Three Gorges Dam Research Articles

Disproportional erosion of the middle-lower Yangtze River following the operation of the Three Gorges Dam

The operation of the Three Gorges Dam (TGD) modifies downstream flow and sediment regimes, triggering disproportional fluvial responses at different distances downstream. However, our understanding of the downstream geomorphic changes in the middle-lower Yangtze River remains incomplete due to the complexity of the river responses across temporal and spatial scales. Here, we leverage data on discharge, suspended sediment concentration (SSC), riverbed grain size, cross-sectional profiles and high-resolution channel bathymetric maps at different locations downstream of the TGD to investigate geomorphic responses. The results show that the magnitude of fluvial erosion decreases downstream, with the Yichang-Luoshan Reach (the first ~500 km downstream) experiencing the most severe erosion in 2003–2020 (~9.05 × 104 t/km/yr). Local changes in riverbed morphology include channel bar erosion, channel incision (~0.43 m/yr in CS1 near the dam site over 2002–2019), riverbank retreat and bed material coarsening (an increase in D50 from 0.175 to 43.1 mm at Yichang station from 2002 to 2017). Such marked erosion is caused by the sharply reduced SSC in the dominant discharge range (10,000–30,000 m3/s) and the extended duration of this dominant discharge range. The sediment erosive magnitude in the Luoshan-Datong Reach is relatively small (3.85 × 104 t/km/yr) in 2002–2020. The Luoshan-Hukou Reach (~500–1000 km downstream) exhibits moderate channel incision, minor bed material coarsening and moderate mid-channel bar lateral erosion. The Hukou-Datong Reach (below 1000 km downstream) experienced minor geomorphic change without significant evidence of bed material coarsening. The relatively small impact of the TGD on the lower reach from Luoshan to Datong can be mainly attributed to the progressive SSC recovery along the river induced by upstream channel erosion providing sediment replenishment. These findings have significant implications for estimating geomorphic changes in response to upstream damming and thus could inform better river management and ecological assessment in other similar alluvial rivers.

Impact of upstream reservoirs on geomorphic evolution in the middle and lower reaches of the Yangtze River

AbstractIntensive human activities in river basins have substantial effects on fluvial hydrological and morphological processes and developmental trends. In this study, we assessed the evolutionary characteristics and developmental trends of the fluvial morphological system of the Three Gorges Dam (TGD) downstream to understand the impacts of human activities on the fluvial hydrological–morphological system of the region. The results indicated that the Three Gorges Project (TGP) and other upstream cascade reservoirs promoted sediment retention, which in turn aggravated the trend of declining sediment load and cumulative scouring over a long distance in the channels downstream of the TGD. Approximately 84% of scouring occurred in the low‐water channels, along with thalweg scouring and riverbank collapse. Notably, extended operation of the TGP and other upstream cascade reservoirs has resulted in gradual changes in the downstream riverbeds. Specifically, we found that the most intensive scouring zone shifted downstream from the near‐dam Yichang–Chenglingji reach (0–408 km) to the Chenglingji–Hankou reach. The scouring–silting distribution in swales has also changed: the scouring of deep swales and silting on floodplains prior to the TGP have been replaced by the scouring of both deep swales and floodplains. The scouring intensity of riverbeds in the Yichang–Hukou reach (954 km) was slightly higher than that forecasted prior to the TGP; the scouring intensities increased significantly during 2013–2021, which could be attributed to the TGP and other upstream cascade reservoirs; river sand mining and waterway dredging are secondary factors. We conclude that, owing to the effects of sediment retention by cascade reservoirs in the upper reaches of the Yangtze River (or Changjiang River), the cumulative scouring intensities of riverbeds in the middle and lower reaches of the Yangtze River will continue to increase in future decades.

Dam construction reshapes sedimentary pollutant distribution along the Yangtze river by regulating sediment composition

Dam construction has far-reaching impacts on pollutant accumulation and the pollutant-induced quality of aquatic environments. Nonetheless, its large-scale effects on pollutant distribution in sediments, which greatly contribute to the environmental impacts of coexisting pollutants, remain poorly understood. We collected sediments from the Yangtze River during the dry and normal seasons (with ‘normal’ defined in terms of precipitation level), and examined how dam construction alters the spatial trajectories of both inorganic and organic pollutants in the sediments. Sediment composition exhibited linear variation from the upper to the lower reaches, with clay and silt particles dominating the sediment in the Three Gorges Reservoir and sand particles dominating in the middle–lower reaches. Accordingly, upstream of the Three Gorges Dam (TGD), sedimentary carbon, nitrogen, phosphorus, heavy metal, polycyclic aromatic hydrocarbons (PAHs), and oxygenated PAHs (OPAHs) contents increased toward the TGD owing to its regulation of the spatial variation in sediment particle size. The TGD caused upstream sedimentary accumulation of pollutants to be higher nearer to the TGD than in the upper reaches by 17%–129% for carbon, nitrogen, and phosphorus, 7%–51% for heavy metals, 30% for PAHs, and 140% for OPAHs. Pollutant content was sharply lower below the TGD, by 0.58–11.15 times for carbon, nitrogen, and phosphorus, 0.1–2.6 times for heavy metals, 1.7 times for PAHs, and 5.6 times for OPAHs. Upstream of the TGD, levels of NH4+-N, the main form of N in the interstitial water of the Yangtze River, increased lineary toward the TGD, whereas those of NO3−-N and NO2−-N decreased. Sedimentary organic matter source contributions were consistent along the Yangtze River, being on an average 46% for C3 plants and 28% for soil organic substances, further confirming the dam's regulatory effect on pollutants. These findings provide a foundation for future assessments of the environmental impact of dam-induced river fragmentation and hydrological alterations, and for developing advanced watershed pollutant management strategies.

Total Phosphorus and Nitrogen Dynamics and Influencing Factors in Dongting Lake Using Landsat Data

Total phosphorus (TP) and total nitrogen (TN) reflect the state of eutrophication. However, traditional point-based water quality monitoring methods are time-consuming and labor-intensive, and insufficient to estimate and assess water quality at a large scale. In this paper, we constructed machine learning models for TP and TN inversion using measured data and satellite imagery band reflectance, and verified it by in situ data. Atmospheric correction was performed on the Landsat Top of Atmosphere (TOP) data by removing the effect of the adjacency effect and correcting differences between Landsat sensors. Then, using the established model, the TP and TN patterns in Dongting Lake with a spatial resolution of 30 m from 1996 to 2021 were derived for the first time. The annual and monthly spatio-temporal variation characteristics of TP and TN in Dongting Lake were investigated in details, and the influences of hydrometeorological elements on water quality variations were analyzed. The results show that the established empirical model can accurately estimate TP with coefficient (R2) ≥ 0.70, root mean square error (RMSE) ≤ 0.057 mg/L, mean relative error (MRE) ≤ 0.23 and TN with R2 ≥ 0.73, RMSE ≤ 0.48 mg/L and MRE ≤ 0.20. From 1996 to 2021, TP in Dongting Lake showed a downward trend and TN showed an upward trend, while the summer value was much higher than the other seasons. Furthermore, the influencing factors on TP and TN variations were investigated and discussed. Between 1996 and 2003, the main contributors to the change of water quality in Dongting Lake were external inputs such as water level and flow. The significant changes in water quantity and sediment characteristics following the operation of the Three Gorges Dam (TGD) in 2003 also had an impact on the water quality in Dongting Lake.

Dam Impacts on Seasonality of Water and Sediment Transport in Intensively Managed River Basins: A Case Study of the Yangtze River

AbstractExtensive studies have been performed on water and sediment transport of the Yangtze River in recent decades, but very few studies were focused on seasonal and daily variations. After the construction of Three Gorges Dam (TGD) in 2003, several key cascade dams were built on the Yangtze upstream. Annual, monthly, and daily data were used to study the seasonality change on water level, water discharge, and sediment load in response to multiple reservoir impoundments. TGD impacted the water level inside the reservoir greatly by shifting the peak water level from summer to winter and made the hydrograph more asymmetrical. In contrast, minor changes in water levels (<2 m) occurred at stations downstream of the TGD mainly due to channel incision. Downstream of TGD at Yichang station, annual hydrograph was flattened slightly: water discharge in the dry season increased 49% whereas that of flood season decreased 14% from 1956–2002 to 2013–2017. Mean annual water discharge downstream of the TGD was slightly impacted because the impounded water was <5% of annual inflow of the reservoir. Comparing with the Yangtze, the annual hydrographs of other intensively managed rivers like the Yellow, Missouri, and Colorado have been flattened more effectively by reservoir impoundments. The sediment rating curves at Yichang and Hankou changed from an oval‐like shape before 2013 to two overlapping and flattened lines in 2013–2017. This shift indicates that the rising and falling limbs of the hydrograph experienced similar sediment concentrations in 2013–2017 and suggests a less obvious hysteresis effect. Over 50% of the sediment load observed at the seaward‐most Datong station was contributed by the large tributaries in the upper reach, like Jinsha and Jialing rivers, before 2003. The channel scour downstream of TGD contributed 49%–64% of the Datong sediment load after 2003.

From the water sources of the Tibetan Plateau to the ocean: State of nutrients in the Changjiang linked to land use changes and climate variability

Anthropogenic activity is an important driver of changes in the chemistry of nutrients (N, P, and Si) over watersheds at the sub-continental scale (e.g., 106 km2) and can markedly modify their seaward fluxes to the global ocean. In the present study, we reviewed the current status of nutrient chemistry in Changjiang (Yangtze River) based on data collected through 11 expeditions along a river course spanning 4,500 km and 15–20 major tributaries during 1997–2016 as well as monthly monitoring at the river mouth since 1980. The data were analyzed together with published results in the literature to synthesize the recent developments and current state of nutrients in the Changjiang. Previously published results from the Qinghai-Tibetan Plateau head waters were included to realize the systematics of nutrients for the whole drainage basin. Here, we showed that tributaries of the upper reaches of watersheds collectively determine the regime with high concentration and skewed species ratio of nutrients in the Changjiang mainstream, producing profound effects over a water course of 2,000–2,500 km further downstream and until the river mouth. Moreover, using data across the Three Gorges Reservoir (TGR) during 2003–2016, we evaluated the trapping and/or amplifying effects of the Three Gorges Dam (TGD) on nutrient chemistry. Tide-influenced river delta contributed an additional 20% dissolved inorganic phosphorus and 5–10% dissolved inorganic nitrogen and dissolved silicates to the seaward flux, dramatically affecting the stoichiometry of nutrients at the river mouth. Next, based on compiled data on supply and export, legacy nutrients were evaluated. Both nitrogen and phosphorus are in the accumulation phase over the watersheds, and the legacy nutrient fluxes are much higher than the annual riverine seaward fluxes. Finally, we demonstrated that the seaward fluxes of anthropogenic nutrients from the Changjiang exceed those from other top 10 largest rivers on this planet, which can be attributed to land use changes in the China over the last three to four decades.

Sediment load crisis in the Mekong River Basin: Severe reductions over the decades

The boom in water infrastructure in the Mekong Basin has raised concerns over the annual supply of water and sediment among its riparian communities. By consolidating various datasets, continuous series of sediment load data were estimated for several stations located within the Lower Mekong Basin. At Chiang Saen in Thailand, the nearest station to the Chinese dams, the average sediment load was 79 ± 32 Mt./yr during the pre-dam period of 1965–1991. However, from 2010 to 2019 – during which a series of mega-dams were built in China – the sediment load decreased drastically by 84 % to only 12.5 ± 4.6 Mt./yr. This phenomenon of reduced annual sediment load during the mega-dam era (2010–2019) as compared to during the pre-dam era (1965–1991) can be observed at stations downstream from Luang Prabang (−53 %) to Nong Khai (−62 %) to Khong Chiam (−33 %).One of the drivers of this sediment load crisis is the rapid development of upstream dams. Especially after 2003, Chinese dams have reduced sediment supply to the downstream Mekong Basin severely. Concurrently, there was an increase in sediment contribution from the stretch of the Mekong River from Chiang Saen to Khong Chiam. A positive outcome of this increased sediment contribution was its buffering effect against the reduction in sediment load from the Chinese part of the Mekong Basin. Although sediment load at Kratie – the gateway station before the alluvial stretch comprising the Cambodian floodplains and Vietnam Mekong Delta – decreased from 78 ± 22 Mt./yr (1995–2009) to only 60 ± 21 Mt./yr (2010–2019), the reduction would have been higher without the increased sediment from the Chiang Saen – Khong Chiam stretch. However, with upcoming planned dams in Laos and Cambodia, this buffering effect is likely to be temporary, implying that the sediment load crisis as already experienced by the downstream communities can only become more severe.

Monitoring the spatiotemporal change of Dongting Lake wetland by integrating Landsat and MODIS images, from 2001 to 2020

Dongting Lake wetland provides critical ecological functions in the Yangtze River Basin, however its landscape pattern has changed during the past 20 years due to both climate change and the human activities such as the operation of the Three Gorges dam (TGD). Numerous studies have used remote sensing technology to monitor such changes. However, most of studies were conducted at low spatial resolutions (250–1000 m) or low temporal resolutions (few images per year), which can introduce a degree of unreliability as regards the associated conclusions. To thoroughly analyze the spatiotemporal characteristics of Dongting Lake wetland, high spatiotemporal resolution images from 2001 to 2020 have been produced by a modified version of the flexible spatiotemporal data fusion method (cuFSDAF 2.0) developed in this paper. This method is able to retrieve both phenology and land-cover changes both quickly and accurately. In addition, a tailored category by category extraction mapping method (CCEMM) is proposed to reliably classify wetland cover types. The mapping results show that the water area expanded by 66.43 Km2 in the wet season, and shrank by 132.86 Km2 in the dry season, between and including the years, 2001 to 2020. It appeared that the water area in the dry season declined obviously from 2003 to 2010 due to the operation of the TGD and is gradually stabilized after 2010. The reed area increased by 279.59 Km2 and the grass area decreased by 220.46 Km2 in the dry season, from 2001 to 2020. The shrinkage of the water area in the dry season may be one of the main factors driving reeds to occupy the previous grass growth area.

Decadal Hydro-Morphodynamic Evolution of a Sandy Meander Bend Downstream of the Three Gorges Dam and Impact on Waterway Navigation

The Three Gorges Dam (TGD) in the Yangtze River profoundly regulates the downstream river discharge and sediment load, causing significant river morphodynamic changes since 2003. Understanding such changes is vital for managing the navigational waterway. Using the Wakouzi waterway 190 km downstream from TGD as an example, we examined its decadal hydro-morphodynamic evolution based on hydrological and topographical data collected from the 1990s onwards. The results showed that the suspended sediment load has reduced by 85.8% downstream of TGD, although the annual runoff has not changed. The suspended sediment has become coarser due to depletion of the fine sediment source. As a result, the river channel has been substantially eroded, particularly the deep-water parts of the channel. Erosion of the elongated sand bars has emerged as well, even in the presence of channel regulation works. Consequently, channel degradation has led to a lower river stage under the same river discharge compared with the pre-TGD condition, particularly in the dry season with a lower river discharge. These changes imply worsened navigability of the examined riverbend. The results of this study have implications for management of other river reaches in the middle Yangtze River and other rivers undergoing similar changes worldwide.

Study on the spawning habitat suitability of four major Chinese carps in the fluctuating backwater area of the Three Gorges Reservoir

The upstream of the Yangtze River has abundant fish resources and is home to the four major Chinese carps (FMCCs), which are economically important fish in the Yangtze River that reside in the fluctuating backwater area (FBA) of the Three Gorges Reservoir (TGR). Their spawning habitat has been influenced by the Three Gorges Dam (TGD) operations. This study investigated the effects of dam operation on spawning habitat suitability and the potential spawning sites of FMCCs, using a physical habitat suitability model combined with a two-dimensional (2D) hydrodynamic numerical model. The habitat model was verified by field measurements of the fish egg density in the FBA, illustrating the capability of predicting the spawning availability of FMCC by comparing the habitat suitability index (HSI) distribution and the weighted usable area (WUA) with the simulation scenarios before and after the TGD construction. The habitat model was then applied to evaluate the effects of TGD operation on spawning habitat suitability and potential spawning grounds in the FBA. These results implied that the TGD operation had negative effects on spawning habitat suitability and produced temporal changes in the available WUA in the FBA. The predictions revealed that impoundment of the TGD decreased the suitability of the hydrodynamic factors. The main spawning period was delayed from April to May, and the variation in the water level induced by the reservoir operation scheme was likely the main factor. The HSI distribution in the FBA showed that the potential spawning grounds decreased to one site located in the Mudong-Luoqi River section. Furthermore, there was a rapid reduction in WUAs, accounting for approximately 18% of that seen in 2001 (before the TGD operation). The results of this study can aid in the management of the spawning grounds of FMCCs in the TGR and provide efficient methods for improving spawning habitat suitability, such as adjusting the pool level of the TGD during the falling stage.

Trends in River Total Suspended Sediments Driven by Dams and Soil Erosion: A Comparison Between the Yangtze and Mekong Rivers

AbstractGlobal river systems are experiencing rapid changes in sediment transport under growing anthropogenic and climatic stresses. However, the response of sediment discharge to the coupled influence of anthropogenic and natural factors and the associated impacts on the fluvial geomorphology in the Yangtze and Mekong rivers are not comprehensively assessed. Here, we recalibrated a seamless retrieval algorithm of the total suspended sediment (TSS) concentrations using in situ data and concurrent satellite data sets to analyze spatiotemporal patterns of the TSS concentrations in the lower Yangtze and Mekong rivers. Combined with soil erosion rates estimated by the Revised Universal Soil Loss Equation for the past 20 years, we examined the contributions of different factors to TSS trends. The results show that TSS concentrations in the Yangtze River decreased from 0.47 g L−1 in 2000 to 0.23 g L−1 in 2018 due to the construction of the Three Gorges Dam (TGD), especially in the Jingjiang reach, with a declining magnitude of 0.3 g L−1 (∼56%) since the TGD began operating. The Mekong River experienced increasing TSS concentration trends upstream and decreasing trends downstream from 2000 to 2018, possibly attributed to increased upstream soil erosion and decreased downstream water discharge. Declining TSS concentrations in both rivers have driven varying degrees of river channel erosion over the past two decades. This study investigated long‐term changes in the TSS concentrations and soil erosion in the Yangtze and Mekong rivers, and the results provide baseline information for the sustainable development of river sediment delivery.

Paleolimnological records for tracking dam-induced changes in the composition and supply of sediment to middle Yangtze floodplain lakes

A global boom in dam construction has reduced sediment loading of large rivers, as well as their connected floodplains. In order to explore potential effects of hydrological regulation on floodplain ecosystems, this study presents concentrations of inorganic elements, organic matter content and grain size spectra in 210Pb-dated sediment cores of five lakes in the middle Yangtze River floodplain. These lakes had free hydrological connectivity with the Yangtze River before the operation of local sluice gates, and hence K-rich but Al-poor particulates from the upper Yangtze reaches can be transported into lakes during periods of high river discharge. After hydrological regulation, sedimentary K/Al ratios registered substantial decreases in the study lakes, probably due to the declining supply of fine-grained and K-enriched riverine particulates from the Yangtze River. In East Dongting Lake, a hydrologically open lake proximal to the Three Gorges Dam (TGD), the sharp decrease in K/Al ratio after 2003 mainly responded to declining sediment supply from the Yangtze River after the TGD operation. In addition, prolonged water retention time probably promoted aquatic production and greater deposition of organic matter, as indicated by recent increases in organic matter content in the upper strata of the dammed lakes. Taken together, sedimentary K/Al ratios provide essential information on effects of past hydrological changes on sediment supply and aquatic productivity over multi-decadal timescales in these floodplain lakes with scarce monitoring data. The impacts of dam construction during recent decades on sediment composition in middle Yangtze floodplain lakes might be widespread in other similar floodplains worldwide. Thus sedimentary records can be utilized to inform and direct floodplain management.

Economic and Agricultural Impacts of Building a Dam—Evidence from Natural Experience of the Three-Gorges Dam

As the largest dam in the world, the impacts of the Three-Gorges Dam (TGD) on economy and agriculture in the counties along the Yangtze River in China have been subject to debates for a long time, but no conclusions have been made. This paper employs panel data with a wide variety of economic and agricultural variables for 751 counties over the period from 1997 to 2010, which covers the whole building period of the TGD. By calculating the distance of these counties to the dam site and applying the differences-in-differences (DID) method, the results generally imply that the TGD negatively affected economic growth in the downstream counties along the mainstream river. Conversely, counties located closer to the dam site in the upstream received economic benefits. Regarding its effects on agricultural productions, cotton yields of the upstream were negatively affected by building the TGD, while oil production planted in the upstream region were stimulated to grow since the functioning of the dam in 2003. This study proves both the economic and agricultural advantages/disadvantages of the dam in different construction periods for different locations of counties, and sheds light on policy implications for compensating the counties from economic and agricultural lenses due to the TGD construction.

Comprehensive Environmental Flows Assessment for Multi‐Guilds in the Riparian Habitats of the Yangtze River

AbstractThe development of new strategies for the operation of existing dams is essential for restoring ecosystems that have already been affected by the construction and current regulation of these dams. The assessment of environmental flows (EFs) is significant in guiding such strategic dam operations. However, most of the existing methods just focus on one species or guild without simultaneously considering the interactions with other aquatic organisms. This study proposes an innovative method to determine comprehensive environmental flows (CEFs) for multi‐guilds (fish, macroinvertebrates, and hygrophytes) based on the habitat simulation model. We focused on the riparian habitats in the middle reaches of the Yangtze River, coupled the biological preferences of multi‐guilds with suitable habitat factors, including velocity, depth, and substrate, and then analyzed the relationships of flow and weighted usable areas to determine the CEFs for multi‐guilds. The results showed that while the optimal CEFs could be satisfied only in Spring and Summer, the threshold CEF conditions could be met in the Jianli reach throughout the year after the impoundment of Three Gorges Dam (TGD). The results obtained by using the CEFs assessment could better satisfy the EFs for multi‐guilds than the ones developed for individual species. The CEFs could be also used to evaluate the impact of TGD on multi‐guilds’ habitats and optimize ecologically‐friendly operations in already designed dams. This study is helpful for maintaining ecosystem integrity in Yangtze River and provides a good example for use in other dams facing similar issues.

Roles of dam and climate change in thermal regime alteration of a large river

Water temperature in the Yangtze River mainstream has been experiencing significant changes due to the climate change and the operation of a series of world-class large dams, i.e. the Three Gorges Dam (TGD) and upstream cascade dams (CDs). However, quantitative effects of these factors are not fully known, which hinders our understanding on the thermal regime alterations and further prediction in ecosystem response. Here, we will simulate the riverine water temperature (RWT) variations by building a physics-based model, and quantify the respective impacts from TGD, CDs and climate change through a model-based framework. In the framework, both the dam-regulated hydro-thermodynamic processes and the spatial heterogeneity of the meteorological condition in this large river-reservoir system are thoroughly considered. The results show a fluvial warming of 0.31 °C–0.56 °C/10a in recent three decades, mainly attributed to climate change (44%–80% for different reaches). The dam has caused a substantial seasonal thermal lag, e.g. ∼40 d near the TGD in the dry season, and accompanying severe alterations in the monthly RWT. A reduction of 10% in seasonal RWT range is identified, which is attributed to both dam and climate change.

Dynamics of insect communities across a unique network of hydrologically altered riparian habitats in central China

AbstractThe establishment of hydroelectric dams can lead to major changes in hydrology for habitats within the reservoir drawdown zones. The Three Gorges Dam (TGD) in China is the world's largest hydroelectric facility, with a reservoir covering 1080 km2 and a drawdown zone covering 350 km2. Since the TGD's establishment in 2008, the formerly terrestrial habitats within the dam's drawdown zone have effectively been converted to riparian habitats. A strong understanding of ecological dynamics in these novel habitats is critical to their ongoing management. Given the major contribution of insects to ecosystem function and biodiversity, we surveyed riparian insect communities semi‐annually from 2011 to 2018 across a 600‐km network of 18 sites in the TGD's drawdown zone. During this period, the total abundance of insects increased 4.4‐fold, and the composition of insect communities shifted significantly, with Diptera increasingly dominant in later years. Insect abundance trajectories were coupled with concomitant changes in local climate and soil chemistry, which have in turn been brought on by the novel hydrological regime in the drawdown zone. Revegetation efforts had limited impacts on riparian insect abundances. Overall, riparian insect communities have undergone marked shifts since the TGD started operating. We cannot explicitly attribute these effects to the TGD's establishment, but our results indicate that they might have been shaped by—or are at least coupled with—the profound changes in hydrological conditions associated with the TGD's establishment.

Monitoring spatial and temporal dynamics of wetland vegetation and their response to hydrological conditions in a large seasonal lake with time series Landsat data

Periodic hydrological processes of seasonal lakes are the dominant factors leading to the development of vegetation communities and ecological processes. Dongting Lake is a typical seasonal lake in the middle and lower reaches of the Yangtze River and its hydrological conditions have changed significantly due to the construction of the Three Gorges Dam (TGD). However, there are knowledge gaps in the responses of wetland vegetation dynamics to the changes of hydrological conditions, especially in cloudy and rainy regions, where continuous large-scale monitoring is still challenging. In this study, by using time series of Landsat data on Google Earth Engine, a long-term cloudless remote sensing images dataset of Dongting Lake during the dry season was constructed. The multi-year wetland vegetation from 2003 to 2020 was mapped using the random forest model. Relationships between wetland vegetation dynamics and hydrological conditions (e.g., water level and inundation character) were examined using correlation and linear analysis. Results revealed that wetland vegetation in the Dongting Lake experienced significant inter-annual spatial heterogeneity and temporal dynamic equilibrium with a slight increase during the post-TGD period. Only Carex communities at elevations above 28 m and below 26 m displayed apparent changes. Reed communities have relatively high selectivity to inundation frequency, and are affected mainly by changes in water levels during the dry season of the current year. Hydrological conditions during the rising period have an important impact on the first growth and reproduction stage of Carex, while higher water levels or more rapid surface submergence lead to a decrease in seed density in the soil of low-elevation floodplains. This study deepens our knowledge of changes of wetland vegetation and their response to hydrological conditions, and is of great significance to the ecological protection and restoration of Dongting Lake.

Seasonal variation of heavy metals in suspended sediments downstream the Three Gorges Dam in the Yangtze River.

High sediment flux in large rivers provide sufficient dilution to the heavy metals' concentration. However, sediment starvation caused by hydrological engineering in recent decades has been reported worldwide. Thus, a study is necessary on the influences of recent declining sediment flux on heavy metal pollution change in the suspended sediments. In this study, heavy metal concentrations and speciation (Cd, Pb, Zn, Cu, Co, Ni, and Cr) in suspended sediments were investigated downstream the Three Gorges Dam (TGD) during dry and flood seasons. Substantial changes of Pb, Zn, Cd, and Cu along the river channel were found which were constrained by the dilution efficiency of suspended sediment during the dry season. High proportion of labile fraction revealed anthropogenic sources of heavy metal. Moreover, the historical trend of metal content illustrated TGD construction together with anthropogenic influx both contribute to the increasing environmental risk in the Yangtze River basin.

A cascading reaction by hydrological spatial dynamics alternation may be neglected

Water regime changes play a critical role in the structure and function of watershed ecosystems. However, most previous studies focused only on static fixed-point quantitative change at a given time, ignoring the hydrological spatial distribution states of wetting and drying and lacking dynamic indicators for characterization. Here, we constructed a new dynamic index to characterize water alternation of wetting and drying of Poyang Lake, the largest freshwater lake in China, using all available Landsat images and Google Earth Engine from 1987 to 2020. In addition, we analyzed the relative contribution of the dam to water regime changes according to geographical characteristics, and a neglected cascading reaction was found between the upstream and downstream of the basin. The results showed that the alternation of Poyang Lake significantly intensified, and varied with different years. Although the apparent regulation of the Three Gorges Dam (TGD) has no significant impact on the maximum storage of the downstream Poyang Lake and the runoff of the further Yangtze Estuary, the TGD has changed the water spatial alternation of Poyang Lake, resulting in a cascading reaction to the runoff of the Yangtze Estuary.

Contributions of different sources to sediment transport in the Middle Yangtze River under intensive channel degradation

Sediment transport in alluvial rivers is critical to reshape channel morphology and to the global geochemical cycle. This study reports a detailed investigation into the contributions of different sources to sediment transport in the Middle Yangtze River (MYR). It is found that: (i) after the operation of the Three Gorges Project (TGP), the dominant sediment source in the MYR changed from the upstream input to the fluvial erosion and tributary input, with the upstream input providing 38% of the outlet sediment amount in the MYR and the fluvial erosion and tributary input providing around 39% and 23%, respectively; (ii) river bed erosion contributed to 21% of the transported sediment after the TGP, whereas bank erosion contributed to around 18%, which was estimated to be 2 times larger if bank protection works were not constructed after the TGP; (iii) in terms of the sediment transport processes of different fractions after the TGP, the fraction of fine sediment (diameter d ≤ 0.31 mm) in the MYR mainly came from the upstream and tributary input, and the fraction of medium sediment (0.31 mm < d ≤ 0.125 mm) almost equally came from each source, whereas the fraction of coarse sediment (d > 0.125 mm) was primarily eroded from the bed and banks. Besides, the MYR was transformed from a transport-limited river system into a supply-limited one after the TGP operation, with the total amount of sediment supply accounting for 21% of its transport capacity under current flow and boundary conditions. The development of bed armoring in the main stream and the construction of various river regulation works, would result in a further decrease in the sediment supply rate of the MYR in the future.