size from dams on farmers'fields to dams more than 100 m high, have been built throughout theYangtze's watershed since 1950 (Yang et al., 2005). As of 2000, theYangtze had 15 dams taller than 100 m in height, and another 20 ormore scheduled to be constructed by 2015. Of particular interest hasbeen the extent to which the world's presently largest dam, the ThreeGorges Dam (TGD), 185 m high, which was closed in 2003, wouldimpact the river and its delta (e.g., Li et al., 2004; Yang et al., 2006,2007a; Xu et al., 2007; Chen et al., 2008).

Thirty-five years after the accelerated dam construction and morethan five years after the commissioning of the TGD, it is perhaps nowtime to reflect on some of the impacts that these activities have hadon the Yangtze and its delta. Any number of previous studies havefocused 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 andMilliman, 2009; Xu et al., 2010a; Yang et al., 2001, 2002, 2005, 2006;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 StudyAreas 1 and 2, the Twin-Jetties Groyne Complex (TJ-GC) and its associated dredged channel.

15S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

knowledge, no previous work has shown the collective impact bothon the middle and lower courses of the river as well as the impact onthe Yangtze's subaqueous delta, the objective of the present study.

2. Materials and methods

For this study we gathered monthly and annual water andsuspended sediment discharges between the years 1953 and 2008at the Yichang, Hankou and Datong gauging stations (Fig. 1)maintained by the Yangtze Water Conservancy Committee, Ministryof Water Conservancy of China. We also utilized the mean grain sizedata of suspended sediments at the Yichang and Hankou stations aswell as the bottom sediment texture between Yichang and Hankou.

To calculate temporal and spatial changes in theYangtze subaqueousdelta, we used bathymetric maps at a scale of 1:75,000 obtained by theMaritime Survey Bureau of Shanghai, Ministry of Communications ofChina. 2000, 2004 and 2007 data were collected using a DESO-17 echo-sounder, precision 0.1 m; navigationwas obtained using a GPS (TrimbleCo., USA)with a horizontal error of±1 m 1958 and 1977 echo-sounderdata had vertical precisions of ~0.1 m; navigation error was ~50 m in1958 (using a sextant) and ~20 m in 1977 (using Loran S). Bathymetric

soundings for the five surveys totaled 1350 (1958), 1580 (1977), 2220(2000), 1570 (2004) and 2590 (2007) km in length. All surveys werecarried out between early May and early June, prior to peak discharge.Tidal corrections used recorded tidal levels in and near the study area.Maps were processed using ArcGIS software developed by ESRI(Environmental Systems Research Institute, USA).

Each set of depth soundings was interpolated to a grid with30×30-m cells using Kriging interpolation technique. Digitized mapswere used to calculate the vertical accretion/erosion rates and fordelineating accretion/erosion areas. At each grid, deduction of thelater depth from the earlier depth gave the thickness of accretion(positive) or erosion (negative). The total volumes of accretion anderosion and the difference between them were then calculated. Therate of annual accretion or erosion rate could then be calculated withthe net volume divided by area and time length (years).

The Kriging interpolation technique is widely used in GIS analysis.The error associated with the value of sediment volume based onbathymetry and the Kriging interpolation technique rests with thedifference in depth between the neighboring bathymetric data pointsand the complexity of the seabed morphology. The greater thedifference in depth between the data points and the more complicated

Fig. 3. A) Down-river change in annual sediment load as delineated at Yichang (upperriver), Hankou (middle river), and Datong (lower river), 1955–2008. The Three GorgesDam (TGD) was closed in 2003. B) Deposition and erosion in the middle section of theriver as represented by the difference between calculated sediment supply (sedimentload at Yichang+sediment load from tributaries and net sediment exchange betweenthe river and Lake Dongting) and sediment load at Hankou.

16 S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

the seabed morphology, the greater the error associated with thecalculated sediment volume. If there are numerous data points in astudy area, however, the overall error is reduced because of the counter-balancing between positive and negative errors. As a result, errorestimation associated with sediment volume using the Kriginginterpolation technique is customarily omitted. In the present study,there are numerous data points on each bathymetricmap (as addressedabove), and the seabed of the subaqueous delta is smooth, withgradients typically b1‰ (See Fig. 1b). So, errors associated withsediment volume estimation using Kriging interpolation is assumed tobe very low (b1%, as we estimated).

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

Except for several extremely wet or dry years, particularly in 1954,Yangtze water discharge has remained relatively constant since the1950s (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 ofthe TGD (Fig. 2). The decline in sediment discharge in the upperreaches of river, as measured at Yichang, 37 km downstream from theTGD and 1800 km from the river mouth (Fig. 1A), has been evengreater, falling from ~530 Mt/yr in the 1950s and 1960s to ~60 Mt/yrafter 2003 (Fig. 3A).

Of particular interest has been the evolving sedimentary regime inthe middle reaches of the river, as measured at Hankou, ~700 kmdownstream fromYichang and 500 kmupstream fromDatong (Fig. 1A).Sediment reachingHankou should reflect the sediment passing Yichangplus sediment introduced from the Han River and ungauged tributaries(Yang et al., 2007a) as well as sediment exchange with Dongting Lake.Between the mid-1950s and mid-1980s, the total amount of sedimentreaching Hankou from these various sources should have totaled~530 Mt/yr, whereas only ~440 Mt/yr were measured at Hankou. This~90 Mt/yr sediment “loss” (Fig. 3) suggests deposition along themiddlereaches of the river. The slightly higher sediment load at Datong duringthe same period, 470 Mt/yr, may reflect the sediment import fromdownstream tributaries (Xu and Milliman, 2009). Between the mid-1980s and late 1990s, sediment deposition in the middle reaches of theriver declined to ~60 Mt/yr, in large part the response to upstreamdamming (Xu et al., 2007), and from 2000 to 2002, total sedimentimport between Yichang and Hankou and the amount reaching Hankouwere essentially equal, suggesting little or no net sediment gain or lossin the middle reaches of the river (Fig. 3A).

For the first six years (2003–2008) following the TGD closure, thesediment discharge past Hankou averaged 125 Mt/y, 50 Mt/yr morethan the sediment input from Yichang and downstream sources(Fig. 3A), suggesting sediment erosion in the middle reaches of the

Fig. 2. Annual water and sediment load for the Yangtze River as measur

river (Fig. 3B). Similarly, between 2003 and 2008 the annual sedimentdischarge past Datong was 154 Mt/y, 29 Mt/yr more than the onemeasured at Hankou, suggesting further erosion in the lower reachesof the river. Using the approach by Yang et al. (2007a), the tributariesbetween Hankou and Datong supplied 18 Mt/yr in 2003−2008. Thatis, the combined erosion rate between Yichang and Datong was61 Mt/yr, ~40% of the amount of sediment trapped behind the TGD(Yang et al., 2007a,b); the erosion along the 700 km reach fromYichang to Hankou (0.071 Mt/yr/km) was three times higher thanalong the 500 km reach from Hankou to Datong (0.024 Mt/yr/km). Itis necessary to indicate that here the erosion, based on the sedimentbudget, reflects only the responses to decline in the sediment supply.Sand extractions in the Yangtze (Chen et al., 2005) and other rivers inChina (e.g. the Pearl River, Lu et al., 2007) presumably also haveresulted in significant additional erosion.

Erosion along themiddle reaches of the river is also suggested by thecoarsening of the Yangtze channel sediment, the mean grain size of

ed at Datong, the farthest downstream gauging station, 1953–2008.

17S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

river-bottom samples taken at fixed locations between Yichang andHankou increasing from 210 μm in 2002 to 300 μm in 2008. The closerthe sampling sites to the TGD, the greater the difference in grain sizebetween 2008 and 2002. At Yichang, the grain size increased from0.36 mm in 2002 to 25 mm in 2008; at Chenglinji, 400 km downstreamfrom Yichang, grain size increased from 0.18 mm in 2002 to 0.19 mm in2008 (Xu et al., 2010b). Another measure of channel erosion along themiddle reaches of the river is seen in the change of the grain size of thesuspended sediment. Between 1987 and 2002, the mean grain size ofthe suspended sediment at Yichang and Hankou averaged ~8–10 μm,whereas after 2003 the suspended sediment particle size at Yichangdeclined to ~3 μm in response to the settling of coarser sedimentsbehind the TGD, but increased to 17–19 μm at Hankou (Fig. 4), weassume because of the aforementioned channel erosion.

Decline in sediment load along the Yangtze also affected sedimen-tation in the linked lakes. A great deal of water and sediment flowsfrom the Yangtze into Lake Dongting via several inlets, although thewater and some of the sediment return to the Yangtze via an outlet(Dai et al., 2005). The deposition rate within Lake Dongting decreasedfrom 146 Mt/yr between 1956 and 1980 to 84 Mt/yr between 1981and 2002 (Yang et al., 2007b). Since the closure of the TGD, thesediment transported from the Yangtze into Lake Dongting has beendrastically reduced, and the deposition rate within Lake Dongting hasdecreased to b10 Mt/yr, in some years (2006 and 2008) showing a neterosion of 2 and 1 Mt/yr, based on our calculated sediment budget.Although there is only one channel linking the Yangtze with LakePoyang, the decline in the Yangtze's sediment load might havesomewhat impacted sedimentation in this lake because water andsediment occasionally flow from the river into the lake, particularlyduring river floods and lake droughts. Similarly, the deposition rate ofLake Poyang decreased from 11 Mt/yr in 1956–1980 to 8 Mt/y 1981–2002 (Yang et al., 2007b). Based on our sediment budget, the lakeeroded at a rate of 7 Mt/yr in 2003–2008,whichmight be relevantwiththe Yangtze's declining sediment load.

4. Morphologic changes in Yangtze Delta

It is still not clear how islands at the seaward edge of the Yangtzeestuary have responded to change in river flow, in part because theislands' shorelines are reinforced by jetties and revetments. Acomparison of transects taken in 1982 and 1990 show a 2-kmprogradation of eastern Chongming Island. By contrast, between 2006and 2010, salt marsh progradation was only ca.100 m and themudflats vertically eroded as much as 30 cm (Fig. 5).

To delineate temporal trends in the Yangtze subaqueous deltabathymetry and morphology, we compared bathymetric data from1958, 1977, 2000, 2004 and 2007 within an 1825-km2 area in theriver's South Branch through which more than 98% of the Yangtzewater and sediment are discharged (Chen et al., 1985). Bathymetric

Fig. 4.Mean grain size of suspended sediment at Yichang and Hankou, 1987–2008. Notethe abrupt grain size change after the 2003 TGD closure.

extensions of Chongming, Hengsha and Jiuduansha define thewestern limits of the study area; 15 to 20-m depth contours definethe eastern limits. Most of the study area, which we term Area 1(1280 km2, Fig. 1B), was surveyed all five years; Area 2 (545 km2),directly off the North Channel of the Yangtze's South Branch, was notsurveyed in 2007. Calculating bathymetric changes based on the lessaccurate 1958 and 1977 navigationwas partly compensated for by thelonger time intervals between subsequent surveys (19 and 23 years,respectively).

Delineating bathymetric change was somewhat complicated in1997 by construction of the Twin Jetty–Groyne Complex (TJ-GC;103 km2 within Area 1) in the North Passage of the South Channel, tofacilitate navigation (Du and Yang, 2007). By 2004 the TJ-GC extendedinto the southwestern portion of the study area (Fig. 1B). Inconjunction with the TJ-GC, a 350-m-wide channel (8.5 km2 inarea) has been dredged regularly to 10 m below lowest tide level,most of the spoil being dumped beyond the 10-m depth contour.

Historically most of the Yangtze's annual sediment load wasdeposited on the subaqueous delta during flood season, much of iteroded during winter storms (DeMaster et al., 1985; Milliman et al.,1985) and transported southward to form an extensive nearshore mudwedge extending into Taiwan Strait (Liu et al., 2007). Isotopic data fromseveral cores suggest that sediment accumulation in the subaqueousdelta in the 1950s–1980s averaged in the range of 3−5 cm/yr(DeMaster et al., 1985; Zhang et al., 2008).

Between 1958 and 1977, when the Yangtze river (as measured atDatong) discharged an average of 470 Mt/yr of suspended sediment,bathymetric data indicate that the 5-m contour east of Hengsha Islandprograded as much as 5 km (Fig. 6). The 10-m isobath progradedoverall more than 12 km although retreating ~2 km in the north. The20-m contour, by contrast, showed little overall change (Fig. 6).During this 19-year period, 55% of the area shoaled by N5 cm/yr,whereas 8% deepened by more than 5 cm/yr, mostly off the NorthChannel and the North Passage of the South Channel (Fig. 7A).Average shoaling within the study area was 6.8 cm/yr.

Between 1977 and 2000, most of the Yangtze subaqueous deltacontinued to prograde, but, presumably because of declining sedimentdischarge (Fig. 2), shoaling was generally b5 cm/yr. Although some ofthe north-central delta and areas off Jiuduansha shoaled by asmuch as15 cm/yr, there was little net change to the south (Fig. 7A). Calculatednet shoaling for the entire study area was 3.2 cm/yr.

Between the 2000 and 2004 bathymetric surveys, Yangtzesediment discharge at Datong fell from 340 (2000) to 205 Mt/yr(2003) (Fig. 2). The b5-m contour east of Hengsha Island continued toprograde — locally as much as 2 km — but showed little change inposition east of Chongming or Jiuduansha islands. The 10-m contour,by contrast, retreated 0.5–1.5 km throughout much of the study area,particularly east of Hengsha (Fig. 6). Overall, between 2000 and 2004,70% of the study area experienced erosion; the average vertical erosionrate for the study area was −3.8 cm/yr. The difference in verticalerosion 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-mcontour off both Henhsha and Chongming islands continued toprograde, locally by asmuch as 1–2 km (Fig. 6), perhaps in response toerosion from the eastern ends of the islands (see above). The 10-misobath, in contrast, retreated on average ~1 km, and locally by nearly2 km. Erosion seems to have been concentrated between 5 and 8 mdepths, 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 spoilremoved from the TJ-GC channel. Calculated net erosion rate for Area1 was −4.5 cm/yr.

Interestingly, since 1958 bathymetric changes within the studyarea have shown a strikingly linear correlation with the Yangtze's

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 lowmarsh edge is 2.8 m. Locationof the transect is shown in Fig. 1B.

18 S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

sediment load as measured at Datong (Figs. 7B, 9). Between 1958 and1977, when the average annual suspended sediment discharge was470 Mt, the subaqueous delta shoaled by ~125 Mm3/yr. In contrast,between 2000 and 2004 an average discharge of 245 Mt/yr resulted in70 Mm3/yr of delta front erosion (Fig. 7B). These data and trendssuggest that the Yangtze delta will continue to erode as long as theriver annually discharges less than ~270 Mt/yr of sediment (SeeFig. 9). In 2007 and 2008, sediment discharge past Datong averagedonly 130–140 Mt/yr; extrapolating the linear trends shown in Fig. 9suggests that erosion in the subaqueous delta may have alreadysurpassed 100 Mm3/yr. Whether the erosion continues to be confinedprimarily to 5–8 m depths remains to be seen.

Fig. 6. Left: 5-, 10- and 20-m bathymetric contours on the Yangtze delta of the years 19

5. Conclusions

The sediment discharge of the Yangtze River, as measured atDatong, fell from 490 Mt/yr in the 1950s and 1960s to ~150 Mt/yrafter the closure of the TGD. In contrast, water discharge remainedrelatively stable, indicating that the decline in sediment load stemmedprimarily from trapping behind the watershed's 50,000 dams, inparticular the TGD. In response to this drastic decrease in sedimentsupply, the river channel downstream from the TGD has changed fromone of net accretion to one of net erosion. Since 2003, the middlestretches of the river have experienced on average ~50 Mt/yr oferosion, compared with a deposition rate of ~90 Mt/yr between the

58, 1977, 2000, 2004 and 2007. Right: changes in bathymetric profiles (A) and (B).

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.

19S.L. Yang et al. / Global and Planetary Change 75 (2011) 14–20

mid-1950s and mid-1980s. River channel erosion, which also isreflected by the coarsening of bottom sediments, has only compen-sated for about 20% of the river's decreased sediment discharge. As aresult, the estuary has experience sediment starvation, and acorresponding decrease in coastal salt marsh accretion and neterosion in subaqueous delta front. The erosion seems to have occurredprimarily between 5 and 8 m below the lowest tide. The conversionfrom delta front accretion to erosion seems to have occurred whenriver's sediment load fell below 270 Mt/yr. Because of the TGDoperation and the construction of new large dams and the South-to-North Water Diversion in the watershed, the sediment load of theYangtze Rivermost likely will continue to decline and thus the erosion

Fig. 8. Depth distributions in subaqueous delta area. Note the marked decrease in arearepresented by 5–8 m depths and increase in 8–11 m.

along the main river channel of the middle and lower reaches and inthe delta front will continue in the coming decades.

Acknowledgements

We are grateful to the Editor and reviewers who provided valuablecomments and suggestions. The study was supported by the Ministry

Fig. 9. Comparison of subaqueous delta accumulation/erosion (Area 1) between variousbathymetric surveys and mean sediment load past Datong during the periods 1958–77,1977–2000, 2000–04, and 2004–07 (r: correlative coefficient; P: level of significance).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),the Natural Science Foundation of China (40721004), and the StateKey Laboratory of Estuarine and Coastal Research of China (SKLEC-2008KYYW01, SKLEC-2009KYYW02). JDM was funded partly by theU.S. National Science Foundation.

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