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Jun 10, 2003 - West Palm Beach, Florida. January 2008. Yangtze River Sediment: In Response to Three Gorges. Reservoir (TGR) Water Impoundment in ...
Journal of Coastal Research

24

1A

30–39

West Palm Beach, Florida

January 2008

Yangtze River Sediment: In Response to Three Gorges Reservoir (TGR) Water Impoundment in June 2003 Zhong-Xin Chu and Shi-Kui Zhai College of Marine Geosciences Ocean University of China Qingdao 266003, China [email protected]

ABSTRACT CHU, Z.-X., and ZHAI, S.-K., 2008. Yangtze River sediment: in response to Three Gorges Reservoir (TGR) water impoundment in June 2003. Journal of Coastal Research, 24(1A), 30–39. West Palm Beach (Florida), ISSN 0749-0208. The well-known and enormously controversial Three Gorges Reservoir (TGR) on the Yangtze River, China, impounded about 9.936 ⫻ 109 m3 of water (between Qingxichang and Huanglingmiaodou) for the first time from May 25 to June 10, 2003, finally elevating the water level to about 135 m above mean sea level at the dam. Mainly on the basis of daily, monthly, and annual water and sediment data sets of the Yangtze River at mainstream hydrological stations above and below the TGR Dam in 2003, we preliminarily examined the effects made by the TGR water impoundment in June 2003 on the Yangtze River sediment. According to sediment transport balance method, the estimated amount of suspended sediment impounded in the TGR is (1) about 3.1816 ⫻ 106 t between Qingxichang and Huanglingmiaodou during June 1 to June 10; (2) about 1.247 ⫻ 108 t between Cuntan and Yichang in 2003, which in June accounted for 25.9% and from June to December accounted for 99.2%; and (3) about 1.359 ⫻ 108 t between Cuntan and Yichang in 2003, if considering eroded sediment from the channel between the TGR Dam and Yichang. The TGR sedimentation resulting from the TGR water impoundment leads to unnaturally clear water discharged from the TGR Dam, which results in the Yangtze River suspended sediment concentration and sediment load downstream of the TGR Dam in 2003 being reduced more than those above the TGR Dam, and the channel just below the TGR Dam significantly eroded (about 4.759 ⫻ 105 t between Yichang and Shashi from June 1 to June 10). ADDITIONAL INDEX WORDS: Yangtze River (Changjiang), Three Gorges Reservoir (TGR), water impoundment, sediment, human activities, China.

INTRODUCTION Human activities have been strongly affecting the world’s river sediment supply to the oceans since the 19th or 20th century (FANOS, 1995; SMITH and WINKLEY, 1996; STANLEY and WARNE, 1993; STANLEY, 1996; YANG et al., 2002, 2004). The sediment trapped by dams along river systems has been a topic of global concern. About 13% of all fluvial discharge is presently dammed (PEARCE, 1991), and moreover, rivers are being dammed at an increasing rate (MILLIMAN and SYVITSKI, 1992). The decreased riverine sediment discharges due to human interference, especially dam constructions, are the main cause for observed deltaic degradation processes worldwide (STANLEY and WARNE, 1993). The Nile sediment transported to the Mediterranean essentially ceased because of the closure of the Aswan High Dam in 1964 (STANLEY, 1988), along with barrages below Aswan and river control structures on the Nile River delta plain (STANLEY, 1996), resulting in erosion of extensive sectors of the Nile delta, which was once the largest depot center in the Mediterranean (BLODGET, TAYLOR, and ROARK, 1991; FANOS, 1995; FRIHY, 1996; FRIHY and DEWIDAR, 2003; FRIHY and KOMAR, 1993; STANLEY, 1996; STANLEY and WARNE, 1994, 1998). Dam and reservoir constructions during the 1950s and 1960s on such DOI:10.2112/05-0547.1 received 11 July 2005; accepted in revision 11 November 2005.

major tributaries as the Missouri and Arkansas rivers, and revetments on the Mississippi River mainstem channel (KESEL, 2003) have reduced suspended sediment loads reaching the Gulf of Mexico by estimates ranging from 50% (KEOWN, DARDEAU, and CAUSEY, 1986; MEADE and PARKER, 1985) to over 70% (KESEL, 1988, 1989), thereby resulting in loss of wetlands and delta (DAY et al., 2000; GAGLIANO, MEYERARENDT, and WICKER, 1981). In Spain, after the construction of the Ribarroja-Mequinenza dam complex in the early 1960s, about 96% of the Ebro river sediment has been trapped in the reservoir (SANCHEZ-ARCILLA, JIMENEZ, and VALDEMO´ N and RO, 1998), leading to erosion of the river delta (GUILLE PALANQUES, 1997). Today, with more than 90% of the Colorado River’s incoming sediment trapped behind the Glen Canyon Dam in Lake Powell, the water flowing through the Grand Canyon is unnaturally clear, unexpectedly affecting the native fish populations there (P OWELL, 2002). There were 85,153 reservoirs at the end of 2003 in mainland China, with total water storage capacity amounting to 8.658 ⫻ 1011 m3 (MINISTRY OF WATER RESOURCES OF CHINA, 2003b), and the number of dams higher than 15 m in China at the end of 1972 was 12,517 (HAN and YANG, 2003), accounting for about half of those in the world (ZHOU, 2000). The sedimentation in reservoirs has been a serious problem, and the total water storage capacity of reservoirs in China has decreased by 40% because of sediment impoundment

Yangtze River Sediment: In Response to TGR Water Impoundment

(WANG and LIN, 2003). Construction of dams is the principal reason for the Yangtze River decreased sediment into the sea after the 1960s (YANG et al., 2004). The Luanhe River delta has an erosion rate of 300 m/a at the mouth, due to the sediment decrease by 95% by water conservancy projects in the basin (QIAN, 1994). Significant erosion has also occurred on extensive sectors of the modern Yellow River delta mainly due to numerous dam constructions and water diversions on the mainstream and its tributaries (LI, ZHUANG, and WEI, 2000; ZHANG et al., 1996). The Yangtze River sediment entering the sea is closely related to the evolution of the Yangtze River mouth, mouth bar, river delta, and even deposition patterns of the East China Sea (CHEN and XU, 1995; CHEN and ZONG, 1998; CHEN et al., 1985; CHEN et al., 2000; CHEN et al., 2001b; HORI, SAITO, and ZHAO, 2001b; HORI et al., 2001a; LI, LI, and SHEN, 2003; MILLIMAN et al., 1985; QIN, 1963; SAITO, YANG, and HORI, 2001; SHEN, ZHANG, and MAO, 2000; SHI, 2004; SUN, FANG, and HUANG, 2000; WANG, REN, and ZHU, 1986; YANG, ZHAO, and BELKIN, 2002; YANG S.L. et al., 2003, 2004; YANG S.Y. et al., 2002; YANG Z.S. et al., 1992). On the other hand, the Yangtze River sediment features are closely related to the particulate geochemical compositions (LIN et al., 2002; MARTIN and MEYBECK, 1979; YANG et al., 2002; ZHANG, 1999), which have important effects on the estuarine, coastal, and even shelf area ecosystems and environments (JIANG and YAN, 2003; ZHANG, 1999). The Three Gorges Reservoir (TGR) is enormously controversial. The project will surely bring about enormous benefits such as flood control, hydropower generation, and navigation; however, there are also many people (e.g., JIN, 1993; LI, 1992, 1993; LI et al., 1993) arguing that this project should be given up or put off. The TGR will submerge two of the three worldfamous gorges, along with irreplaceable cultural and archaeological sites. NOF (2001) feared that the TGR water impoundment, along with other water diversions in China, could significantly change the salt content of the Japan Sea, thereby affecting the climate of that region. Some unexpected aftermaths may have resulted from the TGR water impoundment. For example, according to the Three Gorges Project (TGP) feasibility study, it is believed that an earthquake induced by the TGR is not inevitable (CHEN, 1999); however, the most recent study (LI et al., 2003) showed that the increased microquake swarm activity is related to the TGR water impoundment in June 2003. NING and ZHONG (2004) showed that the TGR water impoundment in June 2003 did have some effects on the water regime of the middle and lower Yangtze River. Sediment research is one of the most important concerns about the TGR (LU, 2003). The TGR sedimentation will affect the normal operation of the TGR, hydropower generation, and navigation. The geological and sediment study in the TGR region can be traced back to the 1920s and 1950s respectively (CHEN, 1999; HU, 2000), and is still continuing. Many scientists and departments already examined the sediment problem of the TGR via prototype observation, numerical modeling, or solid model experimentation (ZHANG, ZHAO, and MAO, 2003) before the TGR water impoundment. The TGR water impoundment in June 2003 provides an

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Figure 1. Location of the Three Gorges Reservoir (TGR) in the Yangtze River drainage basin, major tributaries, and interior lakes (after LU and HIGGITT, 2001; YANG et al., 2002).

excellent opportunity to examine its effects on the Yangtze River sediment. This paper aims to examine daily, monthly, and annual response of the Yangtze River sediment features, including suspended sediment concentration (SSC) and sediment load, above and below the TGR Dam in 2003 to the TGR water impoundment. Another objective is to estimate the amount of sediment impounded in the TGR.

RESEARCH BACKGROUND The Yangtze River is the third longest (WANG and ZHU, 1994), fifth largest in water discharge, and fourth largest in sediment load in the world (EISMA, 1998). It originates in the Qinghai–Tibet Plateau and extends about 6300 km eastward to the East China Sea (Figure 1). The huge Yangtze River drainage basin, with a catchment area of 1.94 ⫻ 106 km2, can be divided into the upper, middle, and lower reaches, primarily on the basis of geology and climate, and secondarily on the basis of the resulting geomorphology of the river (CHEN, YU, and GUPTA, 2001a). The upper Yangtze River is more than 4500 km long from the headwater to Yichang, with a total drainage area of about 1.00 ⫻ 106 km2. There are four major tributaries (Yalong, Min, Jialing, Wu, as shown in Figure 1) joining the mainstream. The middle Yangtze River from Yichang to Hukou is 950 km in length and has a total drainage area of about 6.8 ⫻ 105 km2. Three large inputs join the mainstream in this section: Lake Dongting drainage basin, Han River, and Lake Poyang drainage basin (Figure 1). Lake Dongting and Lake Poyang in middle reaches of the Yangtze River are the two largest freshwater lakes in China. The Yangtze River partially flows into Lake Dongting through Songzi, Hudu, Anxiang, and Ouchi at Xinjiangkou, Shadaoguan, Mituosi, Ouchikang, and Ouchiguan (Tiaoxian was stopped up artificially in 1959), respectively. The water and sediment from the Yangtze River and four other major tributaries (Xiang, Zi, Yuan, Li, as shown in Figure 1) are transported back to the Yangtze River through the outlet at Chenglingji after regulation of Lake Dongting. Previous studies (JIANG and HUANG, 2004; SHI, XIA, and YANG, 1999) showed that the area and volume of Lake Dongting are decreasing because of

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sediment deposition, reclamation, and other natural and human reasons. The water and sediment from five major tributaries (Xiu, Gan, Fu, Xin, Rao, as shown in Figure 1) after regulation of Lake Poyang are transported into the Yangtze River through the only outlet at Hukou. The lower Yangtze River between Hukou and Datong is about 900 km in length and has a drainage area of 1.2 ⫻ 105 km2. About 50% of the Yangtze River water and nearly all its sediment originate from the uplands upstream from the TGR Dam, which has a catchment area of 1.00 ⫻ 106 km2 (55.6% of the whole drainage basin) (CHEN, YU, and GUPTA, 2001a). The 1800-km-long river stretch between the TGR Dam and Datong acts as a regulator for the river sediment (YANG, ZHAO, and BELKIN, 2002). The TGR on the Yangtze River, near Yichang, which is designed for flood control, navigation, and hydropower generation, formally commenced in December 1994 (CHEN, 1999), and the whole project is to be completed in 2009 (CAI, 2002). When the reservoir reaches the full height of 175 m at the dam, it will extend 600 km upstream, with a reservoir surface area of 1060 km2, stretching from Yichang to Chongqing (LU and HIGGITT, 2001), and has a world-record hydropower capacity of 17,860 MW (GAO and CHEN, 1994). Cuntan, near the upper part of the TGR region (LU and HIGGITT, 2001), is the controlling station of water and sediment into the TGR (GUO et al., 2004; LIU, CHEN, and WANG, 2004), which can be viewed as an inlet of the TGR. On the other hand, Yichang, about 42 km below the TGR Dam, is the controlling station out of the TGR, which can be treated as the outlet of the TGR (LI et al., 2004; LU and HIGGITT, 2001). Because Datong, over 1800 km downstream of the TGR Dam, is located at the tidal limit of the river mouth in the dry season and because it is located downstream of 94% of the drainage area, it is the controlling station for the measurements of the Yangtze River water and sediment discharges into the sea (YANG, ZHAO, and BELKIN, 2002). The TGR impounded water for the first time from June 1 to June 10 of 2003, finally elevating the water level to about 135 m above mean sea level at the dam. Because of natural high altitude, the water level at the dam was 80.13 m at 0000 on May 25 (ZHANG, ZHAO, and MAO, 2003). Some preparations for the TGR water impoundment were made from May 25 to May 31, mainly including gradually adjusting diversion bottom outlets of the Gezhou Dam, 40 km downstream of the TGR Dam, to reduce downstream water discharge (ZHANG, ZHAO, and MAO, 2003). From June 1 to June 10, the TGR Dam decreased downstream water discharge by alternating two or three diversion bottom outlets, which are the only passages for upstream water passing the TGR Dam before deep outlets opening (ZHANG, ZHAO, and MAO, 2003). During this period, downstream water discharge from the TGR Dam is often controlled at 2500–4900 m3/s for downstream navigation (ZHANG, ZHAO, and MAO, 2003). When the water level at the TGR Dam reached 135 m, the TGR backwater traced above Fuling, about 500 km upstream of the TGR Dam (LI, 2003). After the water level reached 135.79 m at the dam at 2200 on June 10, five days earlier than predicted, 13 deep outlets were gradually opened beginning at 0900 on June 10 to increase downstream water discharge, and the water dis-

charges at the inlet and outlet of the TGR were gradually balanced (ZHAO, 2004). From then on, the TGR is in normal dispatching stage, with the water level at the dam ranging from 134.9–135.4 m until the end of flood season (Z HAO, 2004). When the water level at Yichang is lower than 38 m in the dry season, it will affect navigation through the Gezhou Dam (ZHAO and ZHANG, 2003). To compensate discharge for navigation downstream of the Gezhou Dam in the dry season, the TGR impounded water for the second time from October 25 to November 5 of 2003, finally elevating the water level to 139 m at the dam and impounding 1.48 ⫻ 109 m3 of water (NING and ZHONG, 2004). During this period, water discharge from the TGR Dam was controlled at 6800 m3/s at the least for downstream navigation (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). The water level dropped to 135 m again before May 2004 (NING and ZHONG, 2004).

DATA COLLECTION A daily record data set of water level, water discharge, and SSC of the Yangtze River above and below the TGR Dam (Qingxichang, Wanxian, Huanglingmiaodou, Yichang, Zhicheng, and Shashi), generally measured one time each day during May 25 to June 10, 2003, were collected from public data published by the Bureau of Hydrology, Changjiang Water Resources Commission. In addition, monthly runoff and sediment load of the Yangtze River in 2003 above and below the TGR Dam (Cuntan, Yichang, Shashi, Hankou, Datong, as shown in Figure 1) were derived from maps published by the MINISTRY OF WATER RESOURCES OF CHINA (2003a). Besides, annual runoff and sediment load of the Yangtze River at the five mainstream hydrological stations as mentioned above in 2001, 2002, and 2003, as well as the multiyearly averages [Cuntan, Yichang, Shashi, and Datong (1950–2000); Hankou (1954–2000)] were collected from data published by the MINISTRY OF WATER RESOURCES OF CHINA (2002, 2003a). Finally, monthly runoff and sediment load of main tributaries of the Yangtze River and those at outlets of Lake Dongting and Lake Poyang in 2003 were also collected from maps published by the MINISTRY OF WATER RESOURCES OF CHINA (2003a).

RESULTS Daily Response Daily Water Level As for the Yangtze River water level upstream of the TGR Dam, the farther from the TGR Dam, the fewer effects made by the TGR backwater (Figure 2A). Water level at Qingxichang, about 472 km upstream of the TGR Dam, was at 142.0 m on May 25, and basically remained the same until 142.04 m on June 6, finally reaching 147.11 m on June 12, which suggests that the water level at Qingxichang was hardly affected by the TGR backwater until June 6. The water level at Wanxian, 288 km upstream of the TGR Dam, rose more quickly than that at Qingxichang, from 107.93 m on May 25 to 110.3 m on June 1, 134.45 m on June 10, and 137.15 m on June 11. Figure 2A also shows that the water level at Wanx-

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levels at Zhicheng and Shashi, absent from May 25 to May 31, increased by 5.28 m and 4.81 m, respectively, from the lowest levels on June 10 to the highest levels on June 12 during June 1 to June 12 (Figure 2A).

Daily Water Discharge Downstream water discharges at Yichang, Zhicheng, and Shashi below the TGR Dam were dramatically smaller than those at Qingxichang and Wanxian above the TGR Dam from May 28 to June 10, and they are almost equivalent to each other above and below the TGR Dam on other days (as shown in Figure 2B). From June 3 to June 11, the water discharge at Qingxichang was significantly larger than that at Wanxian (Figure 2B), which means the TGR backwater affected the former less than the latter during this period. This can be easily understood, because Qingxichang is farther than Wanxian upstream from the TGR Dam. The water discharges at Yichang, Zhicheng, and Shashi downstream of the TGR Dam rose quickly after deep outlets gradually opened on June 10, and they were almost equivalent to those at upstream stations, Qingxichang and Wanxian, on June 12 (Figure 2B).

Daily SSC

Figure 2. Time series of daily water level (A), water discharge (B), suspended sediment concentration (SSC) (C), runoff (D), and sediment load (E) of the Yangtze River from May 25 to June 12, 2003, measured commonly one time each day (generally at 0900 ⫾ 1 h) at hydrological stations above and below the TGR Dam.

ian was significantly affected by the TGR backwater after June 3, and its curve line is perfectly parallel to that at Miaohe after June 3. The water level at Miaohe, only 11.5 km upstream of the TGR Dam, rose very quickly, from 84.74 m on May 26 to 106.64 m on June 1, 121.41 m on June 6, and 135.85 m on 11 June. The water level at Huanglingmiaodou, 13 km downstream of the TGR Dam and 27 km upstream of the Gezhou Dam, dropped 1.2 m from 66.57 m on May 25 to the lowest point, 65.37 m on June 6. The Gezhou Reservoir may have buffered such little water level drop. The water level at Yichang, about 42 km and 2.4 km downstream of the TGR dam and the Gezhou Dam, respectively, decreased by 4.74 m from 43.54 m on May 25 to the lowest level of 38.80 m on June 6. The water

SSC at Qingxichang first increased slowly from 0.075 kg/ m3 on May 28 to 0.147 kg/m3 on June 5, then increased quickly to 0.608 kg/m3 on June 8, then decreased to 0.306 kg/m3 on June 10, and finally increased to 0.808 kg/m 3 on June 12. SSC at Wanxian first increased to 0.199 kg/m 3 on May 30, then decreased to 0.042 kg/m3 on June 8, and then increased to 0.143 kg/m3 on June 10. The similar SSC trend (increase– decrease–increase) at Qingxichang and Wanxian (Figure 2C) may suggest that upstream SSC from the TGR Dam first increased because of increased suspended sediment in the TGR, then decreased because of suspended sediment rapid settling, and then increased because of suspended sediment slowed settling. SSCs at Huanglingmiaodou, Yichang, Zhicheng, and Shashi downstream from the TGR Dam decreased before June 11, and then increased because of increased water discharge (Figure 2C). Figure 2C also shows that the SSC at Shashi is significantly larger than those at Huanglingmiaodou, Yichang, and Zhicheng, which means the riverbed between Zhicheng and Shashi was badly eroded.

Daily Runoff Daily runoffs, product of water discharge at Huanglingmiaodou and Yichang downstream of the TGR Dam, were dramatically smaller than those at Qingxichang and Wanxian upstream of the TGR Dam from May 28 to June 10, and they are almost equivalent to each other above and below the TGR Dam on other days (as shown in Figure 2D). From May 25 to June 12, about 2.1656 ⫻ 1010 m3 water (at Qingxichang) was input in the TGR, and only 1.172 ⫻ 1010 m3 (at Huanglingmiaodou) came out of the TGR, so about 9.936 ⫻ 109 m3 of water was impounded in the TGR between Qingxichang and Huanglingmiaodou. Thus, about 24.7%, or

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2.456 ⫻ 109 m3, was distributed between Qingxichang and Wanxian. The result of 9.936 ⫻ 109 m3 is very similar to the estimate of 1.00 ⫻ 1010 m3 by the MINISTRY OF WATER RESOURCES OF CHINA (2003a). According to the same method, about 8.037 ⫻ 109 m3 of water was impounded in the TGR during June 1 to June 10. At that time, about 20.3%, or 2.013 ⫻ 109 m3, was distributed between Qingxichang and Wanxian.

Daily Sediment Load Daily sediment loads, product of SSC, and water discharge at Huanglingmiaodou and Yichang downstream of the TGR Dam were dramatically smaller than those at Qingxichang and Wanxian upstream of the TGR Dam from May 28 to June 10, and they were almost equivalent to each other above and below the TGR Dam on other days (as shown in Figure 4E). During May 25 to June 10, about 3.9277 ⫻ 106 t of suspended sediment (at Qingxichang) was input in the TGR, and only 7.312 ⫻ 105 t (at Huanglingmiaodou) came out of the TGR, so about 3.1965 ⫻ 106 t was impounded in the TGR. Thus, about 52%, or 1.6626 ⫻ 106 t, was distributed between Qingxichang and Wanxian. During this period, suspended sediment load at Yichang is 7.303 ⫻ 105 t, almost equal to that at Huanglingmiaodou. According to such a method, about 3.1816 ⫻ 106 t was impounded in the TGR from June 1 to June 10. Thus, about 74.6%, or 2.3739 ⫻ 106 t, was distributed between Qingxichang and Wanxian. During this period, suspended sediment load at Yichang is 1.063 ⫻ 105 t, decreased by 2.6 ⫻ 104 t from that at Huanglingmiaodou, which maybe means that some suspended sediment was impounded in the Gezhou Reservoir (Huanglingmiaodou is above the Gezhou Dam and Yichang is below the Gezhou Dam). The SSC and water discharge data at Zhicheng and Shashi were only from June 1 to June 10. They were generally measured one time per one or two days, and moreover, SSC at Shashi was measured only one time at intervals of three days. Therefore, it is difficult to quantify the exact daily sediment loads. Coupled with our knowledge of water level and water discharge above Zhicheng as mentioned above, sediment loads at Zhicheng and Shashi were also acquired by linear interpolation. The roughly calculated sediment loads at Zhicheng and Shashi from June 1 to June 10 are 2.955 and 5.822 ⫻ 105 t, respectively. When comparing with that (1.063 ⫻ 105 t) at Yichang during this period, we believe with a grain of salt that about 1.892 ⫻ 105 t and 2.867 ⫻ 105 t of sediment were eroded from the riverbed between Yichang and Zhicheng and Zhicheng and Shashi, respectively. Mainly because of some water and sediment of the Yangtze River delivered into Lake Dongting through Songzi and Hudu, respectively, between Zhicheng and Shashi, the eroded sediment from the riverbed between Zhicheng and Shashi should be somewhat larger than 2.867 ⫻ 105 t during this period.

Monthly Response Monthly Runoff The monthly Yangtze River runoffs at all five main hydrological stations above and below the TGR Dam in flood season

Figure 3. Monthly runoff (A), SSC (B), and sediment load (C) of the Yangtze River in 2003 at five mainstream stations (their locations are in Figure 1) above and below the TGR Dam. Note: SSC and sediment load below the TGR Dam are evidently smaller than those at Cuntan above the TGR Dam from June to September. Smaller monthly runoffs at Cuntan from October to December lead to smaller monthly sediment loads, although monthly SSCs are still high.

(from May to October) are significantly larger than those in nonflood season (from January to April, November and December) in 2003 (Figure 3A), and so are monthly SSCs (Figure 3B) and monthly sediment loads (Figure 3C). Monthly runoffs at Datong and Hankou are significantly larger than those above Hankou, mainly because of water supplemented to the Yangtze River mainstream by the Han River, Lake Poyang, and Lake Poyang (their locations in Figure 1). The monthly runoff at Yichang, outlet of the TGR, should be somewhat larger than that at Cuntan, inlet of the TGR, in June 2003 mainly because of supplement to the TGR by the Wu River, but they are almost equal (Figure 3A). This is probably due to 8.037 ⫻ 109m3 of water impounded in the TGR from June 1 to June 10, 2003 as mentioned above.

Monthly SSC Downstream monthly SSCs at Yichang, Shashi, Hankou, and Datong are dramatically smaller than that at Cuntan above the TGR Dam after the TGR water impoundment in June 2003, especially in flood season (Figure 3B). The monthly SSC at Shashi below Yichang is significantly larger than that at Yichang (Figure 3B), probably indicating that the channel between Yichang and Shashi was badly eroded.

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Monthly Sediment Load Downstream sediment loads at Yichang, Shashi, Hankou, and Datong are markedly smaller than those at Cuntan above the TGR Dam from June to September in 2003 (Figure 3C). Although monthly SSCs at Cuntan from October to December are still higher than those downstream of the TGR Dam (Figure 3B), relatively smaller monthly runoffs lead to smaller monthly sediment loads (Figures 3A and 3C). The sediment load at Shashi is significantly larger than that at Yichang from June to October, probably indicating that the channel between Yichang and Shashi was badly eroded.

Annual Response Annual Runoff The annual runoff at Cuntan above the TGR Dam in 2003 is 95.2% and 112.9% of the multiyearly average and that in 2002, respectively. Annual runoffs at Yichang, Shashi, Hankou, and Datong below the TGR Dam in 2003 are 93.1%, 94.1%, 103.8%, and 102.2% of their multiyearly averages, respectively, and 103.8%, 104.8%, 96.0%, and 93.2% of those in 2002, respectively. When comparing with those in 2002 and 2001, or multiyearly averages in the past five decades, in general, it is noted that the annual Yangtze River runoffs at all five mainstream stations above and below the TGR Dam in 2003 did not change largely (Figure 4A).

Annual SSC Annual SSC at Cuntan above the TGR Dam in 2003 is 51.2% and 93.6% of the multiyearly average and that in 2002, respectively. Annual SSCs at Yichang, Shashi, Hankou, and Datong below the TGR Dam in 2003 are 20.9%, 39.4%, 39.4%, and 45.9% of their multiyearly averages, respectively, and 41.2%, 54.7%, 72.0%, and 80.5% of those in 2002, respectively. When comparing with those in 2002, 2001, or the multiyearly averages, it can be seen that SSCs downstream of the TGR Dam reduced more than those upstream of the TGR Dam in 2003 (Figure 4B).

Annual Sediment Load Annual sediment load at Cuntan above the TGR Dam in 2003 is 48.7% and 105.6% of the multiyearly average and that in 2002, respectively. Annual sediment loads at Yichang, Shashi, Hankou, and Datong below the TGR in 2003 are 19.5%, 37.0%, 40.8%, and 47.6% of their multiyearly averages, respectively, and 42.8%, 57.3%, 69.0%, and 74.9% of those in 2002, respectively. In 2003, the annual Yangtze River sediment load at Cuntan above the TGR Dam increased by 5.6% over that in 2002, whereas those at Yichang, Shashi, Hankou, and Datong below the TGR Dam decreased by 57.2%, 42.7%, 31.0%, and 25.1%, respectively, from those in 2002 (Figure 4C).

TGR Sedimentation According to sediment transport balance method (GUILLE´N and PALANQUES, 1997), sediment impounded in the TGR was estimated as follows. Sediment load at Cuntan (inlet of the

Figure 4. Annual runoff (A), SSC (B), and sediment load (C) of the Yangtze River at five mainstream stations (their locations are in Figure 1) above and below the TGR Dam in 2003, 2002, and 2001, and their multiyearly averages. Note: SSC and sediment load downstream of the TGR Dam in 2003 reduced more than those at Cuntan above the TGR Dam when comparing with those in 2002, 2001, and the multiyearly averages.

TGR) plus that delivered into the TGR by the Wu River, and minus that at Yichang (outlet of the TGR) is sediment budget of suspended sediment impounded in the TGR, and the calculated results are shown in Table 1. Table 1 shows that about 1.247 ⫻108 t of suspended sediment was impounded in the TGR in 2003, and that in June accounts for 25.9%, and that from June to December accounts for 99.2%. Suspended sediment was impounded in the TGR mainly during June to September (Table 1). Our estimation of 1.2365 ⫻ 108 t of suspended sediment impounded in the TGR between Cuntan and Yichang from June to December is very similar to the estimate of 1.24 ⫻ 108 t between Qingxichang and Huan-

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Table 1. Sediment budget of suspended sediment (unit: ⫻10 4 t) impounded in the TGR (between Cuntan and Yichang).

Date

Sediment Budget

Percentage of Total in 2003

June July August September October November December June–December 2003

3224.5 2250.5 2917.7 2935.6 793.8 134.4 108.7 12,365.2 12,470

25.9% 18.0% 23.4% 23.5% 6.4% 1.1% 0.9% 99.2% 100.0%

glingmiao during this period by the MINISTRY OF WATER RESOURCES OF CHINA (2003a). From December 2002 to November 2003, the Yangtze River channel between Letianxi (just below the TGR Dam) and Yichang was eroded about 8.50 ⫻ 106 m3 (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). According to the dry bulk density of the Yangtze River sediment, 1.32 t/m3, used by YANG et al. (2002), the eroded sediment from this channel is 1.122 ⫻ 107 t. In addition, sediment supplemented to the TGR by the Wu River (at Wulong station) in 2003 was 1.63 ⫻ 107 t (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). The sediment load (2.06 ⫻ 108 t) at Cuntan (inlet of the TGR) plus that (1.63 ⫻ 107 t) supplemented to the TGR by the Wu River and plus that (1.122 ⫻ 107 t) eroded from the channel between Letianxi and Yichang (assumed as eroded from the channel between the TGR Dam and Yichang in 2003), and minus that (9.76 ⫻ 107 t) at Yichang (outlet of the TGR) is 1.359 ⫻ 108 t (2.06 ⫹ 0.163 ⫹ 0.1122 ⫺ 0.976 ⫽ 1.359). In addition, the sediment delivered into the TGR is also generated from the immediate vicinity of the TGR area beside the large upstream catchment area (HIGGITT and LU, 2001; LU and HIGGITT, 2001). So the amount of suspended sediment impounded in the TGR in 2003 should be slightly larger than 1.359 ⫻108 t.

DISCUSSION Three large inputs join the Yangtze River mainstream below the TGR Dam: Lake Dongting drainage basin, Han River, and Lake Poyang drainage basin (their locations in Figure 1). The Han River just above Hankou offered the Yangtze River 1.4 ⫻ 107 t of suspended sediment (measured at Huangzhuang) in 2003 (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). About 2.050 ⫻ 107 t of suspended sediment from the Yangtze River was delivered into Lake Dongting through four inlets of Songzi, Hudu, Anxiang, and Ouchi, respectively, and only 1.750 ⫻ 107 t were transported back to the Yangtze River mainstream through the outlet (at Chenlingji) after the regulation of Lake Dongting (MINISTRY OF WATER RESOURCES OF CHINA, 2003a), which means 3.00 ⫻ 106 t of suspended sediment from the Yangtze River was trapped in Lake Dongting in 2003. Lake Poyang offered the Yangtze River mainstream 1.760 ⫻ 107 t of suspended sediment (measured at Hukou) in 2003 (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). So the three inputs below the TGR Dam supplied the

Yangtze River about 2.86 ⫻ 107 t (0.14 ⫺ 0.03 ⫹ 0.176 ⫽ 0.286) of suspended sediment in 2003. Previous studies (CHEN et al., 2001b; YANG et al., 2002) showed that sediment load at Datong is much less than that at Yichang because of siltation in the 1800-km-long channel, but slightly higher than that at Hankou. Why has the sediment load at Datong in 2003 increased by 111%, or 1.804 ⫻ 108 t, over that at Yichang, although still slightly increased by 25%, or 4.1 ⫻ 107 t, over that at Hankou? This is probably due to channel erosion below the TGR Dam, especially between Yichang and Shashi, by unnaturally clear water from the TGR Dam resulting from TGR sedimentation after TGR water impoundment. In fact, the channel between Letianxian (just below TGP Dam) and Yichang was eroded 8.50 ⫻ 106 m3 from December 2002 to November 2003, and the channel between Yichang and Zhicheng (stretching about 61 km) from September 2002 to October 2003 was eroded about 2.917 ⫻ 107 m3 (MINISTRY OF WATER RESOURCES OF CHINA, 2003a). The annual Yangtze River SSCs and sediment loads at mainstream stations (Cuntan, Yichang, Shashi, Hankou, and Datong) above and below the TGR Dam in 2001, 2002, and 2003 were evidently smaller than the multiyearly averages in the past five decades, although annual runoffs did not change largely (as shown in Figure 4). In fact, the Yangtze River sediment entering the sea tends to decrease, mainly because of numerous dam constructions and water diversions for agriculture and industry on the Yangtze River mainstream and its tributaries since in the 1960s (YANG et al., 2004). But annual SSC and sediment load at stations below the TGR Dam reduced more than those above the TGR Dam in 2003 (Figure 4), which means downstream sediment from the TGR Dam is significantly affected by the TGR sedimentation resulting from the TGR water impoundment in June 2003.

CONCLUDING REMARKS From May 25 to June 10, about 9.936 ⫻ 109 m3 water was impounded in the TGR between Qingxichang and Huanglingmiaodou, and about 80.9%, or 8.037 ⫻ 109 m3, was impounded from June 1 to June 10. According to sediment transport balance method, the estimated amount of suspended sediment impounded in the TGR is as follows: (1) from May 25 to June 10, about 3.1965 ⫻ 106 t between Qingxichang and Huanglingmiaodou, accounting for about 52% between Qingxichang and Wanxian; (2) from June 1 to June 10, about 3.1816 ⫻ 106 t between Qingxichang and Huanglingmiaodou, accounting for about 74.6% between Qingxichang and Wanxian; (3) about 1.247 ⫻ 108 t between Cuntan and Yichang in 2003, in June accounting for 25.9%, and from June to December accounting for 99.2%; and (4) about 1.359 ⫻ 108 t between Cuntan and Yichang in 2003, if considering eroded sediment from the channel between the TGR Dam and Yichang. During the TGR water impoundment in June 2003, the farther the channel from the TGR Dam, the fewer effects made by the TGR water impoundment, except for the river channel between the TGR Dam and the Gezhou Dam, maybe due to buffering by the Gezhou Reservoir. Water level and SSC at

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Wanxian and Qingxichang upstream of the TGR Dam began to be significantly affected by the TGR backwater after June 2 and June 5, respectively (Figure 2). Although the Yangtze River sediment entering the sea tends to decrease because of numerous dam constructions and water diversions since in the 1960s (YANG et al., 2004), sediment load and SSC downstream of the TGR Dam reduced more than those upstream of the TGR Dam in 2003, when compared with those in 2002, 2001, or the multiyearly averages (Figure 4). This is probably due to TGR sedimentation resulting from TGR water impoundment. The fact that SSC and sediment load at Shashi are significantly larger than those at Yichang after the TGR water impoundment indicates that the channel between Yichang and Shashi was badly eroded, and the estimated amount of eroded sediment from the channel from June 1 to June 10 is about 4.759 ⫻ 105 t. It seems that the Yangtze River sediment at Yichang is first and foremost prone to be affected by the TGR water impoundment, and those at Shashi, Hankou, and Datong are ordinal. Besides monthly SSC and sediment load data above and below the TGR Dam in 2003 (Figure 2), it is difficult to see the exact effects made by the TGR water impoundment from October 25 to November 5, 2003, on the Yangtze River sediment. It must be pointed out that the sediment data in this study were suspended sediment data. JI and GUO (1998) showed that suspended sediment is ⬎99% of the total sediment delivered into the TGR. YANG et al. (2002) also showed bed load, usually absent, at Datong is 0.044% of the total sediment load. So, suspended sediment load is dominant when compared with the bed load. In view of its dominance, the suspended sediment was used to substitute for the total sediment in this study. The Yangtze River decreased sediment delivering into the estuary because of the TGR sedimentation resulting from the TGR water impoundment, which would increase sedimentcarrying capacity of the water in the estuary (CHEN and XU, 1995). Thus, the effects made by the TGR water impoundment on the estuarine coast, riverbed, mouth bar, turbidity maximum, sediment transport, and adjacent oceans need further investigation.

ACKNOWLEDGMENTS This study is supported by National Basic Research Program of China for Land–Sea interaction and its effect on the environment in the typical estuaries and offshore areas of China (Project No: 2002CB412400).

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