Environmental Earth Sciences (2018) 77:394 https://doi.org/10.1007/s12665-018-7576-2

ORIGINAL ARTICLE

Evolution of the mid-channel bars in the middle and lower reaches of the Changjiang () River from 1989 to 2014 based on the Landsat satellite images: impact of the

Yaying Lou1 · Xuefei Mei1 · Zhijun Dai1,2 · Jie Wang1 · Wen Wei1

Received: 11 August 2017 / Accepted: 16 May 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The mid-channel bars have long been identified as essential landforms in the large rivers of the world, and the significance of connectivity between morphology and flow-sediment dynamics has been intensively emphasized. In this study, remote sensing images and associated hydrological data from 1989 to 2014 were used to explore mid-channel bars evolution in the middle and lower reach of the Changjiang and their responses to the Three Gorges Dam (TGD), the world’s largest hydrologi- cal engineering. The results indicated that mid-channel bars, respectively, exhibited deposition and erosion in the flood and dry season in pre-TGD period, while mild deposition in flood season and deposition in dry season were found in post-TGD period. As a consequence, mid-channel bars area was characterized by ‘remarkable seasonal differences in pre-TGD period, mild seasonal pattern in post-TGD period’. The obvious shift in seasonal features could be attributed to the TGD operation in 2003. Specifically, flood duration decrease and sediment load reduction following TGD regulation suppressed the bars growth in flood season. TGD-induced variations in differences between sediment carry capacity and suspended sediment concentration resulted in the bars transformation in dry season. Meanwhile, the change trends of downstream mid-channel bars became weak as their locations’ distance to TGD increases because of the river adjustment and tributaries supplement. Moreover, mid-channel bars in different river patterns presented various change trends with the most remarkable variation being detected in goose-head-shaped river pattern. The results of this paper provide a theoretical basis for the river channel improvement in the middle and lower reaches of the Changjiang River.

Keywords Mid-channel bars · Morphodynamic process · Sediment · Changjiang (Yangtze) River · Three Gorges Dam (TGD)

Introduction bars can be vegetated, with flow discharge passing through both sides (Hooke and Yorke 2011). As unique geomorphic Mid-channel bars, i.e., a free bar in the middle of a chan- cells and components are present in braided river and estuary nel, are typical accumulation landforms that are distributed area, mid-channel bars are of vital importance to maintain worldwide over river channels (Bristow and Best 1993). the river and estuarine channel stability and protect adja- Generally seen in diamond or lozenge-shaped, mid-channel cent wetlands (Hooke 1986; Bristow and Best 1993; Sanford 2007). Thus, morphodynamic evolution of the mid-channel bars along fluvial rivers has received attention in the litera- * Zhijun Dai ture (Leopold and Wolman 1957; Hooke 1986; Lippmann [email protected] and Holman 1990). Yaying Lou The earlier studies about river bars leaned to analyze [email protected] their formation mechanism and natural evolution and aggradation (Knighton 1972; Church and Jones 1982; Ash- 1 State Key Lab of Estuarine and Coastal Research, East Normal University, Shanghai 200062, China worth 1996). Knighton (1972) recognized that a number of mid-channel bars on the Bollin River were arisen where 2 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, the bank was rapidly eroded and the channel became wid- Qingdao 266100, China ened. More and more researches have focused on the

Vol.:(0123456789)1 3 394 Page 2 of 18 Environmental Earth Sciences (2018) 77:394 contribution of detached bars to the meander form devel- Study area opment and variations. Church and Jones (1982) identi- fied three major causes of bars’ formation: reduce in shear Changjiang River is the longest river (about 6300 km) in stress, widening of the channel and tributary entrances Asia with a drainage area of about 1.8 × 107 km2 (Wang (all of which lead to flow divergence). Thereafter, Ash- et al. 2009; Dai and Liu 2013; Dai et al. 2014; Mei et al. worth (1996) demonstrated the bars’ development process 2016). The reach between Chenglingji and Datong (CD) and linked the process to changes in channel geometry belongs to the middle and lower reach of the Changji- and local flow strength and direction. In addition, a series ang River, which is one of the densely populated area of of recent research works further conducted the specific Changjiang River (Dai and Liu 2013). Chenglingji–Datong bars’ formation and evolution mechanisms globally, and (CD) reach is about 753.4 km in length with the Han showed that their evolutions were related to the hydrologi- River and Poyang Lake mingling in at Hankou and cal regime (Ashworth et al. 2000; Lunt and Bridge 2004; Hukou, respectively (Fig. 1). Accordingly, this reach Burge 2006; Szupiany et al. 2009; Baubinienė et al. 2015). can be further divided into three reaches: CH reach from With intensive human intervention and occupation, the Chenglingji to Hankou, HJ reach from Hankou to Jiuji- natural river regimes were gradually replaced by the regu- ang and JD reach from Jiujiang to Datong. The hydrologic lated flow, which dramatically affected the mid-channel regimes in these reaches are monitored by five hydrologi- bars’ evolution and gained international concerns (Sanford cal stations, namely, Luoshan, Hankou, , Jiujiang 2007; Skalak et al. 2013; Kiss and Balogh 2015; Raška and Datong. The annual runoff (from 1951 to 2002) in et al. 2016). For example, Sanford (2007) showed that in Luoshan, Hankou and Datong were about 6.5 × 1012 m3, the Missouri River, dam regulated rate of flow resulted in 7.1 × 1012 m3 and 9 × 1012 m3, respectively, while the con- the decrease in the sand bars’ area, less centroid migra- temporary sediment load were 4.15 × 109 t, 3.98 × 109 t and tion and more easy bars aggregation. Kiss and Balogh 4.28 × 109 t. Due to the effect of monsoon, the runoff and (2015) showed the point-bars developed quickly upstream sediment in Chenglingji–Datong reach are mainly con- and laterally in the Dráva River, because of the coarse centrated in the flood season (May to October), when the sediment supply and the decreasing stream energy by the runoff, respectively, accounted for 74.2, 73.3 and 71.1% dam effect. Raška et al. (2016) discovered most islands of the total runoff in the three reaches from upstream to have disappeared because of the construction of dams downstream, while the sediment took up 85.5, 87.6, 87.7% and lock chambers in Elbe River. Meanwhile, a series of of the total sediment load (Pan 2011). water conservancy projects have been constructed along As a braided river with sandy beds, CD reach has devel- the Changjiang River, especially, the Three Gorges Dam oped a number of mid-channel bars (Fig. 1) (Yu 2005). To (TGD), currently the world’s largest water conservancy facilitate statistic, stable and relatively large bars that can project, which influences the hydrological regime (Chen be observed in both flood and dry seasons were selected et al. 2017) and may affect dozens of sand bars in the in this study. Besides, based on the channel classification downstream reaches (Xu 2013; Zhu et al. 2015; Li 2011). theory of Rust (1978) and Chien (1987), it was further pro- For instance, Zhu et al. (2015) analyzed the evolution posed that braided river with different sinuosities can be characteristics and trend of the braided channels in the divided into straight braided river, bending braided river middle Changjiang reach, and found that the mid-channel and goose-head-shaped braided river. These three river bars in lower Jingjiang reach experienced the most severe types are described in Fig. 2, while the related specific erosion after the TGD impoundment. However, the previ- mid-channel bars number in each river type are shown in ous understanding on bars change have been mainly con- Table 1. ducted along Jingjiang reach and ignored variations along the further downstream reach (Shao et al. 2005; Jiang et al. 2010). Besides, according to classification of river chan- nel by Rust (1978), Chenglingji–Datong reach belongs Materials and methods to a braided river with different sinuosities, which could also affect the bars’ development which is why it needs an Traditional mid-channel bars’ research methods included urgent research. model simulation (Leopold and Wolman 1957), numeri- Therefore, the objectives of this paper are to (1) explore cal method (Davoren and Mosley 1986; Ashworth et al. the evolution of braided bars along the Chenglingji–Datong 2000), historical maps and aerial photos analysis (Hooke reach of Changjiang River, (2) identify the change character- 1986; Sanford 2007; Raška et al. 2016). The methods of istics of the mid-channel bars in braided river with different model simulation and numerical method are prone on the sinuosities, (3) discuss associated variation mechanism of theoretical analysis, which, however, cannot document the mid-channel bars evolution.

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Fig. 1 Study area of Chenglingji to Datong reaches

Fig. 2 Three kinds of braided river diagram; a straight braided river; b bending braided river; c goose-head-shaped braided river

real situation properly. Besides, although historical maps used to examine the significance of area changing between and aerial photos are more intuitive in exploring the bars’ two periods (Makwana et al. 2016). development, there are no lasting yearly observed maps or photos in this study. In recent decades, with technological advancement, remote sensing images have been widely Satellite imagery selection and data source used in bars’ observation and study because of high resolu- tion and easy accessibility (Moretto et al. 2012; Yang et al. According to the Worldwide Reference System, three TM 2015; Deng et al. 2017), which, therefore, was applied to scenes (path 123, row 39; path 122, row 39; path 121, row this study as well. 39) can absolutely cover the study area. TM scene selection Water depth has an inevitable effect on bars’ exposed was controlled by the water level. Only those correspond- area, which, accordingly, was used to select the Landsat ing to the same or similar water level could be used in this satellite images in this study. To improve the results preci- study (Tables 2, 3). Considering the water and sediment sion and reduce the comparison errors, the selected inter- differences between flood and dry season, two remote sens- annual images should correspond to the constant water level ing images with high and low water levels were selected (Wang et al. 2013a, b). Besides, the threshold segmentation within 1 year. According to the hydrological regime along was adopted to distinguish the mid-channel bars from water. the Changjiang River, the dry season ranges from Novem- The bars area thereafter could be extracted to compare the ber to April, while the flood season covers May to October. changing tendencies between pre- and post-TGD periods. Occasional remote sensing images were missing over the Furthermore, the independent samples T test method was study period because of the weather limits. Detailed remote

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Table 1 The information of channel bars and corresponding unified water levels Bar Name River pattern Long axis slope Short axis slope Unified water level Unified water level (flood season,m ) (dry season, m)

Nanyang bar Straight 1/60 1/23 22.5 15.0 Nanmen and Xinyu bar Straight 1/75 1/40 22.5 15.0 Zhong bar Goose-head-shaped 2/5 2/5 22.5 15.0 Xin bar Goose-head-shaped 1/60 1/20 22.5 15.0 Fuxing bar Straight 1/208 1/39 22.5 15.0 Tuan bar Goose-head-shaped 1/56 1/100 22.5 15.0 Tianxing bar Bending 1/160 1/10 22.5 15.0 Dongcao bar Goose-head-shaped 1/80 1/50 19.0 11.5 Daijia bar Bending 1/40 1/20 19.0 11.5 Xin bar (Longpin) Goose-head-shaped 1/30 1/30 19.0 11.5 Zhangjia bar Bending 1/100 1/30 15.5 10.0 Xiasanhao bar Straight 1/60 1/30 15.5 10.0 Mianchuan bar Bending 1/100 1/30 15.5 10.0 Yudai bar Straight 1/90 1/20 15.5 10.0 Xin bar (Guanzhou) Bending 1/130 1/40 15.5 10.0 Meimao bar Bending 1/335 1/140 15.5 10.0 Jiangxin bar Bending 1/18 1/14 15.5 10.0 Tieban bar Goose-head-shaped 1/20 1/60 15.5 10.0 Fenghuang bar Bending 1/160 1/10 15.5 10.0 Chongwen bar Bending 1/96 1/40 15.5 10.0

Table 2 List of satellite images Remote sensing image number Remote sensing image number Remote sensing image number used in the present work (dry (123/39) (122/39) (121/39) season) Acquisition date Hankou water Acquisition date Huangshi Acquisition date Jiujiang level (m) water level (m) water level (m)

1989/2/11 14.20 1989/2/4 11.07 1989/2/13 9.72 1994/3/5 15.00 1994/1/25 10.95 1994/1/2 9.97 1995/12/5 15.78 1995/12/30 11.15 1995/12/7 9.92 1996/12/23 15.13 1997/3/6 12.1 1997/2/11 10.35 1999/12/24 15.47 1999/12/25 11.98 1999/12/18 10.11 2000/2/26 15.17 2000/3/6 11.98 2000/1/27 9.84 2001/12/29 15.13 2001/12/22 11.93 2002/2/1 8.51 2003/1/17 15.95 2003/2/19 12.13 2003/1/27 9.92 2003/12/27 14.85 2004/3/9 12.35 2004/1/30 8.15 2004/12/13 15.92 2005/1/7 11.58 2005/1/16 8.78 2007/4/10 15.38 2007/4/19 11.54 2007/2/23 9.08 2008/12/24 14.75 2008/12/17 12.66 2009/2/12 8.31 2009/11/25 14.49 2010/2/22 11.31 2010/1/14 8.53 2011/3/4 14.92 2011/1/8 12.23 2011/2/2 9.84 2013/11/20 15.77 2014/3/5 12.34 2013/11/6 8.89

sensing images and their water level information were shown chosen to control the TM scenes selection. All these data in Tables 2 and 3. were obtained from Changjiang Water Resources Commis- Three hydrological stations (Hankou, Huangshi, Datong) sion, China (http://www.Cjh.com.cn). In addition, monthly with daily water level information during 1989–2014 were discharge and sediment load data at , Luoshan,

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Table 3 List of satellite images Remote sensing image number Remote sensing image number Remote sensing image number used in the present work (flood (123/39) (122/39) (121/39) season) Acquisition date Hankou water Acquisition date Huangshi Acquisition date Jiujiang level (m) water level (m) water level (m)

1991/9/21 22 1991/6/26 19.5 1991/10/9 15.03 1993/10/12 22.1 1993/7/17 19.67 1993/6/8 15.55 1994/7/27 22.56 1994/10/24 18.7 1994/8/30 15.68 1995/8/31 23.46 1995/6/5 19.54 1995/9/18 15.27 1997/6/17 22.1 1997/8/29 18.21 1997/9/7 15.37 2000/9/13 23.75 2000/9/22 19.68 2000/9/23 16.71 2002/8/2 24.5 2002/8/3 21.3 2002/6/17 16.33 2003/9/22 23.43 2003/5/26 20.34 2003/8/23 15.92 2005/6/23 22.52 2005/9/20 19.92 2005/9/29 15.49 2007/6/29 22.81 2007/8/25 19.66 2007/10/5 15.27 2009/8/21 23.73 2009/7/13 19.45 2009/6/4 15.75 2013/6/13 23.11 2013/7/8 20.41 2013/8/18 14.7

Hankou and Datong were also acquired from Changjiang To reduce these data errors, some larger bars were selected Water Resources Commission. The Landsat data from a for analysis. Because, it is difficult to require the bars eleva- Thematic Mapper (TM) and an Enhanced Thematic Mapper tion change information from the remote sensing images, (ETM+) were provided by the Earth Resources Observation the exposed bar areas were extracted from different remote and Science Center (http://glovi​s.usgs.gov/). sensing images to analyze the bar dynamics.

Exposed area calculation Methods In spite of careful selection of the remote sensing images Waterlines extraction to avoid the effect of water level, differences still existed in the inter-annual water level among the images (Tables 2, 3). In this paper, threshold segmentation method was utilized To decrease such uncertainties, the generalized geometric for extracting the waterlines to separate the land and water model was utilized to simplify the shape of mid-channel through ENVI and ArcGIS. Specifically, the near infrared bars (Fig. 3) for a better unifying of water levels. Then, the band that has high resolution for water and land was chosen horizontal distances of bars’ isobaths between 0 and 2 m as the research object, first. Then, gray histogram of each could be measured by the Changjiang river channel graph band could be obtained using ENVI software. Thereafter, (http://www.cjien​c.com/), thus the slopes of sand bars can the gray value was set as the bottom between the two peaks be calculated. of the histogram, which was used as the threshold to separate the land from water. For the selection of the bottom value, the minimum threshold value method was utilized (Abutaleb 1989; Guo and Pandit 1998; Cuevas et al. 2010): 𝜕h(z) 𝜕2h(z) = 0 > 0, 𝜕z 𝜕z2 (1) where h(z) represents the histogram. Then, the image was loaded into the ArcGIS to carry out the image binary based on the threshold. Eventually, mid-channel bars’ exposed area data could be obtained in the ArcGIS. Considering the resolution of remote sensing images is 30 m, if mid-channel bars with small areas will be used, the situation would lead to comparatively larger relative errors. Fig. 3 The mid-channel bars generalization

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The bars outcrop area changes with the length and width of (9.8 m/s2); h is the water depth (m); ω is the sediment set- quadrilateral, which could be adjusted through the horizontal tling velocity; k and m are empirical coefficients. Here k is and vertical slopes. The specific formulas are as follows: equal to 0.07 and m is 1.14 (Zhang 1998; Gao et al. 2009). U k = S∕(ab), Then, is determined from the fitting curve between dis- (2) charge and flow velocity (Wang et al. 2009). h is obtained 2, a1 = a +(hf − hu)∕i × (3) through the same way. ω can be calculated by the following formulas (Rich- 2, b1 = b +(hf − hu)∕j × (4) ardson and Zaki 1954):

, s −  S1 = ka1b1 (5) = 1.72 gD, 0  (7) where S is the bars area under the actual water level ­(m2), a is the length of channel bar, b is the bar’s width (m), k is =(1 − S )m , a coefficient representing the ratio between the actual bars v 0 (8) area and the parallelogram area, which is set as a constant where ω0 is the settling velocity of a sphere of diameter D; value for the same bar. And, i represents the bar’s horizontal D is the grain diameter; γs is the sediment density (2300 kg/ 3 3 slope (m), j represents the bar’s vertical slope, hf is the actual m ); γ is the density of water (1000 kg/m ); Sv is the sus- 3 water level (m), hu is the unitive water level (m), subscript pended sediment concentration (kg/m ) (Yuan et al. 2012). ‘1’ represents the value under the unitive water level. Fur- therly, each TM scene contains a water level control station, and corresponding unitive water level can be seen in Table 1. Results Independent samples T test Seasonal changes in channel bar area Comparison of mid-channel bars area variation rates between pre- and post-TGD periods was carried out through According to the T test results in dry season, the best cut- independent samples T test. Independent samples T test pro- off point for the entire 1989–2014 series is 2003 (Tables 4, cedure compares the means of two or more groups of cases 5, 6). The bars area variations have significant difference (Larjani et al. 2014; Makwana et al. 2016). In this study, the in the two sub-periods of 1989–2002 and 2003–2014 entire data set was classified into two groups by a specify- (Sig = 0.16, ­Sig2-tailed = 0.011 < 0.05) (Table 4). A clear ing cutoff point. Here, 2002, 2003 and 2005 was selected as downward trend was detected in the dry season with a cutoff points, respectively. Then, the area variation rates for mean decrease rate of 0.89 km2/year before 2003 (Fig. 4a). the two sub-groups were computed and compared. However, during the post-TGD period, the bars area had By default, a 95% confidence interval for the difference observable increase trend with a rate around 0.815 km2/ in means is displayed (Larjani et al. 2014; Makwana et al. year. 2016). If p ≤ 0.05, there is a significant difference between During the flood season, the whole area of mid-channel the two groups, otherwise, there is no significant difference. bars between CD reach presented upward trend with an increased rate of 1.07 km2/year between 1991 and 2014 Sediment carrying capacity formula (Fig. 4b). The T test shows that the bars’ variation rates have a statistical significant variance between the two sub- Flow hydraulics, sediment transport and mid-channel bars stages (Sig = 0.16, ­Sig2-tailed = 0.021 < 0.05) (Table 4). It evolution in braided rivers were closely related (Bridge means that compared to those in natural situation with- 1993; Ashworth 1996; Ashworth et al. 2000). In this paper, out TGD, the mid-channel bars still kept growing in flood sediment carrying capacity was used to quantitative explore season during 2003–2014, but the increase tendency was of the variation mechanism of mid-channel bars before and relatively insignificant (Fig. 4b). after the TGD operation. Thus, mid-channel bars’ evolution was entitled as According to the gravitation theory, the sediment-carry- ‘deposition in the flood season, erosion in the dry season’ ing capacity could be estimated by (Chien and Wan 1983): during the pre-TGD stage, indicating remarkable seasonal changes. Nevertheless, in post-TGD stage, mid-channel 3 m U bars’ variation was summarized as ‘mild deposition in the Svm = k , gh  (6) flood season, deposition in the dry season’. Taken alto- gether, mid-channel bars area showed a notable difference where Svm is the suspended sediment carrying capacity; U following the construction of TGD in 2003. is the flow velocity (m/s); g is the gravitational acceleration

1 3 Environmental Earth Sciences (2018) 77:394 Page 7 of 18 394 1.77 1.87 3.42 3.38 Upper − 0.43 − 0.38 − 0.04 − 0.15 0.36 0.40 95% confidence interval of the dif - ference Lower − 1.20 − 1.29 − 2.67 − 2.73 − 2.55 − 2.45 0.68 0.70 0.51 Std. errorStd. dif - ference 0.51 0.58 0.51 0.68 0.66 0.29 0.29 1.89 1.89 Mean difference − 1.55 − 1.55 − 1.30 − 1.30 0.680 0.690 0.011 Sig (2-tailed) 0.020 0.044 0.031 0.021 0.018 9.5 df 12 8.3 12 9.1 12 9.0 8.9 0.42 0.41 2.80 2.88 test for equality of means equality for T test t − 3.03 − 3.03 − 2.25 − 2.54 0.54 Sig 0.16 0.07 0.54 0.40 Levene’s test for for test Levene’s - of vari equality ances F 2.24 4.12 0.40 Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal test for mid-channel bars variation rates between Chenglingji-Datong reaches Chenglingji-Datong between rates bars variation mid-channel for T test Independent samples 4 Table of significance Sig level Area variation (dry variation Area season) 1989–2000 Group1: 2002–2014 Group2: Area variation (dry variation Area season) 1989–2002 Group1: 2003–2014 Group2: Area variation (dry variation Area season) 1989–2005 Group1: 2007–2014 Group2: Area variation (flood season) variation Area 1991–2000 Group1: 2003–2014 Group2:

1 3 394 Page 8 of 18 Environmental Earth Sciences (2018) 77:394 1.85 1.89 1.01 1.00 0.73 0.70 1.63 1.71 − 0.27 − 0.29 − 0.28 − 0.28 Upper 0.01 0.32 0.25 − 0.06 − 0.10 − 0.06 − 0.06 − 0.02 − 1.48 − 1.45 − 1.03 − 1.03 95% confidence interval of the dif - ference Lower 0.42 0.44 0.23 0.23 0.16 0.15 0.28 0.33 0.27 0.27 0.17 0.17 Std. errorStd. difference 0.89 0.89 0.47 0.47 0.35 0.35 0.98 0.98 − 0.87 − 0.87 − 0.66 − 0.66 Mean difference 0.060 0.070 0.080 0.070 0.060 0.045 0.009 0.014 0.010 0.006 0.003 0.002 Sig (2-tailed) 8.6 9.0 8.7 9.0 6.8 9.0 7.8 8.5 10.0 12.0 10.7 12.0 df 2.13 2.04 2.02 2.02 2.24 2.33 3.46 2.99 − 3.29 − 3.29 − 3.83 − 3.83 test for equality of means equality for T test t 0.50 0.74 0.36 0.24 0.22 0.13 Sig 0.49 0.12 0.92 1.55 1.65 2.66 Levene’s test for for test Levene’s - of vari equality ances F Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal test for mid-channel bars variation rates between Chenglingji-Hankou, Hankou-Jiujiang and Jiujiang-Datong reaches and Jiujiang-Datong Hankou-Jiujiang Chenglingji-Hankou, between rates bars variation mid-channel for T test Independent samples Area variation (flood season JD) variation Area 1991–2000 Group1: 2003–2014 Group2: Area variation (flood season HJ) variation Area 1991–2000 Group1: 2003–2014 Group2: Area variation (flood season CJ) variation Area 1991–2000 Group1: 2003–2014 Group2: Area variation (dry variation Area season JD) 1994–2002 Group1: 2003–2014 Group2: Area variation (dry variation Area season HJ) 1989–2002 Group1: 2003–2014 Group2: 5 Table Area variation (dry variation Area season CJ) 1989–2002 Group1: 2003–2014 Group2:

1 3 Environmental Earth Sciences (2018) 77:394 Page 9 of 18 394 0.44 0.46 0.62 0.70 2.10 2.06 1.01 0.10 Upper − 0.11 − 0.03 − 0.26 − 0.26 0.19 0.21 95% confidence interval of the dif - ference Lower − 0.28 − 0.30 − 1.63 − 1.71 − 1.27 − 1.27 − 0.04 − 0.12 − 0.09 − 0.06 0.17 0.17 0.35 0.35 0.23 0.23 0.15 0.16 0.49 0.46 0.18 0.17 Std. errorStd. difference 0.08 0.08 0.29 0.29 1.00 1.00 0.60 0.60 Mean difference − 0.87 − 0.87 − 0.76 − 0.76 0.640 0.640 0.030 0.040 0.010 0.010 0.080 0.130 0.070 0.060 0.009 0.010 Sig (2-tailed) 8.6 6.4 9 4.8 9 8.5 9.0 8.0 12.0 12.0 12.0 11.7 df 0.48 0.48 1.99 1.84 2.07 2.16 3.33 3.51 test for equality of means equality for T test t − 2.50 − 2.50 − 3.29 − 3.29 0.03 0.01 0.78 0.15 0.51 0.12 Sig 6.32 9.88 0.08 2.45 0.48 2.90 F Levene’s test test Levene’s of equality for variances Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal Equal variances assumed variances Equal Equal variances not assumed not variances Equal test for mid-channel bars variation rates in straight, bending and goose-head-shape braided rivers bending and goose-head-shape braided in straight, rates bars variation mid-channel for T test Independent samples 6 Table Area variation (dry variation Area season straight) 1991–2000 Group1: 2003–2014 Group2: Area variation (dry variation Area season bending) 1991–2000 Group1: 2003–2014 Group2: Area variation (dry variation Area season goose-head) 1991–2000 Group1: 2003–2014 Group2: Area variation (flood season straight) (flood season variation Area 1991–2000 Group1: 2003–2014 Group2: Area variation (flood season bending) variation Area 1991–2000 Group1: 2003–2014 Group2: Area variation (flood season goose-head) variation Area 1991–2000 Group1: 2003–2014 Group2:

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Fig. 4 The overall change trend of mid-channel bars from Chenglingji to Datong: a in flood season from 1991 to 2014. b in dry season from 1989 to 2014

Fig. 5 The mid-channel bars area statistic in different reaches in dry/ reach in dry season; d CH reach in flood season. e HJ reach in flood flood season;a Chenglingji–Hankou (CH) reach in dry season. b season. f JD reach in flood season Hankou–Jiujiang (HJ) reach in dry season. c Jiujiang–Datong (JD)

Changes in the mid‑channel bars along the river Specifically, in the dry season before TGD operation, the course total area of the mid-channel bars was about 55 km2 in CH, 95 km2 in HJ, and 325 km2 in JD reach, respectively. In flood There are obvious spatial differences of the mid-channel season, the mid-channel bars areas in the three reaches were bars along the course of the Changjiang River (Fig. 5). approximately 37, 75 and 280 km2, respectively. Similar

1 3 Environmental Earth Sciences (2018) 77:394 Page 11 of 18 394 seasonal variety can be found in flood/dry seasons during variations in the three reaches basically maintains the previ- post-TGD period (Fig. 5). ous forms, but with smaller variation rates (Fig. 5d–f). By Meanwhile, in dry season, the mid-channel bars area the independent samples of T test, there were insignificant in CH and HJ decreased significantly before the impound- differences in bars variations between pre- and post-TGD ment of TGD, with the inter-annual reduction rate reach- stages along the Changjiang River, except the CH reach 2 ing 0.67, and 0.21 km /year, respectively, (Fig. 5a–c). Bars (CH reach: Sig = 0.36, Sig­ 2-tailed = 0.045 < 0.05; HJ reach: area in JD reach presented significant increase with a rate Sig = 0.74, ­Sig2-tailed = 0.07 > 0.05; JD reach: Sig = 0.5, 2 of 0.79 km /year during 1994–2002. It was noted that the ­Sig2-tailed = 0.07 > 0.05) (Table 5). year of 1989 was excluded from the analysis because of the abnormal broken of Mianchuan bar and Xiasanhao bar, Changes of mid‑channel bars located in different which can affect the accuracy of the result. Nevertheless, river patterns after the TGD operation, mid-channel bars in CH and HJ reach turned from erosion to deposition with an increas- In dry season, mid-channel bars area in straight and bend- ing rate of 0.3 and 0.49 km2/year, respectively. On the ing braided river manifested a decreased trend between contrary, mid-channel bars in JD reach experienced ero- 1989 and 2002 with a decreasing rate of 0.62 and 0.42 km2/ sion, indicating an average yearly declining area of 0.3 km2 year, respectively (Fig. 6a, b). However, goose-head- (Fig. 5a–c). Moreover, all reaches had significant difference shaped braided river presented slight deposition at a rate between pre- and post-TGD periods according to T test of 0.15 km2/year in bars area (Fig. 6c). Along with the (CH reach: Sig = 0.13, Sig­ 2-tailed = 0.002 < 0.05; HJ reach: TGD operation in 2003, change tendencies of the three Sig = 0.22, ­Sig2-tailed = 0.006 < 0.05; JD reach: Sig = 0.24, river types had all changed. There were obvious devia- ­Sig2-tailed = 0.010 < 0.05) (Table 5). tion of bars variation rates between pre- and post-TGD Nevertheless, in flood season, the growth area of mid- stages, except for straight braided river (straight braided: channel bars mainly concentrates on the JD reach with an Sig = 0.03, ­Sig2-tailed = 0.64 > 0.05; bending braided: 2 increasing rate of 0.86 km /year during 1991–2014. The Sig = 0.01, ­Sig2-tailed = 0.04 < 0.05; goose-head-shaped other two reaches showed slight deposition or even ero- braided: Sig = 0.78, ­Sig2-tailed = 0.007 < 0.05) (Table 6). sion with an increasing rate of 0.068 and − 0.039 km2/year, Specifically, for the mid-channel bars in the straight braided respectively. After TGD construction, the mid-channel bars river, while remaining erosion, their average inter-annual

Fig. 6 The mid-channel bars area statistic under three river patterns season. d Straight braided river in flood season.e Bending braided in dry/flood season;a Straight braided river in dry season. b Bending river in flood season.f Goose-head-shaped braided river in flood sea- braided river in dry season. c Goose-head-shaped braided river in dry son

1 3 394 Page 12 of 18 Environmental Earth Sciences (2018) 77:394 area reduction rate was significantly lower (the decreasing Discussion rate was 0.131 km2/year). Mid-channel bars in the bending braided river turned from erosion to deposition (the depo- Variations in water and sediment pre‑ and post‑dam sition rate could reach 0.3 km2/year). In the goose-head- periods shaped braided river, mid-channel bars area kept the growth trend with the yearly average deposition area increasing from The natural hydrological regime and sediment transport pro- 2 2 0.15 km during 1989–2002 to 0.65 km during 2003–2014 cesses along the Changjiang River have been altered by the (Fig. 6a–c). TGD operation (Yang et al. 2006; Mei et al. 2015; Dai et al. However, during 1991–2014, mid-channel bars area in the 2016), which could be likely responsible for the changes in three river patterns all showed increasing trends in flood sea- mid-channel bars. Runoff and sediment in Changjiang River 2 son, with an annual increase rate of 0.27, 0.74 and 0.04 km / were mainly concentrated in the flood season (Table 7). year, respectively (Fig. 6d–f). Unlike the bars’ variations in Mid-channel bars in CD reach generally indicated deposi- dry season, only goose-head-shaped braided river showed tion in flood season before TGD operation due to rich sedi- significant difference following the operation of TGD (T ment materials and decreasing flow velocity when the high test, Sig = 0.12, ­Sig2-tailed = 0.009 < 0.05) (Table 6), with water flows over the bars surface (Yu 2005; Li 2011; Pan 2 bars area decreasing by 0.17 km /year (Fig. 6f). Meanwhile, 2011). While the annual discharge revealed slight change straight and bending braided river experienced insignificant during 1991–2014, the seasonal features indicated signifi- difference in bars area between pre- and post-TGD peri- cant variations. For instance, the regulation of TGD not only ods (T test, Sig = 0.15, Sig­ 2-tailed = 0.08 > 0.05; Sig = 0.51, decreased the runoff amount in flood season (Fig. 7), but Sig­ 2-tailed = 0.07 > 0.05) (Table 6). Mid-channel bars in above also cut down the magnitude and frequency of high flow two river types still kept growth trends in post-TGD period, (Mei et al. 2015). Specifically, the days with great discharge 2 but the increasing rates reduced to 0.32 and 0.66 km /year at Luoshan (> 30,000 m3/s), Hankou (> 35,000 m3/s) and (Fig. 6d, e), respectively. Datong (> 40,000 m3/s) fall from 91 in pre-TGD stage to 66 In summary, goose-head-shaped braided river indicated in post-TGD stage, from 74 to 49 and from 90 to 66, in turn the most remarkable variation in these three river patterns, (Fig. 8). Moreover, since TGD began to operate in 2003, which deposited in both flood and dry season during pre- the sediment load has reduced drastically (Yang et al. 2006, TGD stage, but turned into erosion in flood season and more 2007a; Dai and Liu 2013; Dai and Lu 2014; Dai et al. 2016), deposition in dry season after 2003. especially in flood season (Fig. 9). For example, the sedi- ment load at Yichang, Luoshan, Hankou, Datong stations decreased by 89, 73, 68, 61% following the construction of TGD in comparison with the natural situation (Table 7; Fig. 9). Due to shorter duration of high discharge and sharp reduction in fluvial sediment load, the bars’ growth trend in the flood season weakened, relatively (Fig. 4b). On the other hand, if sediment release is coupled with relatively large

Table 7 The flood and dry Runoff volume dry Runoff volume flood Sediment load dry Sediment load season data about flow and season ­(109 m3) season ­(109 m3) season ­(104 t) flood season sediment in four main stations ­(104 t)

Yichang Pre-dam 957 3377 976 39,523 Post-dam 988 3005 56 4301 Luoshan Pre-dam 1798 4822 4933 29,261 Post-dam 1702 4220 1957 7630 Hankou Pre-dam 2034 5273 3839 28,987 Post-dam 1850 4342 1943 8993 Datong Pre-dam 2848 6656 4293 29,329 Post-dam 2630 5779 2799 11,214

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Fig. 7 The monthly average flow of four hydrologic stations in pre- and post-TGD periods (pre-TGD: 1989–2002; post-TGD: 2003–2014)

Fig. 8 The duration of high discharge in three stations in pre-and post-TGD periods

Fig. 9 The monthly average sediment load at four hydrologic stations in pre- and post-TGD periods (pre-TGD: 1989–2002; post-TGD: 2003– 2014)

1 3 394 Page 14 of 18 Environmental Earth Sciences (2018) 77:394 floods, their deposition occurs on a relatively elevated area (Table 7). These hydrological differences were consistent of the bars, which increase the bars elevations instead of the with the mid-channel bars area spatial variations, namely, bars area (Asaeda and Rashid 2012). Under this condition, the further away from the dam, a weaker effect is reflected. the lateral increasing tendencies of sandbar area could be In addition, mid-channel bars area in JD reach had different unobservable (Fig. 5d–f, 6d–f). change trend from the upstream owing to the supplement of Besides, TGD’s influence on flow and sediment exhib- flow and sediment from the Poyang Lake. ited spatial differences along the Changjiang River because of downstream tributary input and riverbed compensation Variation of sediment carrying capacity (Yang et al. 2007a, b) (Table 7), which made the bars in along the river course lower reach more developed. Specifically, Yichang station, 37 km immediate to the TGD, showed the most remark- River sediment concentration and sediment carrying able hydrological variations along with TGD construc- capacity directly dominated scour and deposition in the tion, where the annual sediment load reduced to 4 × 108 t river channel (Petts 1979; Curtis et al. 2010), which, in the post-TGD period, only about one-tenth of its original accordingly, could quantitatively explain the mid-channel value. At Jingjiang reach, directly below TGD, occurred bars’ area change. Based on the linear simulation curves of the greatest erosion after 2003 (Yang et al. 2015; Zhang discharge with suspended sediment concentration (SSC), et al. 2017). Meanwhile, annual sediment load in Luoshan, water depth and average flow velocity (Fig. 10), the sedi- Hankou and Datong reduced by 71, 66, 58% respectively ment carry capacities in Luoshan, Hankou and Datong

Fig. 10 Relationships between discharge and SSC, and flow velocity (U), and water depth at three hydrologic stations in pre-and post-TGD peri- ods

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Fig. 11 Sediment carrying capacity and actual suspended sediment concentration (measured SSC) at three hydrologic stations in pre- and post- TGD periods could be obtained under different hydrological condi- Impact of geomorphic conditions tions (Fig. 11). Compared with pre-TGD period, the sedi- ment carrying capacity decreased obviously in post-TGD As for the alluvial rivers, the lateral changes of river bed not period, as sediment grain in middle and lower reaches of only depended on the sediment and flow from upstream, but the Changjiang River have increased gradually (Fig. 11). also were affected by the original environment conditions, In dry season, the SSC is generally smaller than the such as the river patterns and river bed forms (Friedman sediment carry capacity before TGD operation, suggest- et al. 1998). ing that the sediment in water was starving (Fig. 11). In The channel nodes are typical topographic conditions in such case, mid-channel bars would be eroded to keep the the middle and lower reaches of the Changjiang river, which balance between flow and sediment, which satisfies the can control the river regimes and affect the channel evolution concepts of the graded river (Hoover Mackin 1948) and (Chien 1987). Due to the effect of deflecting flow of channel dynamic equilibrium of Hack (1960). However, after TGD nodes under different discharges, various sediment scour- operation, sediment carrying capacity of CH, HJ and JD ing and depositing degrees could occur (Yu 1987). Under reach dropped to 23, 31, 76% of the initial values in turn, low flow conditions, due to the constraint of channel nodes, while their corresponding SSC just reduced to 38, 42, discharges are concentrated in the river channel and corre- 55%, respectively. These variations were in agreement spondingly water velocity increases, which together led to with the bars changes along the course of the Changji- the bars erosion. The restraint effect of channel nodes turns ang River in dry season. Furthermore, sediment carrying out to be relatively weak under the high discharge conditions capacity of CH reach reduced to 14% of the pre-TGD level (Xia and Yan 2000; Zhu et al. 2015). With the TGD opera- in flood season, while that in HJ and JD reduced by 87 and tion, the natural flow regime was replaced by the regulated 51%, respectively. In the meantime, the suspended sedi- one (Curtis et al. 2010; Mei et al. 2018). The effect of the ment concentrations of these three reaches declined to 36, channel nodes, as a result, also has been adjusted, which 38, 61% of the pre-TGD value. further caused the variation of mid-channel bars. Besides, the clear water released from TGD would scour downstream channels and as a consequence, take the coarse riverbed particles to the downstream, especially the coarse Table 8 The geometrical morphology of different river patterns sand (D > 0.125 mm) (Zhang et al. 2017). Sediment coars- ening along the downstream reach coupled with sediment River pattern Widening rate Tortuous rate Branch- ing carrying capacity decrease in post-TGD period were likely numbers to promote the sediment deposition in the mid-channel bars (Yuan and Li 2016; Yuan et al. 2012). To make things worse, Straight braided 2.17 1.08 2.3 the shallow water in dry season mainly covered coarser par- Bending braided 4.21 1.27 2.3 ticles (Rust 1972; Ashworth 1996), which further contrib- Goose-head-shaped 6.72 2.04 3.4 braided uted to, mid-channel bars deposition.

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Expect the influence of channel nodes, river patterns, lowing the construction of TGD. Therefore, mid-channel including the widening rate (the ratio of full branches’ bars in Chenglingji–Datong reach is characterized by widths and import straight channel width), tortuous rate (the ‘remarkable seasonal difference in pre-TGD period, and ratio of the branch channel arc length and its straight length) mild seasonal pattern in post-TGD period’. and branching numbers (Yu 2005; Li 2011), also affected 2. Owing to the effect of tributary input, mid-channel the bars development. Geometrical morphology of straight, bars in CH, HJ, JD reaches showed different degrees of bending and goose-head-shaped braided river patterns in the variation in response to the regulation of TGD. Farther middle and lower reaches of Changjiang river are summa- downstream from TGD, the bars variation is weaker. rized in Table 8 (Chien 1987). Besides, mid-channel bars variation in JD reach in dry Generally, the route of discharge tends to be straight in season was contrary to the upstream as a result of the flood season and bent in dry season (Chien1987 ). Accord- Poyang Lake influence, indicating the self-adjustment ingly, in flood season, mid-channel bars in straight braided of the river system. river with the least tortuous rate (1.08) could obtain a sig- 3. Mid-channel bars in different river patterns had different nificant amount of sediment from flood. Besides, it generally responses to TGD-induced water and sediment varia- developed the narrow and low mid-channel bars, and had tion. Among the three river types, goose-head-shaped poor stability. Therefore, straight braided river presented river pattern indicated the most remarkable alteration larger growth rate than the other two river patterns in flood in mid-channel bars. Specifically, the bars’ deposition season. In dry season, due to greater tortuous rate (2.04) and rate increased from 0.155 km2/year during 1989–2002 the existence of the natural rocky nodes, the import flow to 0.53 km2/year in dry season following the construc- could be restricted, which facilitated the settling of sedi- tion of TGD. In flood season, however, the bars showed ment in goose-head-shaped braided river and make the bars erosion with a rate of 0.17 km2/year after 2003. develop (Ma and Gao 2001). In addition, comparing with the other two river patterns, Above all, this study shows the changing trend of mid- goose-head-shaped braided river had greater widening rate, channel bars in the middle and lower reaches of the Changji- tortuous rate and more branch channels (Table 8), which ang River in response to TGD regulation. Such knowledge could result in a shorter cycle of mutual transformation is essential for the sustainable management of channel bars between the main branch and other branches, as well as more and can provide scientific support to other mega rivers under unstable mid-channel bars evolution (Brice 1982). There- a similar context. fore, goose-head-shaped braided river was most sensitive to the water and sediment variation, where the mid-channel Acknowledgements This study was supported by the National Science Foundation of China (NSFC) (41706093). The Laboratory for Marine bars had the greatest variation since the TGD construction Geology, Qingdao National Laboratory for Marine Science and Tech- in 2003 (Ma and Gao 2001). nology (MGQNLM201706) and the Key Laboratory of Coastal Science and Engineering, Beibu Gulf, Guangxi (2016KYB01).

Conclusion Author contributions ZD jointly conceived the study. JW collected the field data and processed the remote sensing images. WW and YL undertook the sediment carry capacity computation. XM provided As important geomorphic features and components are valuable suggestions on the mid-channel bars evolution analysis. All present in middle and lower reaches of Changjiang River, co-authors contributed to the discussion. YL drafted the main manu- mid-channel bars development has important influence on script, which was then commented and edited by XM and ZD. the river channel stability. However, under the influence of the TGD, the downstream hydrologic regime have dramati- Compliance with ethical standards cally changed, which also affected the mid-channel bars. Conflict of interest The authors declare no conflict of interest. Therefore, this paper explored the variation of mid-channel bars and potential driving factors of these changes using the remote sensing method. The main conclusions were sum- marized below: References 1. The mid-channel bars in Chenglingji–Datong reach 2 Abutaleb AS (1989) Automatic thresholding of gray-level pictures experienced erosion in dry season (− 0.89 km /year) using two-dimensional entropy. Comput Vis Graph Image Pro- during 1989–2003 and turned from erosion to deposition cess 47:22–32 with a growth rate of 0.82 km2/year after 2003. In flood Asaeda T, Rashid MH (2012) The impacts of sediment released from dams on downstream sediment bar vegetation. J Hydrol season, bars kept silting up state with an increasing rate 430–431(8):25–38 of 1.05 km2/year. However, the growth rate reduced fol-

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