Human development and rivers have been closely linked since ancient times. Rivers have created the ecological environment for human survival, and in turn, human development of civilization has been facilitated by the increasing understanding and use of rivers [1
]. In recent years, the water–sand combination of the Yangtze River channel has been changed due to anthropogenic factors, especially the construction of water conservancy facilities such as the Three Gorges Project, and the changes are not transient. Changing water–sand conditions have a very long-term impact on the shaping of river morphology in the middle and lower reaches of the Yangtze River [2
]. The main river channels in the middle and lower reaches of the Yangtze River have undergone drastic evolution, showing serious bank collapses, which pose a threat to flood safety and affect the operation of national economic facilities along the river [3
]. For the management and control of unstable river sections, the law of river evolution and its evolution mechanism need to be used as a reference, so that reasonable planning and remediation can be made to stabilize the river [4
The changes in water and sand in the river channel are closely related to the river evolution process. Once the river water and sand conditions change, the river channel adjusts accordingly [5
]. River evolution can be studied from both horizontal and vertical dimensions. Studies on the evolution of the Yellow River have focused on its vertical changes and have analyzed changes in the scouring and deposition of the river channel, the shape of the longitudinal section, and the spatial and temporal evolution of the flow and water level [6
]. There are also some studies on the horizontal evolution, such as shoreline change, beach scouring, and deposition [10
]. The analysis of river channel evolution of small rivers in China, such as the Gan and Xiang Rivers, has also focused on river channel scouring and deposition changes [15
]. Outside of China, river channel evolution studies are also about horizontal changes (e.g., shoreline migration and river area) and vertical changes (e.g., channel scouring, deposition changes, and river section shape), such as the Mississippi, Nile, the Ganges, and other large rivers [18
]. On the whole, many scholars tend to analyze river evolution from both horizontal and vertical points of view.
As remote sensing and GIS technologies advance by leaps and bounds, they have been increasingly used in river evolution studies.more and more widely used in the field of river evolution research. Remote sensing and GIS technologies have become the main research tools in the evolution of the middle and lower reaches of the Yangtze River in terms of shoreline evolution, the evolution of beaches, and continental siltation. The research results have important reference values for river regulations. Landsat imagery and related actual measurement data were used to calculate the meander coefficient, bank, and island erosion accumulation area of the Nile River in Egypt from 1987 to 2000, and to analyze the processes associated with sediment accumulation and new islands formation [22
]. The corresponding Landsat images were selected from the upper Amazon River water level data based on the principle of homogeneous water level interpretation [23
]. The interpreted data were compared to analyze the characteristics of the evolution of the river planform. The relationship between runoff volume and river evolution was also explored. The CAESAR-Lisflood model was used to study the channel evolution of ten river floodplains in northern England, quantifying channel erosion, accretion, and lateral migration rates [24
]. The driving forces behind the evolution of the Sharda River were analyzed using GIS techniques based on remote sensing data collected between 1977 and 2001 [25
]. The morphological characteristics of the Gumara River watercourse in the upper Blue Nile basin, Ethiopia, over 50 years were analyzed through GIS using multi-year measured data from hydrological stations [26
]. The change in channel planform of the Ganges and Padma rivers in Bangladesh was studied by GIS technique using multi-period satellite remote sensing imagery from 1973 to 2011 as the base data [27
]. It was found that the barrage in the upper Ganges had a negative impact on the downstream channel through the study of the relationship between multi-year flow data and channel evolution characteristics. The GIS technique was used to calculate the river migration rate, meander coefficient of erosion-prone channels, and to predict the distance of the lateral movement of the river centerline based on multi-temporal satellite remote sensing imagery of the Pajilti River in West Bengal, India. Multi-temporal Landsat remote sensing images and Google Earth (GE) images were used to digitize the river channel on the ArcGIS 10.2 platform according to the image features [28
]. The vector layer of the river channel in different years was obtained, and the evolution characteristics of the river channel and the change pattern of flushing and siltation in the first big bend of the Yellow River during the past 26 years were obtained through comparative analysis. Remote sensing technologies have outstanding advantages such as large area monitoring, simultaneous monitoring, long time series monitoring, and repeatable observation of the same area, which can be used for long-term and flexible investigation independent of the sea and surface conditions. The use of remote sensing technology can effectively monitor the evolution of sandbars, and by superimposing remote sensing images of different phases, the evolution of sandbars can be quickly detected.
The river channel oscillation is vulnerable to bank erosion and collapse, and a large amount of agricultural land in the beach area is destroyed. Compared with the previous methods of river evolution research, remote sensing application technology, with its advantages of wide monitoring range, long time series of data archiving, and low application costs, provides an intuitive and complete interpretation of river shape characteristics and has great potential for historical river change mapping. The Yangtze River is the first in Asia and the third largest in the world for length and flow, after the Amazon and the Nile. Although its natural conditions are superior, there are some problems that need to be solved: the mainstream line of each local section of the river is still oscillating; the erosion of local sections of the river is very harmful to the lives of residents on both sides of the river and embankment projects; the swimming accumulation of some sections of the river adversely affects the setting of ports, docks, and factories. The Three Gorges Dam is the world’s largest river hydroelectric project. Its impact on the Yangtze River and its basin environment has been the focus of research in China and abroad [29
The study of the evolution mechanism of the middle and lower reaches of the Yangtze River is mainly approached from the following five directions: the influencing factors of scouring-deposition evolution of the beach in the Yangtze River [31
]; the influence of water and sediment transport on the evolution of the branch channel in the braided reach of the Yangtze River [32
]; the planar morphological characteristics of the evolution of the Yangtze River channel in terms of riverbanks [33
], mid-channel bars [34
], and point bars [35
]; the longitudinal evolution of the river channel through changes in river scouring and siltation [36
]; the mechanism of the river evolution through both natural and human factors. Additionally, some scholars focus on the evolution of local river sections with severe local changes, mainly analyzing the longitudinal thalweg, beach area, and the rise and fall of branch channels, predicting river changes by exploring its evolution mechanisms, as it washas been done for Qigongling River [37
] and Chengtong River [38
Based on remote sensing images, the GIS technology is used to extract the evolution of the river planes. Combined with the measured data, the characteristics of the river evolution are analysed to investigate the inner mechanism behind the evolution, which is a relatively scientific research process of river evolution [39
]. Socioeconomic development has caused and increased pressure on the water and sand conditions of the river through the effect of human activities, such as the construction of defense structures, land reclamation, water conservancy projects, and sand mining [41
]. For example, the construction and use of the Three Gorges Dam inevitably led to changes in the water and sand content in the river section near the dam [42
]. Therefore, it is urgent to study the impact on the river section at a certain distance from the dam.
These studies on the analysis of the river channel evolution have informed the research about the evolution of the Chenglingji–Datong Channel in the middle and lower reaches of the Yangtze River and its driving mechanisms. Previous studies have mainly focused on water and sand characteristics, tidal currents, riverbed, and river evolution. There are fewer studies on the evolution of the sandbar in the northern branch of the Yangtze estuary, and they are mainly based on information on hydrology, sediment, and river topography. It is difficult to obtain field data for large areas of the river channel [43
]. Remote sensing imagery can address this problem. Current research methods are based on remote sensing imagery combined with visual interpretation, using a single image acquisition [44
], whereas we have combined the modified normalized difference water index (MNDWI), taking into account precipitation and runoff, to analyze channel variability. Previous studies on the Yangtze River have focused on the Jiangxi and Jiangsu sections in the lower reaches of the Yangtze [46
], with less emphasis on the section of the river between Chenglingji and Datong, which links the middle and lower reaches of the Yangtze.
This study used GIS and remote sensing techniques to analyze multi-temporal satellite images of the middle and lower reaches of the Yangtze River from Chenglingji to Datong from 1989 to 2019, to study the river evolution patterns and water–sand characteristics, as well as the relationship between the two, and to explore the mechanisms and driving forces behind the river evolution. This study focuses on quantifying the morphological changes in the riverbank movement, erosion/absorption rates, and the lateral alluvial area over more than 30 years. By quantifying the spatial extent, rate of shoreline changes, and river cores in the middle and lower reaches of the Yangtze River from Chenglingji to Datong, we aim to better understand the erosion and siltation processes in the Yangtze River from the perspective of river width and curvature, as well as the impact of human interventions, such as the construction of the Three Gorges Dam. The results of this study will help us to understand and predict the morphological behavior of the Chenglingji to Datong River section, which can help mitigate the potential adverse socioeconomic impacts of river channel changes in the Yangtze. The study also aims at developing a set of intelligent and rapid tracking methods for river channel changes, explore the driving factors of river channel changes from natural and human factors, and provide decision-making support for the Yangtze River channel to achieve a new balance of water and sand. The research results will provide a reference and a theoretical basis for the regulation of the Yangtze River waterway, the utilization of shoreline resources, flood control safety, river management, and other aspects.
2. Study Area and Data
With a length of more than 6300 km, the Yangtze River is the longest river in both China and Asia and the third longest in the world. The reaches of the Yangtze River are divided into middle and lower at the boundary with Hukou County. The study area of this paper is the Chenglingji to Datong River section, which contains both the middle and lower reaches of the river (Figure 1
). The channel section from Chenglingji (29°26′ N, 113°08′ E) to Datong River (30°46′ N, 117°37′ E) is about 753.4 km long and is one of the most densely populated areas in the middle and lower reaches of the Yangtze River. The evolution process of the river course is very complicated, even in the same river, but because of the different river reach, its evolution law will be different. The rivers in the study area are so long that they need to be studied in segments. The main purpose of studying the river course evolution is to provide a reference for river course management. Therefore, the river course management plan of the middle and lower reaches of the Yangtze River is referred to when the river course is segmented. According to its division principle, the section of the river is divided into three categories, of which the first category is the river segment that needs to be controlled, the second category and the third category are the river segment that needs to be controlled generally [48
]. River channel changes refer to the changes that occur in the river channel under natural conditions or under the influence of human factors. River channel changes can be divided into two categories, vertical and horizontal changes, from the manifestation. The riverbed in the middle and lower reaches of the Yangtze River from Chenglingji to Datong has an overburden layer on top of the bedrock with an average thickness of about 10–12 m. The overburden layer consists of sand, gravel, and pebbles. We conducted a comparative analysis of the changes in river morphology and the changes in the amount of flushing and siltation. Most of the areas on the left and right sides of the river in the study area are alluvial plains, and the coastal boundary is mainly made up of the river terraces, hills, and low hills; the width of the riverbanks varies, with different widths, showing a coupling-like bending pattern. A typical geomorphic feature exists in the section from Chenglingji to Datong, namely nodes, which makes the riverbanks with nodes have strong scouring resistance, can make the riverbanks have strong stability, and plays a stabilizing role in the evolution of the river. The shoreline of the middle and lower reaches of the Yangtze River from Chenglingji to Datong is mainly an alluvial shoreline composed of Holocene loose sediments, and is weak in alluvial resistance, with frequent shoreline oscillations under the action of water and sand, and obvious changes in lateral alluvium and siltation of the river channel, leaving the river in an unstable state, and sometimes even the banks collapse, causing threats to people’s personal safety and property.
The section of the Chenglingji to Datong River in the middle and lower reaches of the Yangtze River spans the footprint of three Landsat scenes (path/row numbers 123/39, 122/39, and 121/39). In order to analyze the temporal and spatial evolution of the river, we selected seven time-series images (1989, 1994, 1999, 2004, 2009, 2013, 2019).
This paper used the dry season water level data of Hankou, Huangshi, and Jiujiang hydrological stations in the study area. Normally, the dry season is from December to April each year. The daily water levels were very close to each other. The daily water levels for the days of the selected remote sensing images are shown in Table 1
. The maximum water level difference during these days was less than 2 m.
This study also collected a series of additional hydrological data. The annual runoff volume, annual sand transport volume, and annual median particle size data came from the Yangtze River Sediment Bulletin; beach flushing volume and flushing intensity data come from the Statistical Yearbook; annual precipitation data from 68 precipitation stations in the middle and lower reaches of the Yangtze River region were derived from the National Meteorological Information Center. All data are from official sources, with high accuracy and authority. The data periods are shown in Table 2
5.1. Morphological Evolution Characteristics of River Reach
Currently, there is a wide range of applications for the study of the river planform through remote sensing imagery from several time periods. The planform changes of the riverbanks can be analyzed by estimating the oscillation rates of the left and right banks. The calculation of this parameter allows both quantitative analysis of the overall trend of riverbank changes and the identification of local areas of dramatic change, while avoiding the inconvenience caused by the small number of measured river cross-sections [60
]. The analysis of Figure 11
and the left and right shoreline changes show that there are phases and regions of shoreline oscillation. For example, the shoreline of the Luxikou River section shifts sharply between 1989 and 1999, but changes return to equilibrium with less oscillation in the later stages. The Jiujiang River section has less shoreline changes from 1989 to 1994, but more dramatic changes with larger oscillations from 1994 to 2019. These reflect the regional nature of shoreline oscillations.
By quantifying the lateral erosion/accretion area of the river and analyzing its spatial and temporal evolution, the evolutionary characteristics of the river can be revealed [62
]. Combining the results of lateral alluvial area changes with Figure 12
, the maximum lateral erosion on the left bank occurred from 1994 to 1999 and the maximum lateral accretion occurred from 1999 to 2004, with little change in erosion accretion rates between the periods. On the right bank, the maximum erosion occurred in 1989–1994, the maximum accretion occurred in 2013–2019, and the area and rate of lateral erosion and accretion were smaller than those on the left bank.
We analyzed the evolution of the middle and lower reaches of the Yangtze River from Chenglingji to Datong. The results show that the overall change of river shoreline is not significant, but the local area changes dramatically; the lateral scouring and siltation area of the river is basically the same; the area of the river core continents has been reduced in recent years and is in a shrinking state; the amount of siltation in the riverbank has changed from siltation to scouring. The river channel from Chenglingji to Datong is mainly of the goose head branching type, with the river channel separated by a river core continent to form a branching section, which is often unstable and evolves periodically, and during the alternating evolution of the branching channel, the continental beaches in the river channel will also change in position, merge, and grow with the changes.
5.2. Effects of Human Activities on River Channel Change
Based on the above evolution characteristics, we discuss the driving force of the river channel evolution from natural and human factors. The results show that the geological conditions on both sides of the river, the implementation of the revetment project, and the improvement of the vegetation cover all enhanced the scour resistance of the riverbank [63
], while rainstorm floods, the operation of the Three Gorges Dam, and sand mining activities caused scouring of the river channel. The construction of water conservancy projects can lead to discontinuous changes in river morphology, destroying the stability of the ecosystem. They can also change the relationship between water and sand in downstream rivers. For example, after the completion of the key project hub to restore navigation in the Shaying River, a large amount of siltation occurred in the approach channel, which seriously affected the restoration of navigation in the Shaying River and restricted the economic development [64
]. Because the Three Gorges Dam intercepted a large amount of sediments in recent years, the sediment load from Chenglingji to Datong in the middle and lower reaches of the Yangtze River has decreased sharply. There was an increasing trend of sand mining in the river that caused the release of clear water and led to an unsaturated state of the sediment-carrying capacity [65
]. In order to achieve a balanced state, the riverbank, the bar, and the riverbed were scoured to recharge the sediment [66
Sand mining in the river will cause the riverbed in the sand mining area to be undercut and create a height difference with the surrounding area, forming a sand mining pit. It will make the overwater area increase, and the sand from the upstream will tend to silt in this area, thus causing the sediment content of the downstream river to decrease. Looking at the process of sand mining activities in the middle and lower reaches of the Yangtze River, it can be divided into three main stages. From the beginning of indiscriminate mining, to the ban, and then to the orderly lifting of the ban, each process will cause changes in the amount of sand transport in the river [67
In the process of river scouring, the riverbank is generally maintained in a stable state, mainly because there are many natural nodes on the riverbank from the Chenglingji to Datong River section, and the stability of the riverbank is strong. Secondly, a series of revetment projects have been implemented to maintain the stability of the river. The roots of riparian vegetation have the effect of stabilizing the riverbank silt, while the presence of vegetation reduces the velocity of the water flow, making it easier for river sediment to silt up there. Compared to 1989, the vegetation cover on both sides of the river has improved. It was found that the riparian vegetation not only stabilized the riverbanks, but also inhibited the bank erosion, thus slowing down the lateral evolution of the river channel. The improvement of the vegetation cover in the section from Chenglingji to Datong has led to an increase in streambank impact resistance and streambank stability.
The scouring of the river channel was mainly manifested in the scour depth and coarsening of the riverbed. Over time, the vertical scour drops the water level, reduces the water surface slope, weakens the scour capacity, and finally the channel begins to silt up.
5.3. Uncertainty Analysis
Remote sensing images from several periods were used, but the accuracy of the results was low due to some positional biases in local areas. The water extraction was based on the use of MNDWI, which, compared to NDWI, has higher values over water and lower values over buildings. Thus, MNDWI facilitates the distinction between buildings and water bodies, increasing the accuracy of the extracting water. Bathymetric data were too expensive to obtain for the study of the mid-channel bar evolution, and its quantification by remote sensing is not accurate. Therefore, the measured data will need to be supplemented in subsequent research to enhance the accuracy of the results. The acquisition and quantification of natural factor data for the analysis of their impact on the river channel evolution were challenging. Therefore, we only considered precipitation as a component of climate change because water evapotranspiration, which is difficult to obtain, has little effect on runoff and sediment, and groundwater has little effect on inter-annual changes, mainly manifesting as intra-annual distribution. Additional influencing factors should be taken into consideration in a follow-up study. The impoundment of the Three Gorges Dam sharply decreased the sediment content in the river, while the runoff changed little. In future research, we should continue to pay attention to the water and sediment situation in the river, especially the impact of reservoir impoundment.
In recent years, the total lateral erosion and total accretion area of the riverbank in the middle and lower reaches of the Yangtze River from Chenglingji to Datong has been basically the same in terms of the evolution of the river channel. The lateral erosion and siltation of riverbanks have not changed significantly. The river core is in the process of shrinkage by scouring in recent years. The mid-channel bars have been shrinking by scouring in recent years. The river trough was silting before 2003, with a siltation volume of about 553 million m3; after 2003, the process reverted to scouring, with a volume of about 1.278 billion m3. The longitudinal evolution of the river channel changed from the original siltation state to the scouring state. The annual river runoff has not changed much in recent years, but the amount of sand transported has decreased sharply since 2003. The annual runoff variation is mainly influenced by precipitation, while the annual sand transport variation is mainly influenced by human activities. Based on the analysis of the evolutionary characteristics and mechanisms of the river channel, we expect it to remain in a scouring state for some time.
The analysis of the morphological evolution characteristics and driving forces of the Chenglingji to Datong River section indicates that it is influenced by fluvial geomorphological processes and human activities at different spatial and temporal scales. Different geomorphological processes and human activities have led to an increase in siltation and a sudden decrease in sand transport, due to the felling and erosion of loose sediments in the catchment area, and the erosion and excavation of riverbanks by an increasing amount of engineering constructions. These have changed the morphology and dynamics of the river. Changes in vegetation cover and rainfall on both sides of the river reflect the influence of natural factors on the river morphology. Changes in river water and sand conditions will inevitably lead to adjustments in the river morphology, which will have a negative impact on the river embankments, navigation channels, and shoreline resource use. Studies on the influence of geomorphic processes and anthropogenic factors on flood behavior in the Chenglingji–Datong River section should inform and guide flood prevention and mitigation strategies for the entire Yangtze River and other river systems with similar geomorphic, tectonic, and socioeconomic contexts.
Based on Landsat series satellite remote sensing images, MNDWI is used to extract the river potential of the middle and lower reaches of Yangtze River. It can overcome the limitations of various factors, such as natural conditions and hardware equipment, track the river potential changes in real time, and provide systematic scientific support for river potential evolution research. The continuous development of China’s domestic satellite remote sensing program will lead to an increase in the number of remote sensing platforms, the abundance of remote sensing data sources, and the spatial and temporal resolution of remote sensing images, making more accurate river remote sensing monitoring results possible. Additionally, the study of this section of the river complements related studies of the middle and lower reaches of the Yangtze River, contributing to a comprehensive understanding of the relevant Yangtze River channels.
The development of the Yangtze River section has gone through a stage from natural role as the dominant role, to joint role of nature and human, to human control as the dominant role, and natural influence as the secondary role. It also adds more uncertainties to the prediction of river channel changes. Therefore, it is necessary to adhere to the principle of comprehensive management in accordance with the branching river section: stabilize the branching river potential, rationalize the use of side beaches, reduce harmful branching, strengthen supervision and cooperation, and improve management. The whole river potential will remain relatively stable.