Quaternary International

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1 Quaternary International 23 (211) Contents lists available at ScienceDirect Quaternary International journal homepage: Temporal and spatial variations of sediment rating curves in the Changjiang (Yangtze River) basin and their implications Bangqi Hu a,b, Houjie Wang a,b, *, Zuosheng Yang a,b, Xiaoxia Sun a,b a College of Marine Geosciences, Ocean University of China, 238 Songling Rd., Qingdao 2661, China b Key Lab of Submarine Sciences & Prospecting Techniques, MOE, Ocean University of China, 238 Songling Road, Qingdao 2661, China article info abstract Article history: Available online 1 September 29 The natural hydrologic regime of the Changjiang (Yangtze River) in China has been disturbed considerably by intensified human activities over the past five decades. In this study, sediment rating curves were analyzed based on monthly data of water discharge and suspended-sediment concentration at stations in the upper (Yichang station), middle (Hankou station) and lower (Datong station) reaches of the Changjiang in different periods from 1955 to 27. Temporal and spatial variations of the sediment rating curves were analyzed with respect to the impact of human activities and watershed characteristics. Results indicate that human disturbances have had a substantial impact on sediment rating parameters, with the magnitude of the impact related to the scale of the river sections. As indicated by the sediment rating parameters, the sediment transport regimes between the upper and the mid-lower reaches differ significantly. In particular, the impoundment of the Three Gorges Dam in 23 changed the sediment transport regimes in the upper and mid-lower reaches of Changjiang into similar regimes characterized by a decrease in the sediment transport capacity of high water discharge. Furthermore, the sediment rating curve at the Datong station in was applied to estimate the potential sediment load ( ) in the absence of human influences. The mean annual sediment deficit caused by human activities has increased from 8 Mt/yr ( ) to 25 Mt/yr (23 27). This indicates that intensified human activities in the Changjiang basin, especially the construction of the Three Gorges Dam, have altered the natural sediment transport process, and have thus become a dominant force in Changjiang sediment delivery to the sea. Ó 29 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction In a natural river system, the river channel acts as a conveyor belt that continuously delivers erosional materials downstream to ultimate depositional sites below sea level (Kondolf, 1997). However, humans have altered global natural river systems by dam construction and water diversion in order to meet the increasing demands of a rapidly growing global population (Nilsson et al., 25; Syvitski et al., 25). Dams disrupt the continuity of the sediment delivered in the river systems and cause substantial changes to the flow and sediment regimes. In the global scale, there are many examples of changes in sediment transport regimes due to the construction of dams, including the Nile River in Egypt (Stanley, 1996), the Colorado (Carriquiry et al., 21) and Mississippi (Coleman et al., 1998) Rivers in the United States, the Indus River in India (Giosan et al., 26), the Ebro River in Spain (Batalla * Corresponding author. Tel.: þ ; fax: þ address: hjwang@ouc.edu.cn (H. Wang). et al., 24), and the Red River in Vietnam (Le et al., 27), as well as the Huanghe (Yellow River) (Yang et al., 1998, 28; Wang et al., 26, 27), Changjiang (Yangtze River) (Yang et al., 22, 26; Chen et al., 25) and the Zhujiang (Pearl River) (Dai et al., 28a; Zhang et al., 28) in China. The Changjiang (Fig. 1) is one of the largest rivers in the world both in terms of water discharge (92 km 3 /yr) and sediment load (48 Mt/yr) (Milliman and Meade, 1983; Milliman and Syvitski, 1992). Draining a wide basin of km 2, the Changjiang basin accounts for nearly 2% of the total area of China. More than 4 million inhabitants live in the Changjiang basin, and >5, reservoirs were constructed within this watershed. As a consequence, the Changjiang has become one of the most highly impacted rivers in the world (Nilsson et al., 25). Yang et al. (26) used the annual water and sediment discharge data from the upper, middle, and lower reaches of Changjiang to analyze the impact of dams on the Changjiang sediment during the past five decades. The results of Yang et al. (26) indicated that the sediment reduction of the Changjiang has displayed two phases during the period of /$ see front matter Ó 29 Elsevier Ltd and INQUA. All rights reserved. doi:1.116/j.quaint

2 B. Hu et al. / Quaternary International 23 (211) Fig. 1. Map of the Changjiang drainage basin, major tributaries, key gauging stations and reservoirs. The Changjiang is traditionally divided into upper (above Yichang), middle (Yichang to Hukou) and lower (Hukou to Datong) reaches : following the impoundment of Danjiangkou Reservoir on the Hanjiang in 1968 and following the construction of numerous dams and water and soil-conservation works within the upper Changjiang basin after The development of Three Gorges Dam in 23 has further caused a third phase of sediment reduction. Similar studies have also been done by Chen et al. (21), Yang et al. (22, 23, 25), Chen et al. (25, 28), Xu et al. (26), and Zhang et al. (26, 29). These previous studies have documented that the volume of Changjiang sediment drastically declined over the past five decades, mainly as a result of extensive human activities (e.g., reservoirs, reforestation, sand mining). However, few of these studies were able to differentiate and quantify climate impacts and anthropogenic drivers (Xu et al., 27; Dai et al., 28b). The sediment rating curve empirically describes the relationship between suspended-sediment concentration and water discharge for a specific location (Walling 1974, 1978; Fenn et al., 1985; Jansson, 1996; Mossa, 1996; Asselman 1999, 2; Syvitski and Morehead 1999; Syvitski et al., 2; Morehead et al., 23). It is generally expressed as a power function: C S ¼ aq b (1) This is also expressed as a linearized equation based on logarithm transformation: logðc s Þ¼logðaÞþb logðqþ (2) Where C S is suspended-sediment concentration (g/m 3 ), Q is water discharge (m 3 /s), and a, b are the rating coefficient and rating exponent, respectively. The sediment rating curve can be considered as a black box model and the parameters a and b are estimated by regression analysis without any physical meaning (Asselman, 2). However, the sediment rating parameters are often associated with riverbed morphology (e.g., channel shape, slope, and unit stream power) or soil erodibility and erosivity of the river sections (Peters-Kümmerly, 1973; Rannie, 1978; Fenn et al., 1985; Thomas, 1988; Morgan, 1995; Asselman, 1999, 2; Syvitski et al., 2; Morehead et al., 23; Yang et al., 27a; Wang et al., 28). The sediment rating curve technique has been widely used for a variety of scientific and engineering purposes (Syvitski et al., 2). Once a sediment rating curve has been developed it can be applied to water discharge data to reconstruct the long-term sediment transport records (Syvitski and Morehead 1999; Asselman 2; Morehead et al., 23; Iadanza and Napolitano 26; Wang, et al., 28) or to quantitatively estimate sediment deficits during catastrophic flooding events (Xu et al., 25). Recently, Yang et al. (27a) calculated the sediment rating parameters at stations along the mainstream of the Changjiang, based on the daily data of water discharge and suspended-sediment concentration from the 195s to the 198s, and discussed implications for hydrological processes, human activities and the East Asian Monsoon. Wang et al. (28) used the monthly water discharge and suspended-sediment concentration data at Hankou station from 1954 to 25 to calculate the sediment rating parameters in different periods; the impact of human activities on the sediment rating parameters was then discussed. However, temporally limited data sets (195s 198s) and spatially limited locations (only Hankou station) made it difficult to obtain a comprehensive conclusion about the temporal and spatial variation of the sediment rating curves in response to extensive human activities within the Changjiang basin during the past five decades. Therefore, in this study, based on the consecutive and updated monthly data of water discharge and suspended-sediment concentration from 1955 to 27, the sediment rating curves were derived at three key gauging stations along the Changjiang, i.e., Yichang (the upper reaches), Hankou (the middle reaches), and Datong (the lower reaches) (Fig. 1 and Table 1). The objectives were (1) to determine temporal and spatial differences in the sediment rating curves for different locations; (2) to analyze the changes in the sediment rating curves in different periods in relation to the variation in the sediment transport regime of the Changjiang; and (3) to quantitatively assess the magnitude of human impacts on the decreasing volume of the Changjiang sediment by excluding the influence of climate change.

3 36 B. Hu et al. / Quaternary International 23 (211) Table 1 Hydrological Settings of the Changjiang (Location of stations presented in Fig. 1). 2. Changjiang basin 2.1. Geography Upper reaches (Above Yichang) Middle reaches (Yichang Hukou) The Changjiang originates from the Qinghai Tibetan Plateau at an elevation of 66 m, and it flows eastward 63 km through eleven provinces (Qinghai, Tibet, Sichuan, Yunnan, Chongqing, Hubei, Hunan, Jiangxi, Anhui, Jiangsu and Shanghai) before debouching into the East China Sea (ECS) (Fig. 1). The Changjiang basin is characterized by a subtropical monsoon climate initiated from the southeast Pacific Ocean and Indian Ocean, with an average rainfall of 1 14 mm/yr for the whole basin (Shen, 1986). The precipitation patterns throughout the Changjiang basin vary largely, increasing eastwards from 3 mm/yr in the upper reaches to more than 18 mm/yr in the mid-lower reaches. The monsoon-driven precipitation causes seasonal variability in the river flow, with high water and sediment discharge in the flood season from May to October. The Changjiang is generally divided into three major reaches (Fig. 1 and Table 1). The first is the upper reaches, which cover a drainage area of km 2 and flow 45 km through mountains and hills from the river source to Yichang station, with a steep channel slope of (Chen et al., 21). The major tributaries in the upper reaches are the Jinshajiang, Jialingjiang, Wujiang and Minjiang, of which the Jialingjiang and the lower Jinshajiang are two major sediment-producing areas (sediment yield >2 t/km 2 yr) in the Changjiang basin (Xiong et al., 29; Xu, 29). The second is the middle reaches, which extend 95 km through flat fluvial plains (slope of ) from Yichang to Hukou and cover a drainage area of km 2. There are two large lakes (Dongting Lake and Poyang Lake) that link with the mainstream, and a major tributary (Hanjiang) joins in the middle reaches at Hankou station. Dongting Lake and Poyang Lake, with watersheds of 2691 and 384 km 2,bothretainmost of the sediment from their own drainage basins and play an important role in regulating the Changjiang sediment discharge (Dai et al., 25b; Chen, et al., 28). The final major reach is the low reaches, which lie between Hukou and Datong, extending a distance of 93 km and coveringadrainageareaof km 2. This segment is characterized by an anabranching river patternwith the lowest channel slope of (Chen et al., 21). No large tributaries join the lower reaches, and downstream of Datong station, the river is tidally influenced. The Datong station is the last gauging station of the Changjiang basin; the water and sediment discharge records of Datong are generally used to represent the terrestrial flux from the Changjiang to the ECS. The hydrological data at three key stations (Yichang, Hankou and Datong) along the Changjiang indicate that most of the Changjiang sediment originates from the upper reaches, whereas the water discharge from the upper reaches accounts for only half of the water discharge from the Changjiang to the ECS (Table 1) Human activities: reservoirs, soil conservation Lower reaches (Hukou Datong) Gauging station Yichang Hankou Datong Length (km) Basin area (1 4 km 2 ) Water discharge (km 3 /yr) Sediment load (Mt/yr) Since the late 195s, many dams have been constructed in the Changjiang basin. The total water storage capacity of the reservoirs in the upper Changjiang in the 195s was only.6 km 3 and had increased to 23.4 km 3 by 199 (Yang et al., 25). By 1995, 45,628 reservoirs had been constructed in the Changjiang basin, with a total water storage capacity of 142 km 3. There are more than 5, reservoirs in the Changjiang basin with a total water storage capacity of 2 km 3, representing 22% of the annual water discharge from the Changjiang to the sea (Yang et al., 25). Currently, the annual sediment deposited in the reservoirs exceeds 9 Mt (Dai et al., 28b). Because of the unique landforms, climate and deforestation, the effect of reservoirs in the upper Changjiang on the sediment transport is greater than in the mid-lower Changjiang (Chen et al., 25). For example, the Bikou Reservoir (.52 km 3 ) in 1975 and Baozhushi Reservoir (2.6 km 3 ) in 1996 (Fig. 1) essentially retained all sediment from the Bailongjiang basin, which is one of the important sediment-yield areas in the Jialingjiang basin. The Wujiangdu Reservoir (Fig. 1) in the Wujiang basin was built in 1979 with a water storage capacity of 2.14 km 3. After it became operational, the sediment discharge from the Wujiang to the mainstream of Changjiang was greatly reduced (Zhang et al., 1999). The Ertan Reservoir, with a water storage capacity of 5.8 km 3, was built in 1998 on the Yalongjiang, which is the largest tributary of the Jinshajiang. From 1998 to 24, Ertan Reservoir completely retained 396 Mt of sediment, 56.6 Mt annually (Xiong et al., 29). In addition, there are two dams located at the end of the upper Changjiang, the Gezhouba Dam (GD) and the Three Gorges Dam (TGD) (Fig. 1). The GD, with a water storage capacity of 1.58 km 3,is located 8 km upstream from Yichang and began operation in From 1981 to 2, 8.3 Mt of sediment was deposited in the GD annually, although the deposition rate was not constant with time (Yang et al., 27c). Although the effect of the GD on annual sediment load is negligible, it has trapped half of the sediment load during the dry season (Dai et al., 25a). The TGD, located at the outlet of the Three Gorges, was built for flood control, navigation and hydropower generation. The TGD is the largest dam in the world (Nilsson et al., 25), with a water storage capacity of 39 km 3. After the TGD began operation in June 23, more than 1 Mt of sediment was trapped annually, and the TGD has strongly influenced the sediment transport process of the Changjiang (Yang et al., 26, 27b,c). One exception in the mid-lower Changjiang is the Danjiangkou Reservoir in the Hanjiang, which was built in 1959 with a water storage capacity of 17 km 3 (Fig. 1). After it began to trap sediment in 1968, more than 9% of the sediment entering the reservoir was retained there; it also greatly reduced the sediment discharge from the Hanjiang to the mainstream of the Changjiang at Hankou (Yang et al., 22, 26). In 1988, the Water and Soil Conservation Project (WSCP) was implemented in high sediment yield regions of the upper Changjiang basin such as the lower Jinshajiang, the Jialingjiang, and the upper Wujiang (Xiong et al., 29). Up to 28, the WSCP recovered a total area of km 2 via reforestation and building of weirs or small coffer dams. Collectively, the WSCP has accounted for 16%, or 17.2 Mt/yr, of the sediment reduction in the Jialingjiang (Xu et al., 28). About 15% of the sediment reduction of the Changjiang sediment, from 54 Mt/yr in to 28 Mt/yr in , was attributed to effects of the WSCP (Dai et al., 28b). The Chinese government has approved a plan to protect and recover an additional area of km 2 within the Changjiang basin in the next decade Variations of the Changjiang sediment since the 195s During the last half-century, human activities, especially the construction of more than 5, reservoirs, have caused an antiphase relationship between the water discharge and sediment load

4 B. Hu et al. / Quaternary International 23 (211) of the Changjiang on long time scales, i.e., water discharge is almost unchanged, whereas sediment load exhibits a steadily decreasing trend (Chen et al., 21, 25; Yang et al., 22, 26; Zhang et al., 29). The decreasing trend in sediment load first occurred in the mid-lower Changjiang (Hankou and Datong) in the late 196s, mainly resulting from the impoundment of the Danjiangkou Reservoir in 1968 on the Hanjiang. The mean annual sediment load at Hankou and Datong in accounts for 93% and 86% of that in In contrast, the mean annual sediment load at Yichang in the upper reaches show no distinct changes during the period from 1969 to 1985; however, half of the suspended-sediment load passing Yichang had been trapped by the GD in the dry season since 1981 (Dai et al., 25a). Subsequently, the WSCP and construction of numerous dams in the upper Changjiang basin caused further sediment reduction at all three stations (Yichang, Hankou and Datong) along the Changjiang after In , the annual sediment load at Yichang, Hankou, and Datong further decreased by 2% of that in This situation became more apparent after the TGD began operations in June 23. The operation of the TGD has significantly reduced the Changjiang sediment discharge to the downstream and coastal region. Annual sediment loads at Yichang, Hankou, and Datong in 23 27, after the construction of the TGD, were 67 Mt, 129 Mt, and 158 Mt, respectively; these values are only 13%, 29%, and 32% of the averages. 3. Data collection and methods The consecutive monthly data of water discharge and suspended-sediment concentration at Yichang, Hankou and Datong stations from 1955 to 27 were mainly supplied by the Changjiang Water Resources Commission (CWRC) and partly collected from the River Sediment Bulletin of China published by the Ministry of Water Resources, China (IRTCES, 2 27). The river s sediment discharge was measured based on the Chinese national standard criterion. The water samples were taken across the full water column at a fixed gauging section, and the flow discharge was recorded at certain times. Then, the daily, monthly and annual suspended-sediment loads passing the gauging section were derived (Yang et al., 26). Bed load was not included in the present study since the contribution to total load is less than 2% in the Changjiang (Yang et al., 22, 23). To quantify the impacts of human activities (e.g., dam construction, soil-conservation works) within the Changjiang basin on the river system, the 53 years of monthly water discharge and suspended-sediment concentration data have been divided into four periods (three pre-tgd periods and one post-tgd period), according to the stepwise decreases in the Changjiang sediment load delivered to the ECS (Yang et al., 26; Wang et al., 28) (Table 2). Sediment rating curves at three key stations (i.e., Yichang, Hankou and Datong) along the Changjiang are obtained for each period. The sediment rating parameters (b and log(a)) have been estimated using an ordinary least-squared regression on logarithms of water discharge and suspended-sediment concentration data. 4. Results and discussion 4.1. Sediment rating curves at three key stations along the Changjiang Sediment rating curves at three key gauging stations (Yichang, Hankou and Datong) along the Changjiang were obtained for the four different periods (i.e., , , and 23 27). All sediment rating curves at the three stations in different periods displayed a downward trend with time (Fig. 2). The data points for lie on the upper part of the plots. The data points display a gradually downward shift in the later periods in response to the sediment reductions induced by human activities, e.g., construction of numerous reservoirs and the WSCP (Yang et al., 22, 26; Wang et al., 28). The sediment rating parameters (b and log(a)) at the three stations in different periods are shown in Table 3. In this study, the spatial distribution of the sediment rating curves at three stations during the pre-tgd periods is consistent with the results of Yang et al. (27a). The steepest rating curves were found at Yichang, followed by Datong, and the flattest rating curves were found at Hankou. This pattern may relate to the river channel morphology of the selected reaches, i.e., the steep rating curves relate to high gradient and bedrock-confined V-shaped channels with higher unit stream power. In contrast, the flat rating curves occur in the mid-lower Changjiang, where the channel is characterized by low gradient and wide U-shaped channels with lower unit stream power (Yang et al., 27a). Wang et al. (28) indicated that human activities in the Changjiang basin, including damming and WSCP, have decreased the sediment supply from the source region and increased the erosive power of the river, thereby decreasing the rating coefficient a and increasing the rating exponent b at Hankou station during the past five decades. In this study, the sediment rating exponent b values at Yichang continuously increased from 1.6 in to 2.33 in , whereas the rating coefficient log(a) values synchronously decreased from 3.84 to 7.9. The sediment rating parameters exhibited the same variation trend at Hankou over the same periods, where the values of b increased from.89 to 1.3 and log(a) decreased from 1.17 to However, the sediment rating parameters at Datong station appear to vary little during the whole pre-tgd periods (b ¼ 1.21w1.25 and log(a) ¼ 3.w 2.91). Obviously, the magnitude of variations in the sediment rating parameters shows a decreasing trend along the downstream direction. This suggests that the variations of the sediment rating parameters are not only related to the sediment reduction caused by human activities but also to the scale effect of the river sections (Asselman, 2; Morehead et al., 23). Thus, larger drainage areas of river sections are related to smaller variations in suspended-sediment concentration (Xu, 29), and therefore to smaller changes in sediment rating parameters. Table 2 Mean water discharge (Q) and mean suspended-sediment concentration (C S ) at three key gauging stations along the Changjiang for the different periods. Periods Yichang Hankou Datong Q (m 3 /s) C S (kg/m 3 ) Q (m 3 /s) C S (kg/m 3 ) Q (m 3 /s) C S (kg/m 3 ) , , , , , , , , , , , , , , ,987.35

5 38 B. Hu et al. / Quaternary International 23 (211) a log( C S ) b log( C S ) c log( C S ) lo g(q) 4 Hankou lo g(q) 4 Datong Yichang lo g(q) Fig. 2. Sediment rating curves in the a) upper (Yichang), b) middle (Hankou) and c) lower (Datong) reaches of the Changjiang for the different time periods. Upper reach Middle reach Lower reach After the operation of TGD in 23, however, the values of log(a) at all three stations increased, whereas b decreased (in contrast to pre-tgd periods). This pattern does not agree well with previous interpretation of the sediment rating parameters (e.g., Yang, et al., 27a; Wang et al., 28). A better indication of the sediment transport regime of the selected river section is obtained when the rating parameters are plotted in a b log(a) plot (Fig. 3). Asselman (2) indicated that all locations that plot on the same line in the b log(a) diagram appear to be characterized by a similar sediment transport regime. Locations plotted on higher lines are characterized by a sediment transport regime where a larger part of the annual sediment load is transported during high discharge. However, the position of the rating parameters on this line varies with time and therefore must be mainly related to watershed characteristics that also vary in time, such as average or maximum discharge, as well as the sediment availability that is influenced by human activities. The sediment rating parameters b and log(a) at Yichang, Hankou and Datong stations for different periods present three lines with almost the same slope (Fig. 3). The upper line (b ¼ log(a)) coincides with the rating parameters obtained for the Yichang during the pre-tgd periods, and the lower line (b ¼ log(a)) coincides with the rating parameters obtained for the Yichang, Hankou and Datong in the period of Between these two lines, a third line (b ¼ log(a)) can be drawn for the rating parameters obtained for Hankou and Datong in the post-tgd periods. Thus, it can be argued that the sediment transport regime in the upper Changjiang (Yichang) is different from that in the mid-lower Changjiang (Hankou and Datong) during the pre-tgd periods. Though a w3% decrease in sediment load occurred at all three stations between the periods and (Yang et al., 26), the sediment transport regimes during both periods did not change. The locations of the lines indicate that high water discharge at Yichang plays a more important role in the suspended sediment transport than at Hankou and Datong during the pre-tgd periods. This can be verified by the distribution pattern of the monthly sediment load at the three stations (Fig. 4). Fig. 4 shows that at Yichang, more than 95% of the annual sediment load is transported during the flood season (May Oct) in the pre-tgd periods, but at Hankou and Datong in the same periods, this percentage decreases to about 85%. These characteristics of suspended sediment transport are related to the sediment erosion process. The topography of the upper Changjiang basin is hills and mountains, where the landslips and debris flow are the dominant erosion processes. In this case, most of sediment transport occurred when a rainfall threshold is exceeded. Since 23, the TGD has trapped w75% of the sediment load coming from the upper Changjiang basin (Hu et al., 29). Correspondingly, the annual sediment load at Yichang, Hankou, and Datong in decreased to 16%, 39%, and 46%, of that in , respectively. The sediment rating parameters obtained for the three stations in are located on the lower line in Fig. 3, indicating that the sediment transport regimes at the three stations have changed to become more similar to each other during Table 3 Sediment rating parameters (log(a) and b) and coefficient of determination (r 2 ) for different periods at Yichang, Hankou and Datong stations. Periods Yichang Hankou Datong log(a) b r 2 log(a) b r 2 log(a) b r

6 B. Hu et al. / Quaternary International 23 (211) b (1) Yichang Pre-TGD Chance of occurrence (%) Datong 1 1 (1): b= log(a), r 2 =.9995 (2): b= log(a), r 2 =.9856 (3): b= log(a), r 2 =.991 (2) Yichang Hankou Post-TGD (3) Datong Hankou Monthly water discharge(q, m 3 /s) Fig. 5. Occurrences of monthly water discharge for the periods of and , illustrating a very similar mode log(a) Fig. 3. Correlation between slope/intercept values of sediment rating curves obtained by least-squared regression on logarithmic transformed data. the post-tgd period. The distribution patterns of monthly sediment load at the three stations in the post-tgd period also display a similar variation trend (Fig. 4). The largest difference in monthly sediment load between the pre-tgd and post-tgd periods occurred in the flood season (May Oct) when more than 7% of the sediment was trapped by the TGD (Xu and Milliman, 29). This indicates that the dominance of high water discharge on river sediment transport has decreased since the TGD began operation. The results presented in this study are comparable those obtained in the Mississippi River (Kesel, 1989) and the Tiber River (Iadanza and Napolitano, 26). Kesel (1989) identified four periods in the annual suspended-sediment concentration water discharge data at New Orleans from 1851 to 1982: a historic period ( ), a pre-dam period ( ) and two post-dam periods ( and ). When the rating parameters of each period were plotted in a slope/intercept graph, the position of the rating parameters of the historic period ( ) and the pre-dam period ( ) are relatively close to each other. Thus, although a 43% decrease in sediment load occurred between the historic and the pre-dam periods due to improved agriculture and land management techniques in the Mississippi Valley, the sediment transport regime did not change drastically. During the succeeding two post-dam periods, the annual sediment load decreased by 51% and 79%, respectively. This observed decrease in sediment loads was attributed to the construction of reservoirs and dams on the tributaries in the Mississippi River, which tend to trap Monthly sediment load (t/s) Yichang, Pre-TGD Yichang, Post-TGD Hankou, Pre-TGD Hankou, Post-TGD Datong, Pre-TGD Datong, Post-TGD Month Fig. 4. Changes in monthly sediment discharge in the upper (Yichang), middle (Hankou) and lower (Datong) reaches of the Changjiang before and after the TGD. Measured monthly sediment load, Q S M (t/s) Q S M=.86Q S E+1.91 R 2 =.76, n=168 95% confidence level 95% confidence level Estimated monthly sediment load, Q S E (t/s) Fig. 6. Estimated monthly sediment load at the Datong station ( ) compared with the measured data, indicating that the estimates explain 76% of the data variance with 95% confidence level.

7 4 B. Hu et al. / Quaternary International 23 (211) a Monthly sediment load (Mt/mo) Estimated 3 Measured Year 12 b Annual sediment load (Mt/yr) Mean annual sediment load (Mt/yr) Year c Periods Fig. 7. (a) Estimated monthly sediment load at the Datong station compared with the measured data from 1955 to 27. The estimates were derived from the rating curves in shown in Fig. 2c; (b) Estimated annual sediment load at the Datong station compared with the measured data from 1955 to 27; (c) Comparison of the estimated mean annual sediment load with the measured data for the four periods: , , and large amounts of sediment with a decrease in suspended sediment transport during high water discharge. Accordingly, the positions of the rating parameters of the two post-dam periods in the slope/ intercept graph are significantly lower than those of the historic and pre-dam periods, indicating a change in the sediment transport regime. Iadanza and Napolitano (26) obtained the annual sediment rating curves at the Ripetta station on the Tiber River from 1934 to When the rating parameters are plotted in a slope/intercept graph, two nearly parallel lines can be drawn: the upper line plots through the rating parameters from 1934 to 1963, whereas the rating parameters obtained from 1964 to 1974 plot on the lower line. The results indicate that although a 4% decrease in sediment load occurred between and , the sediment transport regime did not change. However, the operation of the Corbara Dam since 1963 has had significant effects. The dam caused the annual sediment load in to decrease by 67% of the sediment load in and 8% compared to the period , which led to a change in the sediment transport regime Decrease in sediment transport from the Changjiang to the ECS Factors controlling the sediment flux of rivers to the sea mainly include geomorphic and tectonic influences (basin area and relief),

8 B. Hu et al. / Quaternary International 23 (211) geography (temperature, rainfall), geology (lithology and ice cover), and human activities (damming, deforestation/reforestation, agricultural practices, surface mining, engineering construction) (e.g., Milliman and Meade, 1983; Milliman and Syvitski, 1992; Walling and Fang, 23; Syvitski and Milliman, 27). However, on a time scale of decades, the sediment yield strongly depends on the rainfall (natural factor) and human activities (anthropogenic factor). The effects of rainfall on the sediment load are multiple and complicated, including the location, distribution, duration, frequency, and magnitude of rainstorms (Xiong et al., 29). In a specific drainage basin, without considering the water diversion, however, the impact of rainfall changes on the sediment load can be principally estimated by the variation in water discharge coupled with the relationship between the water discharge and suspendedsediment concentration (e.g., sediment rating curve). In contrast, human activities have a dual effect on the river sediment load; they simultaneously increase the sediment transport through soil erosion, yet reduce it by sediment retention within reservoirs (Syvitski et al., 25). To assess the magnitude of human activities on the decrease of sediment transport in the Changjiang, an estimate of the potential sediment load at the Datong station, which excludes the impacts of human disturbances, is necessary. The sediment rating curve at Datong during (C S ¼ Q ; Fig. 2c andtable 3), when the effects of human activities were nearly negligible (e.g., Yang et al., 26; Wang et al., 28), was used, and the monthly water discharges at Datong in were used as input data. One concern when using such a method is whether the monthly water discharge at Datong has also been disturbed by human activities. However, the occurrences of monthly water discharge for the periods of and demonstrated a very similar pattern (Fig. 5). The monthly water discharge between 8 m 3 /s to 55, m 3 /s covers approximately 9% and 93% of the data sets in and , respectively. This suggests that the monthly water discharge at Datong in the later periods of has probably not been disturbed greatly by human activities. Therefore, the sediment rating parameters in appear to be valid for estimation of the monthly suspended-sediment load at the Datong station for the period of in the absence of human disturbances. Comparison of the measured sediment load and estimates from the sediment rating curve from 1955 to 1968 indicate that this method is effective (Fig. 6). Fig. 6 shows that the estimates from the sediment rating curve explain approximately 76% of the measured data variance within the 95% confidence limits. The estimated monthly and annual sediment loads from 1955 to 27 are compared with the measured monthly and annual sediment loads (Fig. 7). The results show that from 1969 to 1985, the difference between the measured (including the impacts from human activities) and estimated sediment loads (excluding the impacts from human activities) is considerable and mainly due to the construction of the Danjiangkou Reservoir on the Hanjiang in This difference has increased dramatically since the mid-198s, as a result of the construction of numerous dams and the extensive water and soil-conservation works in the upper Changjiang basin. The subsequent operation of the TGD in 23 has caused the difference between the measured and estimated sediment loads to increase further. If the mean annual sediment load at Datong in is set as a baseline for the Changjiang sediment under natural conditions, the changes in sediment load caused by the natural factor can be calculated as 1 Mt/yr for the period of , 3 Mt/yr for the period of and 1 Mt/yr for the period of (positive values represent increased sediment load, and vice-versa) (Fig. 7c). The mean annual sediment deficit caused by human activities was 8 Mt/yr for the period of , 2 Mt/yr for the period of and 25 Mt/yr for the period of After the TGD was put into operation in 23, the sediment discharge from the Changjiang to the sea has decreased by 35 Mt/ yr, from 51 Mt/yr in to only 16 Mt/yr in According to Hu et al. (29), about 75% of the sediment load coming from the upper reaches was trapped by the TGD in and the TGD-induced sediment reduction at Datong was estimated to be around 9 Mt/yr. Thus, both natural and anthropogenic factors are responsible for this drastic decline in Changjiang sediment in Twenty-nine percent (1/35) can be attributed to the decreased rainfall in the Changjiang basin, and the remaining 71% (25/35) is the result of extensive human activities. Out of all the human activities, the TGD alone accounts for 26% (9/35) of the Changjiang sediment reduction in Moreover, sediment loss caused by anthropogenic factors accounted for 15% (8/52) of the natural sediment load in ; this loss increased to 61% (25/41) after the TGD began operations (23 27). This indicates that the intensified human activities in the river basin have altered the natural sediment transport process of the Changjiang since 1968 and become a dominant forcing factor to the sediment delivery from the Changjiang to the ECS. 5. Conclusions This study shows that the sediment rating curves and their parameters have been substantially affected by extensive human activities in the Changjiang basin since the 196s, but the magnitude of variations in sediment rating parameters has been reduced stepwise in the downstream direction. Before the operation of TGD, the sediment transport regime in the upper reaches of the Changjiang (Yichang) was different from the regimes in the middle and lower reaches (Hankou and Datong); i.e., high water discharge at the Yichang station played a more important role in suspended sediment transport than at Hankou and Datong. Although a w3% decrease in sediment load occurred at all three key stations along the Changjiang in the entire pre-tgd period ( ), the sediment transport regimes appear to be unaltered. Since 23, the TGD has trapped around 75% of the sediment coming from the upper reaches, and the sediment transport regime at the three stations has changed into a similar regime characterized by decreased importance of high water discharge in suspended sediment transport. The time series of sediment flux from the Changjiang to the ECS has shown a significant decrease since the late 196s. A comparison between the measured sediment loads and estimates from the sediment rating curves suggests that the mean annual sediment deficit caused by human activities has increased from 8 Mt/yr in to 25 Mt/yr in This indicates that intensified human activities in the river basin have altered the natural sediment transport process of the Changjiang since the late 196s and have become the dominant factor influencing Changjiang sediment flux to the sea. Acknowledgements Two anonymous reviewers are appreciated for their comments on improving the quality and science of the original manuscript. We thank the Changjiang Water Resources Commission (CWRC) for the access to valuable data sets. This work was financially supported by NSFC (Grant No , ).

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