Qingyang Liu, Nov 2016

Downstream geomorphic changes of the Yangtze river:

after the construction of Three Gorges Dam

1. Introduction

Three Gorges Dam (TGD), the world’s largest power station in respect of its installed capacity,

has generated 500 TWh of hydroelectricity (gov.cn), delivering massive amount of clean power

to homes, in comparison to the dirty fossil fuel energy dominates in China’s energy structure.

Chinese government extolled this project as a historical success in terms of engineering,

economy, and social influence, broadcasting for its effectiveness in controlling floods and

drought hazards, decreasing GHG emissions, and increasing the Yangtze river’s shipping

capacity. However, the installation of TGD lead to the submergence of numerous residential

dwellings, agricultural lands, and archeological relics; the drastically changed topography of this

region indicates huge shifts in ecosystems (News.xinhuanet.com). The overall momentous

ecological transformation might ultimately be reciprocal to changing weather patterns and the

global climate change.

2. Related studies

Being such a controversial project, TGD has been discussed in multiple fields. Wu et al (2004)

look at both terrestrial and aquatic habitat fragmentation caused by TGD, revealing the

significant disturbance that biological communities may encounter. Muller et al (2008) examine

the pollutants downstream of TGD and identify the increasing pollutant concentration, which

threatens the health of the East China Sea ecosystem. Park et al (2003) study possibilities of 162

species of fish in the Yangtze river to establish new reserves in tributaries; they suggest that only

slightly higher than half of the endemic species may survive after the dam filling.

1

Qingyang Liu, Nov 2016

Besides those studies directly related to threatens of living creatures and human health, TGD’s

potential impacts on severe geomorphological changes have already drawn scholars’ attention

before its construction. Lu & Higgitt (1999) look at controlling variables of sediment output in

the upper Yangtze. Their study in 2001 (Lu & Higgitt, 2001) further examines the amount of

sediments delivering to TGD’s upstream catchment. During the post TGD period, many

researchers also look at TGD’s impacts on the mid-lower reach of the Yangtze river watershed.

Academic concerns about the delta erosion in coastal regions (Yang et al, 2011; Yang et al,

2006), low sediment diversion to downstream lakes (Chen et al, 2008), riverbed scouring effects

(Xu et al, 2006), etc., all invoke great efforts on research. Considering the fact that TGD is such

an influential project and two decades have passed since its outset, further research should be

conducted. In order to delineate the current knowledge of TGD’s morphologic influences, this

review will focus on two questions: What are the changes of the sediments downstream of the

Yangtze drainage basin after the construction of TGD; what are their effects?

3. History and Geographical Background

2

Qingyang Liu, Nov 2016

The Yangtze river, or Changjiang, which

means “Long river” in Chinese, is the longest

river (6300 km) in Asia and the third in the

world. It ranks the fifth in terms of discharge

capacity (900km3/yr) and fourth in sediment

flux (500 Mt/yr) during the pre-TGD period

[Figure 1]. The mid-lower Yangtze River basin is commonly considered between Yichang and

Datong gauging station [Figure 2]. This basin contributes over half of its water discharge from

the Yangtze into the sea (Chen et al, 2008). Yang et al (2007) find that historical sediment load

changes in this basin, before the TGD program, are marked by two phases: the first phase of

sediment reduction, from 1969-1985, is evident from the decreasing sediment load at Hankou

gauging station; the second phase, from 1986-2002, is distinguished by all four stations’

decreasing sediment load trend. They claim that during the first phase, the decreasing sediment

loads in both Luoshan and Hankou indicate declining sediment sources in Hanjiang, the longest

[Figure 1: Geographical setting of the Yangtze river and its watershed, graph drawn from Chen et al, 2008]

3

Qingyang Liu, Nov 2016

tributary of the Yangtze; this change is largely attributed to the operation of Danjiangkou

Reservoir lies on the upper reach of Hanjiang, trapping 99% of the sediments in the reservoir.

Similarly, for the second stage change, they discover that Shengzhong Dam reservoir completed

in 1986, Baozhushui reservoir in 1996 and other tens of thousands of reservoirs installed on

tributaries not only stored water but sediments inside. Therefore, the sediment load of the

Yangtze had already decreased severely before 2002, and in response, significant morphologic

[Figure 3: Annual water discharges and sediment loads at four key gauging stations: Yichang, Luoshan, Hankou & Datong downstream of TGD, 1950-2004. Figure drawn from Yang et al, 2006]

and environmental changes had emerged due to local and regional erosion.

4. Changes over the post-TGD period

Given the fact that TGD creates numerous new reservoirs, which have a total capacity of

39.3 km3, it is necessary to study its current sediment load change. According to Yang et al

(2011), sediment discharges measured at Datong dropped from 490 Mt/yr in the 1950s to 150

4

Qingyang Liu, Nov 2016

Mt/yr in 2007; even greater influences occurred at the upper channel: Yichang station, 37km

downstream of TGD, experienced a drop from 530 Mt/yr to 60 Mt/yr [Figure 3]. Those harsh

changes in sediments gave rise to morphological changes in multiple regions.

4.1 Middle and lower reach channels

The morphological changes in downstream channels can be traced from sediment budget

changes manifested in gauging stations’ records. Sediment load differences between Hankou and

Yichang stations account for not only the Yangtze’s sediment deposition and erosion within the

channel but sediment exchanges with tributaries and lakes. Similarly, those differences between

Hankou and Datong also reveal sediment budget changes before the Yangtze flow into the sea.

According to data from those gauging stations, prior to 2000, the mid-lower reach of the Yangtze

had a dominantly depositional landscape (Xu et al, 2006). But nowadays, channels are not

depositional but erosional: Yang et al. (2006) claim that the erosion rate between Yichang and

Datong is 18 Mt/yr in 2003-2008, the first six years after the completion of TGD; similarly, the

channel between Hankou and Datong has an erosion rate of 12 Mt/yr. This riverbed erosion

indicates the Yangtze channel’s responses to decreasing sediment supplies upstream from TGD

(Yang et al, 2011). In other words, trapped sediments behind TGD are partially compensated by

channel erosion downstream; this phenomenon would ultimately lead to exhaustion of the

channel’s sediment storage in different time scales: from 10 years in the reach around Lake

Dongting, to 60 years in the reach at Datong (Wu, 2004). This channel erosion can also be

suggested by the coarsening of sediments: Yang et al (2006) find the mean grain size of river-

bottom samples rose from 210 μm in 2002 to 300 μm in 2008.

4.2 Lakes

5

Qingyang Liu, Nov 2016

The declining sediment load along the Yangtze also impacts linked lakes. Lake Dongting’s

depositional rate of sediments decreased from 146 Mt/yr before 1980 to less than 10 Mt/yr after

TGD’s construction; similarly; Lake Poyang has a depositional rate of 11 Mt/yr before 1980, in

contrast with its 7 Mt/yr erosion rate after 2003 (Yang et al, 2011). But it is important to note the

historically geographical function of Lake Dongting and Lake Poyang as sediment storages: over

the period of 1956-2002, Lake Dongting stored 73% of its annual sediment input and Lake

Poyang stored 43% of that (Yang et al, 2007). Those previously high sediment concentrations

generating large-scale sandbars, deltas, and swamps, drastically shank these lakes’ surface area

and water-storage capacities (Du et al, 2001). Therefore, considering the effect of TGD to

decrease sediment deposition rates, TGD would benefit future stormwater regulation in these

lakes (Dai et al, 2005).

4.3 Deltas

According to a stratigraphical study (Shen, 2001), nearly 40% of the Yangtze sediments are

deposited at the subaqueous delta area. Morphological changes of islands, such as the

Chongming island, at the seaward edge of the Yangtze delta, are clear manifestos of the effect of

sediment load changes. Yang et al (2011) find the sediment accumulation of the Yangtze delta

decreases from 3-5cm/yr average deposition in 1950-80s to an average erosion of 3.8 cm/yr after

2003. They propose that, in accordance with the sediment balance, Chongming island continued

to prograde but in a negligible speed, while the coast contour retreated 1km in many areas from

2000 to 2004: this retreat enlarges the gap between the Chongming island and the seawall,

highlighting the net erosion of this delta. In order to keep sediment budgets of the Yangtze delta

in balance, 310 Mt/yr is the critical amount of sediment load at Datong (Yang et al, 2006).

However, from 2000 to 2004, an average discharge of 245 Mt/yr leads to a 70 Mm3/yr front

6

Qingyang Liu, Nov 2016

erosion of the Yangtze delta (Yang et al, 2011) [Figure 4].

4.4 The changing sediment output

Although only 30% of sediments are entrained from TGD in 2006, the ratio would increase to

81% by 2100 (Yang et al, 2006) [Figure 5]. Chen et al (2008) also propose that the estimation of

sediment recovery downstream should not only base on the linear regression of the Yangtze river

[Figure 4: Annual water and sediment load measured at Datong station, 1953-2008, drawn from Yang et al, 2011]

[Figure 5: Estimated sediment discharge at Datong station after TGD completions, drawn from Yang et al, 2006]

as a black box: major physical and engineering controls are influential. In this case, the amount

of riverbed erosion in mid-lower channels should decrease with time as new equilibriums

7

Qingyang Liu, Nov 2016

establish. Likewise, the present erosion of deltas and lakes might be mitigated as the

accumulation gradually balance out the erosion.

5. Conclusion

The current review draws from different studies about geomorphological changes downstream of

TGD, summarizing the decreasing sediment load’s influences on the channel itself, linked lakes,

and the Yangtze delta. Although the ongoing erosion in the Yangtze river is acknowledged in all

research, different numerical estimations of sediment loads and erosion rates are deduced from

those studies. Those differences might derive from the current changing landscape responding to

the rising anthropogenic intervention and the global climate change trend. Therefore, future

studies should look at long-term changes of the Yangtze delta’s morphology in terms of such

radical changes in the fluvial system.

8

Qingyang Liu, Nov 2016

References:

Chen, X., Yan, Y., Fu, R., Dou, X., & Zhang, E. (2008). Sediment transport from the Yangtze

River, China, into the sea over the Post-Three Gorge Dam Period: A discussion. Quaternary

International, 186(1), 55-64.

Dai, S. B., Yang, S. L., Zhu, J., Gao, A., & Li, P. (2005). The role of Lake Dongting in

regulating the sediment budget of the Yangtze River. Hydrology and Earth System Sciences

Discussions, 9(6), 692-698.

Du, Y., Cai, S., Zhang, X., & Zhao, Y. (2001). Interpretation of the environmental change of

Dongting Lake, middle reach of Yangtze River, China, by 210 Pb measurement and satellite

image analysis. Geomorphology, 41(2), 171-181.

Lu, X., & Higgitt, D. L. (1999). Sediment yield variability in the Upper Yangtze, China. Earth

Surface Processes and Landforms, 24(12), 1077-1093.

Lu, X., & Higgitt, D. L. (2001). Sediment delivery to the Three Gorges: 2: Local response.

Geomorphology, 41(2), 157-169.

Mueller, B., Berg, M., Yao, Z. P., Zhang, X. F., Wang, D., & Pfluger, A. (2008). How polluted is

the Yangtze river? Water quality downstream from the Three Gorges Dam. Science of the

total environment, 402(2), 232-247.

9

Qingyang Liu, Nov 2016

Park, Y. S., Chang, J., Lek, S., Cao, W., & Brosse, S. (2003). Conservation strategies for

endemic fish species threatened by the Three Gorges Dam. Conservation biology, 17(6),

1748-1758.

Shen, H. (2001), Materials Flux of the Changjiang Estuary (in Chinese), pp. 21–24, China Ocean

Press, Beijing.

Wu, J., Huang, J., Han, X., Gao, X., He, F., Jiang, M., ... & Shen, Z. (2004). The three gorges

dam: an ecological perspective. Frontiers in Ecology and the Environment, 2(5), 241-248.

Xu, K., Milliman, J. D., Yang, Z., & Wang, H. (2006). Yangtze sediment decline partly from

Three Gorges Dam. Eos, Transactions American Geophysical Union, 87(19), 185-190.

Yang, S. L., Milliman, J. D., Li, P., & Xu, K. (2011). 50,000 dams later: erosion of the Yangtze

River and its delta. Global and Planetary Change, 75(1), 14-20.

Yang, S. L., Zhang, J., Dai, S. B., Li, M., & Xu, X. J. (2007). Effect of deposition and erosion

within the main river channel and large lakes on sediment delivery to the estuary of the

Yangtze River. Journal of Geophysical Research: Earth Surface, 112(F2).

Yang, Z. S., Wang, H. J., Saito, Y., Milliman, J. D., Xu, K., Qiao, S., & Shi, G. (2006). Dam

impacts on the Changjiang (Yangtze) River sediment discharge to the sea: The past 55 years

and after the Three Gorges Dam. Water Resources Research, 42(4).

探访三峡库区云阳故陵滑坡险情. Xinhua News Agency, 29 Apr. 2009. Web. 01 Nov. 2016.

<http://news.xinhuanet.com/newscenter/2009-04/09/content_11157017.htm>.

三峡电站持续安稳运行累计发电突破 5000亿千瓦时. Xinhua News Agency, 16 Aug. 2011.

Web. 01 Nov. 2016. <http://www.gov.cn/jrzg/2011-08/16/content_1926651.htm>.

10