Article Long-Term (1986–2018) Evolution of Bars in Response to Combined Effects of Cascade in the Middle Reaches of the Hanjiang

Yingying Zhang 1,2,3, Xiaobin Cai 1,3,4, Chao Yang 1,3,4, Enhua Li 1,3,4,*, Xinxin Song 5 and Xuan Ban 1,3,4

1 Institute of Geodesy and Geophysics, Chinese Academy of Sciences, 430077, ; [email protected] (Y.Z.); [email protected] (X.C.); [email protected] (C.Y.); [email protected] (X.B.) 2 University of Chinese Academy of Sciences, 100049, China 3 Key Laboratory of Monitoring and Estimate for Environment and Disaster of Province, Wuhan 430077, China 4 Station for Ecosystem Research, Chinese Academy of Sciences, Honghu 433200, China 5 Normal University, Zhengzhou 450044, China; [email protected] * Correspondence: [email protected]

 Received: 20 November 2019; Accepted: 29 December 2019; Published: 31 December 2019 

Abstract: Channel bars are essential and their evolution is crucial to aquatic and riparian biodiversity, river’s water- process, and economic development. With the development of water conservation facilities and projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade across the entire downstream area. Using Landsat images from 1986–2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1–DX7, XZ1, and XZ2), and three group (XZ3–XZ5) dynamics. Results showed that the mid-channel bars’ area in the reach from Danjiangkou to (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars’ area change was mainly influenced by backwater from the Cuijiaying with high water level after 2010 (r = 0.93, − p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = 0.79, p < 0.05), which was partly attributed to upstream − precipitation according to the high correlation between runoff and precipitation (R2 = 0.65). In general, the DX section was under equilibrium between scouring and compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, , flood events, mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.

Keywords: channel bars; morphology; cascade reservoirs; Landsat; hydrological regime; Hanjiang River

Water 2020, 12, 136; doi:10.3390/w12010136 www.mdpi.com/journal/water Water 2020, 12, 136 2 of 18

1. Introduction As an important component of river morphodynamics, channel bars can preserve biodiversity in river corridors and provide for certain organisms [1]. The middle reaches of the Hanjiang River have developed a certain number of river bars, which have important influence on the stability of channel, navigation channel regulation, wetland biodiversity conservation, and agricultural production. Dams and reservoirs interrupt and modify fluvial system downstream flux of sediment through watersheds, impact the flow regime, and can alter the entire hierarchy of channel variables [2,3]. Dams exert significant geomorphic control over as the backwater effects of a downstream reservoir begin to occur [4], creating new geomorphologic situations and directly affecting the channel and riparian environments [5,6]. Bars not only protect the diversity of the , but also provide information about riverine active processes and the sediment regime for understanding fluvial processes and their controlling factors [7]. Therefore, better understanding of dams, especially the cascade reservoirs impacts on morphological adjustments of channel bars, is required in river management. There are many researches using the filed data collection and laboratory analysis about the changes of channel morphology evolution with the influence of dams and other engineering projects [3,8,9]. With the development of remote sensing (RS) technology and availability of both optical and radar RS data, riverbank migration and channel bars evolution have been widely studied before in dammed around world. For instance, Wang and Xu assessed channel bar morphologic changes in the highly regulated lower River using Landsat imagery and river stage data [10,11]. Capolongo et al. coupled multitemporal remote sensing with and hydrological modelling for post-flood recovery in the Strymonas dammed river basin [12]. There also have been many studies on the morphodynamic processes of sandbars in the River, the channel downstream of the (TGD) in China [13–18]. The decreased sediment load in the middle of the Yangtze River was found to be responsible for dramatic changes of channel bar morphodynamics, which could last for a long time, depending on the operation of the Three Gorges Dam in 2003 [19]. Morphological adjustments in the meandering reaches of the middle Yangtze River were caused by dramatic human activities [20]. The Garrison and Oahe dams in the River have been used to demonstrate the impacts of an upstream dam, which maintains significant geomorphic control over river morphology as the backwater effects of downstream reservoir begin to occur [4]. However, previous studies are mostly the effects of single dam and river training works on channel bars, and little attention is paid to the area affected by cascade dams. The Danjiangkou Water Conservancy Project is the water source area for the South-to-North Water Transfer Project middle route and is a key project for comprehensive development and utilization of in the Hanjiang River. The processes of water and sediment enter the lower reaches of the dam have changed fundamentally since the construction of the [21,22]. In addition, six cascade hydropower stations have been planned in the middle reaches: Danjiangkou, Wangfuzhou, Xinji (under construction), Cuijiaying, Yakou (under construction), and Nianpanshan (under construction). Subsequently, more studies have paid attention to the impact of this change from different aspects with the transformation from natural flowing river to dam-reservoir-river system. In the 1990s, there were many studies in the Hanjiang River about the Danjiangkou reservoir effect on suspended sediment [23], zones [24], [25], and adjustment and evolution of mid-channel bars [26]. Recent researches have only focused on the hydrological regime [27–31], hydrological models’ predictions [32], and water resources allocation [33]. However, few studies have extracted information and analyzed the influence factors for channel bars in response to the combined effects of Danjiangkou Reservoir and other cascade reservoirs. In addition, the long-term analysis of channel bars’ evolution from 1986 to 2018 will provide data and support for further research about channel bar morphodynamics in the middle reaches of the Hanjiang River when all the dams are built completely. Water 2020, 12, 136 3 of 18

The main objectives of this study were to: (1) quantitatively evaluate channel bar spatial and temporal variability along the river from 1986 to 2018; (2) evaluate the factors combined with annual runoff and sediment data measured at hydrological stations; and (3) provide suggestions for channel management under the background of cascade reservoirs development from the perspective of river ecology.

2. Study Area and Data

2.1. Study Area As a sand-bed river, the Hanjiang River is the largest of the Yangtze River, and the Danjiangkou Reservoir is a water source area for the South-to-North Water Transfer Project middle route. The middle reaches of the Hanjiang River lie between the Danjiangkou Reservoir and the Huangzhuang hydrological station, which controls a 46,800 km2 catchment area with a length of about 240 km and with a channel width of about 1 km [24,34]. The area has average annual temperatures of 15–17 ◦C, and annual precipitation ranges from approximately 700 mm to 1200 mm [35]. The terrain is made up of hills and plains, of which 51.6% is plains area, 25.4% is mountainous, and 23% is hilly [36]. Six cascade reservoirs [29,37] have been planned in the middle reaches (Table1). The Danjiangkou Reservoir construction began in September 1958 and was stopped in December 1959. After eight years of detention, it was officially put into service in November 1967 and has been impounded for more than 50 years. Construction of a second phase heightening project for the South-to-North Water Transfer Project started in September 2005 and was completed in 2010. At that time, the Danjiangkou Reservoir changed from an annual regulation reservoir into an incomplete multi-year regulation reservoir. The dam crest elevation was increased from 162 m to 176.6 m with a corresponding storage capacity was 29.05 billion cubic meters [30]. Huangjiagang hydrologic station lies 6 km downstream of the and is the outlet control station. The average annual discharge, average annual runoff amount, and average annual sediment concentration of Huangjiagang was 1080 m3/s, 340 × 108 m3, 0.03 kg m 3, respectively, during the impoundment stage (1968–2004) of the Danjiangkou · − Reservoir [38].

Table 1. Information of the cascade reservoirs in the middle reaches of Hanjiang River.

Normal Distance from Normal Year Reservoir Storage Level Danjiangkou Regulation Ability Capacity Constructed (m a.s.l.) * Dam (km) (million m3) 157.0 0.0 year 17,450 1973 Danjiangkou 170.0 0.0 Multi-year 29,050 2013 Danjiangkou Wangfuzhou 86.2 30.0 reservoir reverse 149.5 2000 regulation under Xinji 76.2 89.7 day 301.2 construction Cuijiaying 62.7 134.0 day 245.0 2010 under Yakou 55.2 201.0 day 608.0 construction under Nianpanshan 49.2 263.0 - 877.0 construction Note: * above level (a.s.l.), the sea level is based Wusong datum of the China in here and for the rest of this paper.

The study area can be divided into the branching reach of Danjiangkou-Xiangyang (DX) and the meandering reach of Xiangyang-Huangzhuang (XZ) [26,39]. From Danjiangkou to Xiangyang, the river flows through shallow hills with many branching channels, and the main river changes dramatically. It is the key section for water transport construction in the Tenth Five-Year Plan of Hubei Province. There are 10 original shoal groups, mostly branching and transitional, which hinder Water 2020, 12, 136 4 of 18

navigation due to their shallowness and danger. After the construction of the Wangfuzhou Reservoir, two were inundated. XZ is 153 km long and is a typical meandering reach with a wide and shallow riverbed, scattered flow, and dense [39]. There are two different river types in the middle reaches of the Hanjiang River, therefore DX and XZ channel bars were analyzed separately. Seven typical mid-channel bars (DX1–DX7) were studied in DX, while two channel bars and three Watershoal 2020 groups, 12, x FOR (XZ1–XZ5) PEER REVIEW were chosen in XZ (Figure1). 4 of 18

FigureFigure 1. LocationLocation of of the the cascade cascade reservoirs reservoirs and and channel channel bars bars in in the middle reaches of the Hanjiang RiverRiver (channel (channel bars bars are are hi highlightedghlighted with Landsat-8 (Operational (Operational Land Land Imager, Imager, OL OLI)I) imagery from 20182018 (bands combination of of Short-wave infrared (SWIR1), Near-infrared Near-infrared (NIR), (NIR), and and Red), and and Xiangyang-Huangzhuang 3 (XZ3), XZ4, and XZ5 ar aree shoal groups displayed in the red area).area).

2.2. Data 1. Information of the cascade reservoirs in the middle reaches of Hanjiang River.

To analyze theNormal impacts of cascadeDistance from dams on the temporal and spatialNormal evolution of channel bars sinceReservoir 1986, LandsatStorage Thematic Level Danjiangkou Mapper (TM) Dam/Enhanced Regulation Thematic Ability MapperCapacity Plus (ETMYear+)/ OperationalConstructed 3 Land Imager (OLI)(m a.s.l.) images * were selected(km) during six low-water periods(million in 1986, m 1994,) 2000, 2004, 2010, 157.0 0.0 year 17,450 1973 Danjiangkou 2013, and 2018. They170.0 were available0.0 free of charge fromMulti-year the USGS (https:29,050//glovis.usgs.gov 2013/) and the study area spans two path/row tracks: 124/38 andDanjiangkou 125/38 (Table 2). To eliminate the influence of waterWangfuzhou level on extracted86.2 channel bar 30.0 area, dry periodreservoir images reverse that were available149.5 in the selected2000 years (January–March and October–December) were compared.regulation There were few appropriate dry season Xinji 76.2 89.7 day 301.2 under construction images in 1994 for path 125 and row 38, so February 1995 was used instead. The images with the Cuijiaying 62.7 134.0 day 245.0 2010 smallestYakou inter-annual55.2 water level di 201.0fference within the day available image 608.0 range wereunder used, construction and the correspondingNianpanshan dates49.2 and water levels 263.0 of selected image are- listed in Table 877.02. Hydrologicalunder construction data on the averageNote: * above sediment sea level concentrations (a.s.l.), the sea of level Huangjiagang, is based Wusong Xiangyang, datum of and the Huangzhuang China in here and were for collected the fromrest the of Changjiang this paper. Water Resources Commission of the Ministry of Water Resources, China. Daily

2.2. Data To analyze the impacts of cascade dams on the temporal and spatial evolution of channel bars since 1986, Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper Plus (ETM+)/Operational Land Imager (OLI) images were selected during six low-water periods in 1986, 1994, 2000, 2004, 2010, 2013, and 2018. They were available free of charge from the USGS (https://glovis.usgs.gov/) and the study area spans two path/row tracks: 124/38 and 125/38 (Table 2). To eliminate the influence of water level on extracted channel bar area, dry period images that were available in the selected years (January–March and October–December) were compared. There were few appropriate dry season

Water 2020, 12, 136 5 of 18 water level and runoff data were retrieved from the Hubei Provincial Department of Water Resources (http://www.hubeiwater.gov.cn/).

Table 2. The 30 m resolution remote sensing data and corresponding water levels of Huangjiagang hydrologic station in the study area.

124/38 125/38 Water Level Water Level Data Source Date Data Source Date (m a.s.l.) (m a.s.l.) Landsat-5 TM 3 December 1986 88.09 Landsat-5 TM 10 December 1986 87.79 Landsat-5 TM 7 November 1994 - Landsat-5 TM 2 February 1995 88.01 Landsat-5 TM 23 November 2000 89.03 Landsat-7 ETM+ 24 December 2000 88.82 Landsat-5 TM 18 November 2004 88.89 Landsat-5 TM 8 October 2004 88.86 Landsat-5 TM 5 December 2010 88.68 Landsat-5 TM 26 November 2010 88.53 Landsat-8 OLI 29 December 2013 88.7 Landsat-8 OLI 4 December 2013 88.73 Landsat-8 OLI 26 February 2018 88.83 Landsat-8 OLI 21 March 2018 88.91 Notes: TM, Thematic Mapper; ETM+, Enhanced Thematic Mapper Plus; OLI, Operational Land Imager; and the blank is the data missing.

3. Methods

3.1. Channel Bar Extraction Methods Due to the different spectral information between water and channel bars, the water area in the river channel can be extracted with high accuracy, leaving the bars for easy identification. Therefore, accurate water extraction is key to extracting channel bars. Globally, there have been a number of studies on water extraction with many methods applied. Of these, the modified normalized difference water index (MNDWI) spectral water index can extract water body information more accurately, quickly, and easily than other methods [40–43]. Li et al. used 81 phases of time-series Landsat remote sensing images (TM Landsat-4/5, ETM+ Landsat-7, and OLI Landsat-8) to extract the Danjiangkou Reservoir area from 1993 to 2015 [44]. Therefore, the MNDWI was selected as the primary tool for automatically extracting channel bars in this study. First, the raw calibrated pixel values in the selected TM, ETM+, and OLI images were converted to surface reflectance using ENVI Landsat Calibration (ENVI 5.3 software, ITT Visual Information Solutions, the ). Second, the TM, ETM+, and OLI reflectance data were used to calculate MNDWI for selected images based on Equation (1). The green and mid-infrared (MIR) bands from TM, ETM+, and OLI data are shown in Table3. Third, the water surface was extracted by setting a threshold in the MNDWI images that was determined by the Otsu method [42,44]. Finally, the resulting binary images were converted into vector polygons using the Raster to Polygon tool provided by ArcGIS 10.4 (ESRI, Redlands, CA, USA) (Figure2). Band BandMIR MNDWI = Green − (1) BandGreen + BandMIR

Table 3. Different sensors correspond to band information of Green and MIR.

Sensor Band Wavelength (µm) B2(Green) 0.52–0.60 Landsat-5 TM B5(SWIR) 1.55–1.75 B2(Green) 0.53–0.61 Landsat-7 ETM+ B5(SWIR) 1.55–1.75 B3(Green) 0.53–0.59 Landsat-8 OLI B6(SWIR1) 1.57–1.65 Notes: Mid-infrared MIR; Short-wave infrared, SWIR. Water 2020, 12, x FOR PEER REVIEW 6 of 18

Table 3. Different sensors correspond to band information of Green and MIR.

Sensor Band Wavelength (μm) B2(Green) 0.52–0.60 Landsat-5 TM B5(SWIR) 1.55–1.75 B2(Green) 0.53–0.61 Landsat-7 ETM+ B5(SWIR) 1.55–1.75 B3(Green) 0.53–0.59 Landsat-8 OLI Water 2020, 12, 136 B6(SWIR1) 1.57–1.65 6 of 18 Notes: Mid-infrared MIR; Short-wave infrared, SWIR.

Figure 2. Channel bar automatic extraction steps (example using Landsat-8 image in false color with the Short-wave infrared (SWIR),(SWIR), Near-infraredNear-infrared (NIR),(NIR), andand RedRed bandsbands fromfrom 2626 FebruaryFebruary 2018).2018).

3.2. Analytical Methods Setting the channel bar area extracted for 1986 as a reference, the area change rates of each channel bar were calculated and compared, where positive values indicate an an increase increase and and vice vice versa. versa. Equation (2) for calculating the area change percentage (ŋ) relative to 1986 area is as follows (A is Equation (2) for calculating the area change percentage (ŋ) relative to 1986 area is as follows (A19861986 is the channel bar area in 1986, A is the channel bar area in n): the channel bar area in 1986, An is the channel bar area in n):

𝐴A−n 𝐴A1986 NGŋ== − × 100%100% (2) (2) 𝐴A1986 × The areaarea valuesvalues of of selected selected river river islands during during 1986–2018 1986–2018 were were determined determined by processing by processing remote sensingremote sensing images. Toimages. understand To understand the area change the area trends, change the trends, anomalies the for anomalies the total areafor the of islandstotal area in DX of andislands XZ in were DX calculatedand XZ were using calculated the formula: using the formula: ∆𝐴 = 𝐴 − 𝐴 ∆A = A A (3)(3) yd yd − d where d refers to a given , y refers to a given year, ∆𝐴 is the area anomaly of a given island where d refers to a given island, y refers to a given year, ∆Ayd is the area anomaly of a given island in a in a given year, 𝐴 is the area of a given island in a given year, and 𝐴 is the 33-year average area givenof the year,islands.Ayd Theis the correlation area of a between given island all river in a givenisland year, area andtotalsA dandis the influencing 33-year average factors arealike ofwater the islands.level, runoff, The correlationand sediment between were calculated all using area Pearson’s totals correlation and influencing coefficient. factors like water level, runoffThe, and Mann-Kendall sediment were (M-K) calculated non-parametric using Pearson’s statistical correlation test developed coefficient. by H.B. Mann (1945) and M.G.The Kendall Mann-Kendall (1990) can ha (M-K)ndle non-parametricnon-normal and statisticalcensored testdata developed[45], and is bywidely H.B. used Mann to (1945) assess andthe M.G. Kendall (1990) can handle non-normal and censored data [45], and is widely used to assess the significance of monotonic trends in hydro-meteorological time series [46,47]. We therefore performed M-K testing using the average hydrological data values for the dry season of January–March and October–December from 1986–2018 in MATLAB R2014a (MathWorks Company, the United States). The M-K abrupt change test assumes (null hypothesis) that the time series under investigation shows no beginning of a developing trend. It calculates two standardized statistic series, UF (the forward sequence, which has a standard normal distribution) and UB (the backward sequence and is calculated similarly to UF with an inverse time series), for a data series and plots them with confidence lines [48]. Water 2020, 12, 136 7 of 18

The null hypothesis is rejected and significant abrupt change occurs if the UF curve and the UB curve intersect within the confidence zone. This study takes the confidence level of 95% (1.96 and 1.96) as − the boundary lines of the confidence zone. The detailed formulas of Mann-Kendall rank trend test statistic Z and UF and UB statistic indices are provided in the literature [35].

4. Results

4.1. Long-Term Hydrologic Conditions The Danjiangkou Reservoir was built in 1959 and completed in 1973. From 1959 to 1967, the reservoir was used for flood retention. After that time, sediment concentrations began to decrease and had reached an average annual sediment concentration of about 1%, which was 10% of that pre-dam period at the Huangjiagang and Huangzhuang stations (Table4). Water level variability at the Huangjiagang, Xiangyang, and Huangzhuang stations during the dry season from 1986–2018 as estimated by M-K trend test is shown in Figure3a–c. Only one intersection could be identified as a water level change at the significance level of 0.05, at the Huangjiagang station in 1999 (Figure3a). An increasing trend was after 2003 (p < 0.05). This increase was mainly caused by the construction of the Wangfuzhou Reservoir in 2000. There was another water level mutation in 2014 at the 0.05 significance level at the Xiangyang station, and it clearly increased after 2009 (Figure3b), which may have been related to the construction of the Cuijiaying Reservoir in 2010. However, the water level at Huangzhuang significantly declined after 2006 (Figure3c). Table5 shows the amplitude of annual variability. We found that water levels at Huangjiagang and Xiangyang station increased significantly by 0.04 cm/year and 0.12 cm/year over the study period with Z statistic values of 4.08 and 2.22, respectively. They decreased significantly by 0.06 cm/year at Huangzhuang station with a Z statistic of 4.29. − Table 4. Average annual sediment concentration (kg/m3) at hydrological stations.

Hydrological Pre-Dam Retention Impoundment 1986–1994 1995–2000 2001–2010 2011–2018 Station (1950–1958) (1959–1967) (1968–2018) Huangjiagang 2.92 1.7 0.03 0.01 0.01 0.01 0.04 Huangzhuang 2.62 1.69 0.27 0.23 0.2 0.15 0.07 Notes: The years of missing data at Huangjiagang station were 1986, 1988, 1991–1997, 1999, 2001, 2002, 2004, 2006, 2008, and 2017; the years of missing data at Huangzhuang station were 1959, 1964, 1968, 1972–1973, 1984, and 1993.

Table 5. Mann-Kendall analysis of changing trends in the hydrological regimes.

Factors Z Statistic Sig. Level A Huangjiagang 4.08 0.01 0.04 water level Xiangyang 2.22 0.05 0.12 Huangzhuang 4.29 0.01 0.06 − − Huangjiagang 1.69 0.1 7.17 runoff Huangzhuang 0.29 >0.1 2.19 − Note: A, the amplitude of variation is the slope of the trendline.

Although annual runoff at Huangjiagang and Huangzhuang stations showed an increasing trend from 1986–2018, it was non-significant with Z values of 1.69 and 0.29, respectively. One intersection − in 1994 was observed (Figure3d), meaning that runo ff at Huangjiagang station after 1994 tended to increase at the 0.05 significance level. Figure3e also shows mutations at Huangzhuang station in annual runoff, such as in 1987, 1995, 2003, 2014, and 2018. Water 2020, 12, 136 8 of 18 Water 2020, 12, x FOR PEER REVIEW 8 of 18

FigureFigure 3. TheThe Mann-Kendall Mann-Kendall (M-K) (M-K) trend trend test test fo forr the the hydrologic regime from 1986–2018 (( aa)) annual water level at the Huangjiagang station; ( (bb)) annual annual water water level level at at the Xiangyang station; station; ( (cc)) annual water level at the Huangzhuang station; ( (dd)) annual annual runoff runoff at the Huangjiagang station; ( (ee)) annual annual runoffrunoff at the Huangzhuang station).

4.2. Channel BarTable Area 5. EvolutionMann-Kendall Trend analysis at Spatiotemporal of changing Scales trends in the hydrological regimes.

The channel bar imagesFactors were obtained afterZ classifying, Statistic transferring, Sig. Level andA clustering. Surface area of DX1, DX6, DX7, and XZ3 significantlyHuangjiagang decreased 4.08 (Figure 4). 0.01 We further0.04 know that all channel bars decreased exceptwater DX4, level XZ4, andXiangyang XZ5. The year 2.22 2000 saw a 0.05sudden change 0.12 in surface area at DX1, while for DX6 and DX7, 2010Huangzhuang was the mutation −4.29 year (Figure0.014a). This− may0.06 have been related to the construction of the WangfuzhouHuangjiagang and Cuijiaying 1.69 Dams. Overall, 0.1 we calculated 7.17 the total area runoff in DX, which decreased by 23.19%Huangzhuang from 1986 to 2018,−0.29 and the total>0.1 area in XZ2.19 decreased by 16.63% (Figure4b). The surfaceNote: area A changes, the amplitude of channel of variation bars in is XZ the were slope complex of the trendline. because of the large number of shoals (XZ3–XZ5). To further analyze the trend of channel bar area evolution, we introduced an 4.2.area Channel change Bar rate Area relative Evolution to 1986 Trend and at area Spatiotemporal anomalies (theScales off set value of the annual bar area from the multi-year average). The channel bar images were obtained after classifying, transferring, and clustering. Surface area of DX1, DX6, DX7, and XZ3 significantly decreased (Figure 4). We further know that all channel bars decreased except DX4, XZ4, and XZ5. The year 2000 saw a sudden change in surface area at DX1, while for DX6 and DX7, 2010 was the mutation year (Figure 4a). This may have been related to the construction of the Wangfuzhou and Cuijiaying Dams. Overall, we calculated the total area in DX, which decreased by 23.19% from 1986 to 2018, and the total area in XZ decreased by 16.63% (Figure

Water 2020, 12, x FOR PEER REVIEW 9 of 18

4b). The surface area changes of channel bars in XZ were complex because of the large number of shoals (XZ3–XZ5). To further analyze the trend of channel bar area evolution, we introduced an area Waterchange2020 rate, 12, relative 136 to 1986 and area anomalies (the offset value of the annual bar area from the multi-9 of 18 year average).

Figure 4. Channel bars with obvious changeschanges in area during 1986–2018.

4.3. Long-Term Morphological Change ofof thethe ChannelChannel BarsBars The area change rates in didifferentfferent yearsyears relativerelative toto 19861986 werewere calculatedcalculated byby EquationEquation (2).(2). DX1 is located at a backwaterbackwater area of the Wangfuzhou Reservoir,Reservoir, thethe areaarea ofof whichwhich hadhad slightlyslightly decreaseddecreased by 1994, but in 2000,2000, thethe areaarea decreaseddecreased dramaticallydramatically byby 53.6%.53.6%. After increasing marginally in 2004 and 2010, DX1 area thenthen declineddeclined byby 62.02%62.02% inin 2018.2018. DX2, DX3, and DX4 were situatedsituated betweenbetween downstream of the Wangfuzhou Reservoir and upstreamupstream of the Cuijiaying Reservoir, but were not aaffectedffected byby the Cuijiaying Reservoir backwater. DX2, DX3, and DX4 areas decreased by 7.52%, 1.91%, and 2.52% in 1994, respectively, asas shownshown inin FigureFigure5 5a.a. InIn 2000,2000, thethe areasareas ofof allall threethreehad hadincreased increased slightly, thenthen DX2DX2 areaarea decreaseddecreased whilewhile thethe othersothers increasedincreased inin 2004.2004. DX2 and DX4 area increased in 2010, and then DX2 decreased while DX4 still kept increasing. The average area changes of DX2, DX3, DX3, and DX4 were aaffectedffected byby damdam backwaterbackwater andand werewere relativelyrelatively stablestable atat −3.22%,3.22%, −0.62%, and 6.57%, − − respectively. DX5, DX6, and DX7 area changes were 7.89%, −5.05%,5.05%, and and 2.40% 2.40% in in 1994, 1994, respectively. respectively. − DX5 was relatively stable with an average area changechange ofof 0.31%0.31% fromfrom 1986–2018.1986–2018. DX6 and DX7 had a relatively consistent decreasing trend from 2000–20182000–2018 (Figure5 5b).b). DX6DX6 andand DX7DX7 areare locatedlocated atat thethe backwater of Cuijiaying Reservoir, therefore their area was significantly reduced in 2010. Figure5c shows that bars of XZ1 and XZ2 were relatively stable with average change rates of 6.09% and 1.48%, − − respectively. We also find that the change rate of XZ3 was less than zero and decreasing, while XZ5 Water 2020, 12, x FOR PEER REVIEW 10 of 18 backwater of Cuijiaying Reservoir, therefore their area was significantly reduced in 2010. Figure 5c shows that bars of XZ1 and XZ2 were relatively stable with average change rates of −6.09% and −1.48%, respectively. We also find that the change rate of XZ3 was less than zero and decreasing, while XZ5 was greater than zero and increasing in a ‘zigzag’ pattern after 1986. There was little increase in XZ5 area in 2000, 2010, and 2018. The XZ4 area increased from 2004–2013, and decreased Waterfrom2020 1986–2000, 12, 136 and in 2018. 10 of 18 Channel bar area trends in the two middle reaches of the Hanjiang River were calculated by wasEquation greater (3). than The zero total and channel increasing bar area in a at ‘zigzag’ DX and pattern XZ clearly after 1986.decreased There during was little the increase33 years in(Figure XZ5 area6). Two in 2000, periods 2010, of and 1994–2000 2018. The and XZ4 2004–2010 area increased showed from a rapid 2004–2013, decrease and in the decreased total DX from area. 1986–2000 The total andshoal in group 2018. area significantly decreased during 1994–2000 and 2013–2018 in XZ section. (a) 2018

2013

2010

2004 Year DX4 2000 DX3 1994 DX2 1986 DX1

-80% -60% -40% -20% 0% 20% 40%

2018 (b) 2013 2010 2004 Year 2000 DX7 1994 DX6 1986 DX5

-50% -40% -30% -20% -10% 0% 10% 20%

2018 (c) 2013

2010 XZ5 XZ4 2004

Year XZ3 2000 XZ2

1994 XZ1

1986

-40% -20% 0% 20% 40% 60%

FigureFigure 5. 5.Channel Channel bar bar area area change change rates rates along along the the middle middle reaches reaches of the of Hanjiangthe Hanjiang River River from from 1986–2018 1986– (a2018) Danjiangkou-Xiangyang (a) Danjiangkou-Xiangyang 1 (DX1)–DX4, 1 (DX1)–DX4, (b) DX5–DX7, (b) DX5–DX7, (c) (XZ1–XZ5). (c) (XZ1–XZ5).

Channel bar area trends in the two middle reaches of the Hanjiang River were calculated by Equation (3). The total channel bar area at DX and XZ clearly decreased during the 33 years (Figure6). Two periods of 1994–2000 and 2004–2010 showed a rapid decrease in the total DX area. The total shoal group area significantly decreased during 1994–2000 and 2013–2018 in XZ section. Water 2020, 12, 136 11 of 18 Water 2020, 12, x FOR PEER REVIEW 11 of 18

1000 Total DX anomaly area

Water 2020, 80012, x FOR PEER REVIEW y = -5.93x2 - 304.74x + 1337.6 Total XZ anomaly area11 of 18 600 R² = 0.87 polynomial(DX)

) 1000

2 Total DX anomaly area 400 polynomial(XZ) 800 y = -5.93x2 - 304.74x + 1337.6 Total XZ anomaly area 200 600 R² = 0.87 polynomial(DX) ) 2 0 400 polynomial(XZ) -200 200y = -5.44x2 - 52.26x + 317.85 R² = 0.2 -400 0

Area annomaly (hm annomaly Area 2 -600-200 y = -5.44x - 52.26x + 317.85 R² = 0.2 -400

-800(hm annomaly Area -600 -1000 -800Year1986 1994 2000 2004 2010 2013 2018

-1000 YearFigure1986 6. Channel 1994 bar area 2000 anomalies 2004 in the DX 2010 and XZ 2013 reaches. 2018

Figure 6. Channel bar area anomalies in the DX and XZ reaches. 4.4. Correlation Analysis of Influencing Influencing Factors It4.4. waswas Correlation necessarynecessary Analysis to to analyze analyze of Influencing the the correlation correlation Factors of of channel channel bar bar area area with with both both stations’ stations’ hydrological hydrological data due to different channel bar area trends in DX and XZ section. Hydrological data at Huangjiagang and data dueIt towas different necessary chto annelanalyze bar the correlationarea trends of channelin DX barand area XZ with section. both stations’ Hydrological hydrological data at Huangzhuang stations both included annual runoff, annual maximum discharge, annual maximum Huangjiagangdata due andto different Huangzhuang channel stationsbar area both trends included in DX annual and XZ runoff, section. annual Hydrological maximum data discharge, at dischargeannualHuangjiagang maximum during the dischargeand dry Huangzhuang season, during annual thestations dry average bothseason, waterincluded annual level, annual average dry runoff, season water annual discharge, level, maximum dry daily season discharge, discharge discharge, and daily annual waterdischarge maximum level. and Daily daily discharge and water annual during level. average the Daily dry season, waterand an levelsannualnual average during average the water dry level, levels season dry during atseason Xiangyang the discharge, dry stationseason wereat Xiangyangdaily only discharge used station due and to were the daily lackonly water of used discharge level. due Daily to data.the and lack an Averagenual of dischargeaverage annual water sedimentdata. levels Average during at Huangzhuang annual the dry sediment season station at downstreamHuangzhuangat Xiangyang of station Xiangyang station downstream were reflecting only used of sediment dueXiangyang to the conditionlack reflecting of discharge was sediment also data. took Average condition into accountannual was sediment also and analyzedtook at into Huangzhuang station downstream of Xiangyang reflecting sediment condition was also took into theaccount correlation and analyzed with the the bars’ correlation area. Pearson with the correlation bars’ area. analyses Pearson were correlation carried analyses out in the were DX andcarried XZ account and analyzed the correlation with the bars’ area. Pearson correlation analyses were carried reach,out in respectively,the DX and XZ with reach, respect respectively, to hydrological with respect conditions. to hydrological Specifically, conditions. the factors Specifically, that passed the significanceout in the test DX are and the XZ water reach, level respectively, in the DX with section respect and to runohydrologicalff in the conditions. XZ section Specifically, (Tables S1 andthe S2). factorsfactors that thatpassed passed the the significance significance test test are are thethe waterwater levellevel in in the the DX DX section section and and runoff runoff in the in XZthe XZ Pearson correlations results in DX showed that annual average water levels at Xiangyang station were sectionsection (Tables (Tables S1 andS1 and S2). S2). Pearson Pearson correlations correlations results inin DX DX showed showed that that annual annual average average water water significantly correlated with channel bar area, with a coefficient of 0.93 (p < 0.01) (Figure7). The levelslevels at Xiangyang at Xiangyang station station were were significantly significantly correlatedcorrelated withwith channel channel −bar bar area, area, with with a coefficient a coefficient of of results−0.93 −(0.93p indicate < 0.01)(p < 0.01) (Figure that (Figure channel 7). 7).The areaThe results results was indicate inversely indicate thatthat proportional channel area area to was waterwas inversely inversely level atproportional Xiangyang proportional to station; water to water i.e., arealevel decreasedlevelat Xiangyang at Xiangyang with station; the station; increase i.e., i.e., area inarea waterdecreased decreased level, withwith especially thethe increaseincrease in 2010. in in water water Pearson level, level, correlationespecially especially inresults 2010. in 2010. for channelPearsonPearson barcorrelation area correlation with results hydrological results for for channel channel data bar inbar XZarea area showed withwith hydrological that only daily data data in runo inXZ XZ ffshowedat showed Huangzhuang that that only only daily station daily wasrunoff significantlyrunoff at Huangzhuang at Huangzhuang negatively station station correlated was was significantly (significantlyr = 0.79, p negatively< 0.05). Figure correlated correlated8 shows (r (=r that− =0.79, −0.79, higher p < p0.05). < daily 0.05). Figure discharge Figure 8 8 shows that higher daily discharge at Huangzhuang− station was associated with lower bar area, atshows Huangzhuang that higher station daily was discharge associated at Huangzhuang with lower bar area,station especially was associated during the with two lower mutation bar yearsarea, especially during the two mutation years of 2000 and 2018. ofespecially 2000 and during 2018. the two mutation years of 2000 and 2018.

65 9000 65 80009000 64 70008000 )

64 600070002 63 r=-0.93 (P<0.01)

5000 ) 6000 2 63 r=-0.93 (P<0.01) 4000 62 5000

3000 (hm Area 4000 62 level (m) Water 61 2000

3000 (hm Area 1000 Water level (m) Water 2000 61 60 0 1980 1985 1990 1995 2000 2005 2010 2015 2020 1000

60 XY annual average water level DX total area 0 1980 1985 1990 1995 2000 2005 2010 2015 2020 Figure 7. Changes in annual average of water level at Xiangyang station (XY) and total channel bar Figure 7. Changes in annualXY annualaverage average of water water level atlevel Xiangyang stationDX total (XY) area and total channel bar area inarea DX inreach. DX reach. Figure 7. Changes in annual average of water level at Xiangyang station (XY) and total channel bar area in DX reach.

Water 2020, 12, 136 12 of 18 Water 2020, 12, x FOR PEER REVIEW 12 of 18

1400 7800 1200 7600 7400 /s) )

3 1000 7200 2 800 7000 600 6800 400 r= -0.79 (P<0.05) (hm Area

discharge(m 6600 200 6400 0 6200 1980 1985 1990 1995 2000 2005 2010 2015 2020

HZ daily discharge XZ total area

Figure 8. Changes in daily discharge at Huangzhuang station (HZ) and total channel bar area in Figure 8. Changes in daily discharge at Huangzhuang station (HZ) and total channel bar area in XZ XZ reach.reach. 5. Discussion 5. Discussion WithWith intensive intensive human human interventions, interventions, such such as dam as dam operations, operations, natural natural river river regimes regimes are gradually are replacedgradually by regulated replaced flowsby regulated and sediment flows and loads, sediment which loads, dramatically which dramatically affect the affect channel the channel bar evolution bar and hasevolution been and studied has been in riversstudied system in rivers across system the across globe the [globe49,50 [49,]. Previous50]. Previous studies studies have have foundfound that: operationthat: operation of the Danjiangkou of the Danjiangkou Dam impacted Dam impacted the hydrological the hydrological regime regime with with an overallan overall alteration alteration degree of 40.6%.degree This of 40.6%. slightly This increased slightly increased to 46.7% withto 46.7% the with combined the combined operation operation of the Wangfuzhouof the Wangfuzhou Reservoir, and increasedReservoir, evenand increased more to even 63.5% more combined to 63.5% with combined the Cuijiaying with the ReservoirCuijiaying [Reservoir29]. The [29]. Danjiangkou The ReservoirDanjiangkou regulated Reservoir the hydrologic regulated regime the hydrologic by storing regime water by storing at the endwate ofr at the the wet end season of the wet and season supplying and supplying water in the dry months, resulting in reduced flood peak and streamflow variability water in the dry months, resulting in reduced flood peak and streamflow variability during the during the year [21,28,31,51]. From the flood detention of the Danjiangkou Reservoir to 1989 (30a), year [21,28,31,51]. From the flood detention of the Danjiangkou Reservoir to 1989 (30a), the widening the widening of the river channel in the middle reaches has been remarkable, with the largest change of thein river the XZ3 channel group in the at middle about twice reaches the width has been of the remarkable, pre-dam river with [52]. the From largest 1987 to change 2005, in the XZ3 beachhad group declined at about substantially twice with the widthaverage of annual the pre-dam erosion of river 1.14 million [52]. Fromm3 [34]. 1987 After to the 2005, construction erosion had declinedof the substantially Danjiangkou Reservoir, with average 95% annualor more erosionsediment of was 1.14 intercep millionted m[26,53],3 [34]. and After the thesection construction from of theDanjiangkou Danjiangkou to Xiangyang Reservoir, has 95% been or under more equilibrium sediment between was intercepted scouring and [26 ,deposition,53], and the and section mainly from Danjiangkouscoured downstream to Xiangyang Cuijiaying has been in under the middle equilibrium reaches betweenafter 1985 scouring [34]. Table and 4 shows deposition, that average and mainly scouredannual downstream sediment concentrations Cuijiaying in thefrom middle 1986–2018 reaches were after almost 1985 zero [34]. at Table Huangjiagang4 shows that station. average annualTherefore, sediment the concentrations effect of sediment from in 1986–2018this study is were not significant. almost zero Combined at Huangjiagang cascade dams station. had Therefore,little effect on annual runoff (Figure 9). After cascade dam operation began, the natural flowing river the effect of sediment in this study is not significant. Combined cascade dams had little effect on annual channel was disrupted and broken into several parts due to impoundment effects, and the naturally runoff (Figure9). After cascade dam operation began, the natural flowing river channel was disrupted occurring aquatic were replaced by lacustrine, transitional, and riverine zone habitats in the and brokenriver channel into [54]. several Furthermore, parts due after to operation impoundment of the Wangfuzhou effects, and and the Cuijiaying naturally Reservoirs, occurring water aquatic habitatslevels were increased replaced around by lacustrine, them [22,37,55]. transitional, As shown and in riverineFigure 3, zonethe annual habitats water in thelevel river increased channel at [54]. Furthermore,Huangjiagang after station operation in 1999. of theThere Wangfuzhou was an obvious and increase Cuijiaying after Reservoirs, 2009 and a mutation water levels in 2014 increased at aroundXiangyang them [ 22station.,37,55 Annual]. As shown water level in Figure at Huangzhuang3, the annual significantly water level decreased increased after at2006. Huangjiagang Annual stationrunoff in 1999. at Huangjiagang There was anstation obvious (Figure increase 9a) increased after 2009 after and 1994 a mutationat a significance in 2014 level at Xiangyang of 0.05, while station. Annualthe waterdecrease level in atannual Huangzhuang runoff at significantlyHuangzhuang decreased station (Figure after 2006.9b) was Annual not significant runoff at Huangjiagang(p > 0.1). stationVariability (Figure 9ina) the increased hydrologic after regimes 1994 at at Huangjiagang a significance and level Xiangyang of 0.05, was while closely the decreaserelated with in the annual construction of Wangfuzhou and Cuijiaying Dams, respectively, because the stations were in the runoff at Huangzhuang station (Figure9b) was not significant ( p > 0.1). Variability in the hydrologic reservoir backwaters [22,55]. Indeed, what direct impact on the hydrological regime is still not clear regimes at Huangjiagang and Xiangyang was closely related with the construction of Wangfuzhou and by the reservoirs under construction in the middle reaches of Hanjiang River for dams under Cuijiayingconstruction Dams, have respectively, not yet gotbecause river closure the stationsworks sowere far [29,31]. in the Therefore, reservoir we backwaters did not quantitatively [22,55]. Indeed, whatanalyze directimpact the specific on the effect hydrological of the three regimereservoirs is stillthat still not under clear byconstruction. the reservoirs under construction in the middle reaches of Hanjiang River for dams under construction have not yet got river closure works so far [29,31]. Therefore, we did not quantitatively analyze the specific effect of the three reservoirs that still under construction. Based on the correlations between channel bars and water and sediment, we knew that annual average water levels had significant negative correlations with channel bar area in DX (r = 0.93, − p < 0.01). The primary factor of influence in DX section was water level change due to dam construction. Water 2020Water, 12 2020, 136, 12, x FOR PEER REVIEW 13 of 1318 of 18

The total channel bar area in XZ had a significant negative correlation with daily runoff at Huangzhuang station (r = 0.79, p < 0.05), and the driving factor of runoff was inseparable from precipitation (R2 = 0.65) − as shown in Figure 10. Therefore, channel area change was attributed to upstream precipitation. Before construction of the Danjiangkou Reservoir, the middle Hanjiang River had a meandering braided channel pattern in a quasi-equilibrium state [25]. After reservoir construction, the changing characteristics of mid-channel bars were significantly related to river erosion, because the bank erosion provided both space and materials for the building of mid-channel bars. When the bars were eroded in the upstream sedimentation zone, those in the downstream sediment zone expanded, and when the macroscopic bedload transport ‘wave’ moved further downstream, the mid-channel bar indices declinedFigure 9. [24 Annual]. Obviously, runoff at Huangjiagang as is shown in(HJG) Figure and 5Huan thatgzhuang the evolution (HZ) stations of bars (the in grey XZ range section is during 1986–2018the were 95% complicated confidence interval). because it was upstream eroded and downstream deposited, the XZ1–XZ3 areas have the decrease trend, the XZ4 and XZ5 areas increase generally. Generally, the DX section was under equilibriumBased on the between correlations scouring between and channel deposition bars and compared water and to downstreamsediment, we knew Xiangyang, that annual and the average water levels had significant negative correlations with channel bar area in DX (r = −0.93, p < bars in which were mainly affected by water level. Water0.01). 2020 The, 12 primary, x FOR PEER factor REVIEW of influence in DX section was water level change due to dam construction.13 of 18 The total channel bar area in XZ had a significant negative correlation with daily runoff at Huangzhuang station (r = −0.79, p < 0.05), and the driving factor of runoff was inseparable from precipitation (R2 = 0.65) as shown in Figure 10. Therefore, channel area change was attributed to upstream precipitation. Before construction of the Danjiangkou Reservoir, the middle Hanjiang River had a meandering braided channel pattern in a quasi-equilibrium state [25]. After reservoir construction, the changing characteristics of mid-channel bars were significantly related to river bank erosion, because the bank erosion provided both space and materials for the building of mid-channel bars. When the bars were eroded in the upstream sedimentation zone, those in the downstream sediment zone expanded, and when the macroscopic bedload transport ‘wave’ moved further downstream, the mid-channel bar indices declined [24]. Obviously, as is shown in Figure 5 that the evolution of bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited, the XZ1–XZ3 areas have the decrease trend, the XZ4 and XZ5 areas FigureincreaseFigure 9. Annualgenerally. 9. Annual runo runoffGenerally,ff at Huangjiagang at Huangjiagang the DX(HJG) section (HJG) and and was Huangzhuang Huan undergzhuang equilibrium (HZ)(HZ) stationsstations between (the grey grey scouring range range is is and the 95%deposition confidencethe 95% compared confidence interval). interval).to downstream Xiangyang, and the bars in which were mainly affected by water level. Based on the correlations between channel bars and water and sediment, we knew that annual average water 1200levels had significant negative correlations with channel bar area in DX (r = −0.93, p < 0.01). The primary factor of influence in DX sectiony = 0.52x was + 335.97 water level change due to dam construction. The total channel1000 bar area in XZ had a significantR² = 0.65 negative correlation with daily runoff at Huangzhuang station800 (r = −0.79, p < 0.05), and the driving factor of runoff was inseparable from precipitation (R2 = 0.65) as shown in Figure 10. Therefore, channel area change was attributed to upstream precipitation.600 Before construction of the Danjiangkou Reservoir, the middle Hanjiang River had a meandering braided channel pattern in a quasi-equilibrium state [25]. After reservoir construction, the400 changing characteristics of mid-channel bars were significantly related to river bank erosion, because the bank erosion provided both space and materials for the building of mid-channel 200

bars. When the(mm) precipitation Annual bars were eroded in the upstream sedimentation zone, those in the downstream sediment zone expanded,0 and when the macroscopic bedload transport ‘wave’ moved further downstream, the mid-channel0 200 bar indices 400 declined 600 [24].800 Obviously, 1000 as is shown 1200 in 1400Figure 5 that the 3 evolution of bars in XZ section during 1986–2018Discharge were complicated(m /s) because it was upstream eroded and downstream deposited, the XZ1–XZ3 areas have the decrease trend, the XZ4 and XZ5 areas FigureFigure 10. 10. Regression analysis between discharge at Huangzhuang hydrologic station and annual increase generally.Regression Generally, analysis betweenthe DX dischargesection was at Huangzhuang under equilibrium hydrologic between station scouring and annual and precipitation at meteorological station. precipitationdeposition compared at Laohekou to downstream meteorological Xiangyang, station. and the bars in which were mainly affected by water level. In fact,In therefact, arethere other are explanationsother explanations for the for observed the observed channel channel bar changes bar apartchanges from apart hydrological from hydrological factors. The first is vegetation growth. Shoal groups in XZ3, XZ4, and XZ5 downstream factors. The first is vegetation growth. Shoal groups in XZ3, XZ4, and XZ5 downstream of the of the Cuijiaying1200 hydrological station with changing area and shape had relatively low vegetation Cuijiaying hydrological station with changingy area= 0.52x and + 335.97 shape had relatively low vegetation coverage with sandy in1000 our study, and serious land desertificationR² = 0.65 affected plant composition [56–58]. In turn, vegetation was800 no longer able to stabilize channel material from erosion [59,60]. The second are human interventions with and revetments. Channel bank protection engineering changed the boundary conditions600 of the river and prevented channel bar banks from being eroded [61]. The 400

200 Annual precipitation (mm) precipitation Annual 0 0 200 400 600 800 1000 1200 1400 3 Discharge (m /s) Figure 10. Regression analysis between discharge at Huangzhuang hydrologic station and annual precipitation at Laohekou meteorological station.

In fact, there are other explanations for the observed channel bar changes apart from hydrological factors. The first is vegetation growth. Shoal groups in XZ3, XZ4, and XZ5 downstream of the Cuijiaying hydrological station with changing area and shape had relatively low vegetation

Water 2020, 12, x FOR PEER REVIEW 14 of 18 coverage with sandy soil in our study, and serious land desertification affected plant composition [56–58]. In turn, vegetation was no longer able to stabilize channel material from erosion [59,60]. The Water 2020, 12, 136 14 of 18 second are human interventions with levees and revetments. Channel bank protection engineering changed the boundary conditions of the river and prevented channel bar banks from being eroded [61].Channel The RegulationChannel Regulation Project was Project constructed was construc from 1986–1996ted from to1986–1996 improve to navigation improve safetynavigation and protect safety andriver protect banks from river erosion banks infrom the XZ,erosion where in 168 the dikes XZ, werewhere built 168 [ 62dikes,63]. were Meandering, built [62,63]. branching Meandering, channels branchinggradually becamechannels stable gradually and lost became some stable bends. and As lost the whole,some bends. channel As bar thearea whole, in XZ channel increased bar area except in XZduring increased 2000 and except 2018 during (Figure 20005). and 2018 in 1998 (Figure and 5). 2017 Floods may havein 1998 caused and 2017 the decreases may have in caused those two the years.decreases Therefore, in those the two third years. factor Therefore, is flooding. the third As a factor sediment is flooding. source forAs channela sediment bars, source a flooding for channel event bars,causes a flooding complicated event changes causes incomplicated erosion or changes deposition in erosion [64]. Taking or deposition the flooding [64]. Taking event of the 2017 flooding as an eventexample, of 2017 the channelas an example, bars downstream the channel of bars the Wangfuzhoudownstream andof the upper Wangfuzhou reaches of and Cuijiaying upper reaches decreased of Cuijiayingafter flooding decreased in 2017, after as the flooding heads of in river 2017, islands as the erodedheads of away river (Figure islands 11 eroded). The 2017away flooding (Figure may11). Thehave 2017 been flooding the reason may why have shoal been groups the fromreason Xiangyang why shoal to groups Huangzhuang from Xiangyang significantly to decreasedHuangzhuang from significantly2013–2018. Finally, decreased other from human 2013–2018. activities Finally, cannot othe ber ignored,human activities such as sandcannot mining, be ignored, land use, such and as sandover-exploitation, mining, land which use, and may over-exploitation, play important roles which in changingmay playchannel important bar roles area andin changing shape [15 channel,65]. In baraddition, area and detailed shape quantitative[15,65]. In addition, analysis detailed of each quantitative influencing analysis factor needs of each further influencing analysis factor combined needs withfurther field analysis and laboratory combined survey with field in the and next laboratory study. As survey for the in submerged the next study. bars, As the for water the submerged level of the bars,reservoirs the water should level be adjustedof the reservoi adaptivelyrs should under be theadjusted highest adaptively water level. under In order the highest to protect water the level. channel In orderbars from to protect disappearing, the channel ecological bars from disappearin can beg, adopted ecological and revetment trees and grasses can be couldadopted be plantedand trees in andthe areagrasses with could scour be of planted the head in of the the area bars. with scour of the head of the bars.

Figure 11. ExampleExample of of channel channel bar change in both area and shape after a flooding flooding event.

Water 2020, 12, 136 15 of 18

6. Conclusions This study analyzed the long-term evolution of channel bars in the middle reaches of the Hanjiang River in response to combined cascade dams. The results showed that the main factor influencing mid-channel bars in DX was water level following dam construction (r = 0.93, p < 0.01). The total − channel bar area in XZ was significantly negatively correlated with daily runoff at Huangzhuang station (r = 0.79, p < 0.05), and the channel bar change area was partly attributed to upstream − precipitation due to the high correlation between runoff and precipitation (R2 = 0.65). In addition, the evolution of bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited, the XZ1–XZ3 areas have the decrease trend, the XZ4 and XZ5 areas increase generally. In fact, there are other explanations for channel bar area change apart from hydrological factors, such as vegetation cover, revetments projects, flood events, sand mining, land use, and over-exploitation. Generally, the DX section was under equilibrium between scouring and deposition compared to downstream Xangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated. The total channel bar area in DX and XZ decreased by 23.19% and 16.63%, respectively, from 1986 to 2018. Furthermore, the study demonstrates that the multi-temporal satellite remote sensing images are convenient and practical to assess the morphodynamic evolution of channel bars. In addition, considering the ecological flow of biological habitats should be given enough attention in the reservoir’s joint operation planning for the hydrological regime caused by the dams has a direct impact on the channel bar evolution. In order to protect the channel bars from disappearing, ecological revetment can be adopted and trees and grasses could be planted in the area with scour of the head of the bars. In future research, combined active and passive satellite images with high spatial and temporal resolution, such as Sentinel-1, Sentinel-2, COSMO-Skymed, PlanetScope, and GF-1, are needed. To fully understand channel bar dynamics and the impacts of cascade dams, it is necessary to explore dynamic evolution in 3-D space and the correlations between water and sand changes with bars in the Hanjiang River.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/12/1/136/s1, Table S1: Pearson correlation analyses between total DX channel bar area with hydrological conditions (* Significant correlation at 0.05 level; ** Significant correlation at 0.01 level); Table S2: Pearson correlation analyses between total XZ channel bar area with hydrological conditions (* Significant correlation at 0.05 level; ** Significant correlation at 0.01 level). Author Contributions: Conceptualization, X.C. and E.L.; data curation, Y.Z. and X.S.; formal analysis, Y.Z., C.Y., X.S. and X.B.; funding acquisition, E.L.; investigation, E.L. and X.B.; methodology, Y.Z. and X.C.; project administration, E.L.; resources, X.C.; software, C.Y. and X.S.; visualization, Y.Z.; writing—original draft, Y.Z.; writing—review and editing, Y.Z. and E.L. All authors have read and agreed to the published version of the manuscript. Funding: This study was supported by the National Natural Science Foundation of China (No. 41671512, 41801100). Conflicts of Interest: No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

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