Climate,Ecosystem and Migration:The Three Gorges Dam Study

The Three Gorges Dam,known as one of the biggest project items throughout the history,is a milestone for the development of China.Since it was completed in 2006,the dam has been persistently supplying ample electricity to the southern and eastern part of China.Despite its impressive output and benefits notwithstanding,we have to objectively pay our attention to the long-term impact that the Three Gorges Dam leaves to the place where we live,especially the influence on the climate,the ecosystem and the migration pattern of the reservoir area.

Review of the Relocation of General Zhang Fei’s Temple in View of the Three Gorges Dam Project

As a national cultural property,General Zhang Fei’s Temple is the largest ancient architectural complex in the Three Gorges of the Yangtze River.It was built in the Han Dynasty and has a long history of nearly 2,000 years.In 1994,the Chinese government launched the Three Gorges Dam project at the Yangtze River and implemented the largest relocation project for the General Zhang Fei Temple to prevent it from being submerged.The temple was relocated 32 kilometers upstream along with the local people of Yunyang County,and the historic environment was rebuilt on the opposite side of the migrant city of New Yunyang County.The temple was restored successfully and the tradition of offering sacrifices to General Zhang Fei continued in the local community.The relocation project lasted for 8 years and became the largest cultural heritage conservation project of the People’s Republic of China at the end of the 20th century.This paper comprehensively summarizes and reviews the project goals,implementation process,and project highlights of this relocation,so as to provide an important case reference for heritage conservation projects in the future.

Impact of the Three Gorges Dam on reproduction of four major Chinese carps species in the middle reaches of the Changjiang River

Silver carp(Hypophthalmichthys molitrix),bighead carp(Aristichthys nobilis),grass carp(Ctenopharyngodon idella),and black carp(Mylopharyngodon piceus) are the four major Chinese carps and are commercially important aquaculture species in China.Reproduction of these carp has declined since the construction of the Three Gorges Dam(TGD) due to an altered water flow and thermal regime in the Changjiang(Yangtze) River.However,details of the changes in reproduction of the four species are not well understood.To assess the impact of the TGD on reproduction of the four carp,we investigated their eggs and larvae at Yidu City,which is 80 km below the TGD,during 2005-2012.We examined differences in larval abundance of the four species in the Jianli section(350 km downstream of the TGD) before(1997-2002)and after(2003-2012) construction of the TGD.Based on these observations,the first spawning date of the four species was delayed a mean of about 25 days after the dam was constructed.Mean egg abundance in the Yidu section of the river was 249 million and mean larval abundance was 464 million,which were significant decreases since the 1980 s.Moreover,larval abundance in the Jianli section after the dam was constructed was significantly lower than that before construction(ANCOVA,P<0.05).The observed larval abundance accounted for only 24.66%of the predicted value in 2003 when the dam was first inundated.The present spawning grounds between the TGD and Yidu section of the river are very similar to those described in the 1980 s,and some spawning grounds exist upstream of the TGD.Large free-flowing stretches upstream of the TGD and the creation of artificially flooded downstream reaches are needed to stimulate spawning and effectively conserve these four major Chinese carps species.

Potential impacts of Three Gorges Dam in China on the ecosystem of East China Sea

The Changjiang River in China was dammed in 2003. The possible changes in matters fluxes from the river downstream after the completion of Three Gorges Dam and their potential impacts on the ecosystem of the East China Sea are discussed . The estuarine and coastal waters in the East China Sea were heavily fertilized by the inflow of nutrient-rich freshwater from the Changjiang River, which has led to severe eutrophication and frequent harmful algal blooms ,thus worsening the ecosystem health in this area. Analy- sis showed that the nutrient loadings are very likely to be reduced in the lower Changjiang River due to the construction of Three Gorges Dam. Especially for the total phosphorus, the discharges to the East China Sea will be reduced by one-third, which would relieve the severe eutrophication in this area. However, the expected decrease in the riverine silicate discharge would lead the ratio of silicon to nitrogen to be much less than 1 in the estuarine and coastal waters and thus may cause an elevation of flagellate growth. The changes in the annual water discharges and their seasonal distributions below the dam will be minor. Reduction of suspended particulate matter loading, due to the sedimentation behind the dam, will reduce the nutrient loadings of the particulate form especially for phosphorus, and decrease the turbidity of estuarine and coastal waters. On the other hand, this may enhance the erosion of the delta and the coasts as well as modifythe benthic ecosystem.

Impact of the Three Gorges Dam on the spawning stock and natural reproduction of Chinese sturgeon in the Changjiang River,China

Chinese sturgeon(Acipenser sinensis) is the flagship species of the Changjiang River.The migration route of this species is blocked by the first dam,the Gezhou Dam,and its reproduction is affected by the Three Gorges Dam(TGD),one of the largest dams in the world.We studied the impact of the impoundment of the Three Gorges Reservoir(TGR) since 2003 on the spawning stock and the natural reproduction of the Chinese sturgeon by using our monitoring data from 1997 to 2013.Results indicate that TGR impoundment has delayed the first spawning dates of the fish from middle-late October to late November,decreased the amount of spawning activities from twice to only once each year,and significantly reduced egg production.In particular,the fish did not demonstrate any spawning activities in 2013.Therefore,TGR impoundment significantly affects the natural reproduction of the fish downstream of the TGD.The spawning stock size of the fish is also predicted to further decrease in the future,which will lead to a risk of population extinction.Ecological regulations must be imposed on decreasing the water temperature to 20℃before mid-October and increasing water discharge downstream of the TGD in October to induce spawning of the Chinese sturgeon.

Diversity of microbial plankton across the Three Gorges Dam of the Yangtze River,China

The Three Gorges Dam （TGD） of the Yangtze River, China, is one of the largest irrigation and hydroelectric engineering projects in the world. The effects of huge man-made projects like TGD on fauna and macrophyte are obvious, mainly through changes of water dynamics and flow pattern; however, it is less clear how microorganisms respond to such changes. This research was aimed to examine differences in microbial diversity at different seasons and locations （in front of and behind the TGD）. In addition, differences between particle-attached and free-living communities were also examined. The community structures of total and potentially active microorganisms in the water columns behind and in front of the TGD were analyzed with the DNA- and RNA-based 16S rRNA gene phylogenetic approaches over three different seasons. Clone libraries of 16S rRNA genes were prepared after ampli- fication from extracted DNA and, for some samples, after preparing cDNA from extracted rRNA. Differ- ences were observed between sites at different seasons and between free-living and particle-attached communities. Both bacterial and archaeal communities were more diverse in summer than in winter, due to higher nutrient levels and warmer temperature in summer than in winter. Particle-attached micro- organisms were more diverse than free-living communities, possibly because of higher nutrient levels and heterogeneous geochemical micro-environments in particles. Spatial variations in bacterial community structure were observed, i.e., the water reservoir behind the TGD （upstream） hosted more diverse bacte- rial populations than in front of the dam （downstream）, because of diverse sources of sediments and waters from upstream to the reservoir. These results have important implications for our understanding of responses of microbial communities to environmental changes in river ecosystems affected by dam construction.

Three Gorges Dam Controls Sediment Coarsening of the Mud Patch on the Inner East China Sea Shelf

The well-known Three Gorges Dam(TGD) within the Yangtze catchment launched its operation in 2003. The effect of the TGD operation on the sediment size on the East China Sea shelf is rarely known. High resolution(0.5 cm sampling) grain size analysis and 137 Cs and 210 Pb dating of the DH8-1 core were conducted with core collected from the distal part of a main sink for the modern Yangtze sediment entering the sea, the Min-Zhe Coastal Mud Deposits(MZCMD) on the inner East China Sea shelf. The 137 Cs dating results show that the core DH8-1 formed during 1946–2012 with a mean deposition rate of 0.65 cm yr^(-1), indicating that the 0.5 cm sampling for grain size analysis in this local area could reflect environmental changes generally on a one-year time scale. The mean grain size of DH8-1 core sediment that deposited after 2003 is significantly larger than that deposited during 1988–2002. After ruling out other possible factors, we infer that the sediment coarsening of DH8-1 core after 2003 is attributed to the TGD operation which causes the erosion of the Yangtze subaqueous delta. Specifically, the TGD operation significantly intensifies the declining trend of the Yangtze sediment loads to the sea despite no decreased water discharge, which results in extensive erosion of the Yangtze subaqueous delta. The relatively coarse sediment of the subaqueous delta is eroded and resuspended by ocean dynamics and then transported by coastal current, finally depositing on the MZCMD area. In addition, the general sediment fining of core DH8-1 that deposited during 1988–2002, comparing with 1946–1987, is mainly caused by dam construction and soil and water conservation within the Yangtze catchment. Our findings are helpful for better understanding the effects of such a huge dam as the TGD on a sediment sink like the MZCMD of such a large river as the Yangtze River.

Chemical element transfer of weathering granite regolith in the Three Gorges Dam region of Yangtze River

Clearing up sediment and regolith on the foundation of the dam in the Three Gorges of the Yangtze River in 1999, riverbed were exposed. On the basis of weathering granite regolith sampled from different portions of the valley landforms, by analysing total chemical contents with X rays fluorescent slice and calculating proper value of chemical element transferring ratio and intensity, the transferring law of chemical elements in different portions of the landforms were concluded: 1) In various landforms of the river valley, the process of desilication is not distinct; 2) in weathering granite regolith of riverbed, easy soluble CaO and MgO are relatively enriched whereas Al2O3 tends to decrease. The enriching rate of Fe2O3 is the greatest in various landforms of the river valley; 3) in weathering granite regolith of flood-plain, K2O and MgO contents are relatively enriched; 4) the weathering granite regolith of valley slope is a typical north subtropical weathering regolith, and its chemical weathering degree is in the transition phase from early to middle period; and 5) there is an opposite layer where K2O is relatively leaching and Na2O relatively enriching in 6.5 m depth of all weathering granite regolith.

Impact of the Three Gorges Dam on Regulation and Storage Capacity of Poyang Lake

The regulation and storage capacity of Poyang Lake is infl uenced by the fl ow from the main stream of the Yangtze River and the fi ve rivers in the Poyang Lake basin.After the operation of the Three Gorges Dam(TGD),hydrological changes in the main stream of the Yangtze River impact water exchange between the Yangtze River and Poyang Lake.Based on the analysis of measured data and factors infl uencing outfl ow at Hukou station,a new empirical formula describing outfl ow at Hukou station and critical water level for lake storage capacity is established.The change in monthly storage capacity of Poyang Lake before and after the construction of the TGD is analyzed quantitatively.The results show that the fl ows from the main stream of the Yangtze River and the fi ve rivers in the Poyang Lake basin affect outfl ow and water storage capacity by changing the water level difference between Xingzi and Hukou stations and by changing the water level at Hukou station.But the Yangtze River and the fi ve rivers in the Poyang Lake basin differ in process and degree.If the water level at Hukou station remains consistent,when the fl ow from the fi ver rivers increases by 1,000 m3/s,the outfl ow at Hukou station increases by 304 m3/s.When the fl ow from the main stream of the Yangtze River increases by 1,000 m3/s,the outfl ow at Hukou station decreases by 724 m3/s.In addition,the operation of the TGD affects the water storage capacity of Poyang Lake.The water volume of Poyang Lake decreases by 49.4%in September,but increases by 47.7%in May.

Three gorges Dam work on schedule

Ongoing construction of the Three Gorges Dam is expected to ensure interception of the Yangtze River between October and November of 1997. Overall, the first phase of the construction is being accomplished a bit ahead of schedule. The project officially started in December 1993. By the end of 1995, 108.8 million cubic metres of earth had been removed and 1.35 million cubic metres of cement placed. The only delay experienced was in the building of a highway linking the dam site with the nearest city, Yichang, where the Three Gorges Development Corp. is based. It is estimated the road, originally scheduled

Three Gorges Dam project in great progress as planned

It hasn’t been easy, but almost all of the work on the Three Gorges Project is on schedule, according to a corporation overseeing construction of the dam. The China Yangtze Three Gorges Project Development Corporation has reported that contractors and other staff are working hard to ensure that the flood season does not slow the dam project. The company recently detailed the mammoth tasks that have taken place so

Long-Term Performance and Microstructural Characterization of Dam Concrete in the Three Gorges Project

This study investigates the long-term performance of laboratory dam concrete in different curing environments over ten years and the microstructure of 17-year-old laboratory concrete and actual concrete cores drilled from the Three Gorges Dam.The mechanical properties of the laboratory dam concrete,whether cured in natural or standard environments,continued to improve over time.Furthermore,the laboratory dam concrete exhibited good resistance to diffusion and a refined microstructure after 17 years.However,curing and long-term exposure to the local natural environment reduced the frost resistance.Microstructural analyses of the laboratory concrete samples demonstrated that moderate-heat cement and fine fly ash(FA)particles were almost fully hydrated to form compact micro structures consisting of large quantities of homogeneous calcium(alumino)silicate hydrate(C-(A)-S-H)gels and a few crystals.No obvious interfacial transition zones were observed in the microstructure owing to the longterm pozzolanic reaction.This dense and homogenous microstructure was the crucial reason for the excellent long-term performance of the dam concrete.A high FA volume also played a significant role in the microstructural densification and performance growth of dam concrete at a later age.The concrete drilled from the dam surface exhibited a loose microstructure with higher microporosity,indicating that concrete directly exposed to the actual service environment suffered degradation caused by water and wind attacks.In this study,both macro-performance and microstructural analyses revealed that the application of moderate-heat cement and FA resulted in a dense and homogenous microstructure,which ensured the excellent long-term performance of concrete from the Three Gorges Dam after 17 years.Long-term exposure to an actual service environment may lead to microstructural degradation of the concrete surface.Therefore,the retained long-term dam concrete samples need to be further researched to better understand its microstructural evolution and development of its properties.

Growing impacts of low-flow events on vegetation dynamics in hydrologically connected wetlands downstream Yangtze River Basin after the operation of the Three Gorges Dam

Wetland vegetation is intimately related to floodplain inundations,which can be seriously affected by dam operation.Poyang Lake is the largest floodplain wetland in China and naturally connected with the Yangtze River and the Three Gorges Dam(TGD)upstream.To understand the potential impacts of TGD on Poyang Lake wetlands,we collected remote sensing imagery acquired during dry season from 1987 to 2020 and extracted vegetation coverage data in the Ganjiang Northern-branch Delta(GND)and the Ganjiang Southern-branch Delta(GSD),using the Object-oriented Artificial Neural Network Regression.Principal components analysis,correlation analysis,and the random forest model were used to explore the interactions between vegetation extent in the two deltas and 33 hydrological variables regarding magnitude,duration,timing,and variation.The implementation of the TGD advanced and extended the low-flow periods in Poyang Lake.Vegetation coverage in the GND and GSD increased at the rates of 0.39 and 0.22 km2/year,respectively.The reservoir storage at the end of September accelerated the runoff recession in the GND and the GSD,making low-flow events more influential for vegetation dynamics and shortening the response time of vegetation to the water regime.This study provides an important reference for evaluating the impacts of dam engineering on downstream wetlands.

Three Gorges Dam:the changing trend of snail density in the Yangtze River basin between 1990 and 2019

Background The area of Oncomelania hupensis snail remains around 3.6 billion m^(2),with newly emerging and reemergent habitats continuing to appear in recent years.This study aimed to explore the long-term dynamics of snail density before and after the operation of Three Gorges Dam(TGD).Methods Data of snail survey between 1990 and 2019 were collected from electronic databases and national schistosomiasis surveillance.Meta-analysis was conducted to estimate the snail density.Joinpoint model was used to identify the changing trend and inflection point.Inverse distance weighted interpolation(IDW)was used to determine the spatial distribution of recent snail density.Results A total of 3777 snail survey sites with a precise location of village or beach were identified.For the downstream area,snail density peaked in 1998(1.635/0.11 m^(2),95%CI:1.220,2.189)and fluctuated at a relatively high level before 2003,then declined steadily from 2003 to 2012.Snail density maintained lower than 0.150/0.11 m^(2) between 2012 and 2019.Joinpoint model identified the inflection of 2003,and a significant decreasing trend from 2003 to 2012 with an annual percentage change(APC)being−20.56%(95%CI:−24.15,−16.80).For the upstream area,snail density peaked in 2005(0.760/0.11 m^(2),95%CI:0.479,1.207)and was generally greater than 0.300/0.11 m2 before 2005.Snail density was generally lower than 0.150/0.11 m^(2) after 2011.Snail density showed a significant decreasing trend from 1990 to 2019 with an APC being−6.05%(95%CI:−7.97,−7.09),and no inflection was identified.IDW showed the areas with a high snail density existed in Poyang Lake,Dongting Lake,Jianghan Plain,and the Anhui branch of the Yangtze River between 2015 and 2019.Conclusions Snail density exhibited a fluctuating downward trend in the Yangtze River basin.In the downstream area,the operation of TGD accelerated the decline of snail density during the first decade period,then snail density fluctuated at a relatively low level.There still exists local areas with a high snail density.Long-term control and monitoring of snails need to be insisted on and strengthened.

Analysis and Modeling of Slope Stability in the Three-Gorges Dam Reservoir (China)——The Case of Huangtupo Landslide

The water level in the Three Gorges Dam reservoir is expected to change between the elevations of 145 m and 175 m, as a function of the flood control implementation and the intensity of the annual flood. As a matter of fact, the hydraulical and mechanical loadings, related to the water level modifications, will result in alterations in the slope stability conditions. The town of Badong (Hubei), of 20 000 inhabitants, is one of the towns which was submerged by the impoundment of the reservoir. As a consequence, the new town of Badong was constructed on a nearby site which appeared to be partly an unstable site. A part of this site corresponds to an old landslide, the Huangtupo landslide, the base of which had to be submerged by the water of the reservoir. The analysis of the Huangtupo landslide, taking into account various events scenarios, drainage and reinforcement measures and monitoring devices, allows to illustrate the general process implemented all along the reservoir in order to mitigate the landslide hazard.

The relationship between water level change and river channel geometry adjustment in the downstream of the Three Gorges Dam

In this study, data measured from 1955–2016 were analysed to study the relationship between the water level and river channel geometry adjustment in the downstream of the Three Gorges Dam（TGD） after the impoundment of the dam. The results highlight the following facts：（1） for the same flow, the low water level decreased, flood water level changed little, lowest water level increased, and highest water level decreased at the hydrological stations in the downstream of the dam;（2） the distribution of erosion and deposition along the river channel changed from ＂erosion at channels and deposition at bankfulls＂ to ＂erosion at both channels and bankfulls;＂ the ratio of low-water channel erosion to bankfull channel erosion was 95.5% from October 2002 to October 2015, with variations between different impoundment stages;（3） the low water level decrease slowed down during the channel erosion in the Upper Jingjiang reach and reaches upstream but sped up in the Lower Jingjiang reach and reaches downstream; measures should be taken to prevent the decrease in the channel water level;（4） erosion was the basis for channel dimension upscaling in the middle reaches of the Yangtze River; the low water level decrease was smaller than the thalweg decline; both channel water depth and width increased under the combined effects of channel and waterway regulations; and（5） the geometry of the channels above bankfulls did not significantly change; however, the comprehensive channel resistance increased under the combined effects of riverbed coarsening, beach vegetation, and human activities; as a result, the flood water level increased markedly and moderate flood to high water level phenomena occurred, which should be considered. The Three Gorges Reservoir effectively enhances the flood defense capacity of the middle and lower reaches of the Yangtze River; however, the superposition effect of tributary floods cannot be ruled out.

Immediately downstream effects of Three Gorges Dam on channel sandbars morphodynamics between Yichang-Chenglingji Reach of the Changjiang River, China

Sandbars are of vital ecological and environmental significance, which however, have been intensively influenced by human activities. Morphodynamic processes of sandbars along the Yichang-Chenglingji Reach of the Changjiang River, the channel immediately downstream of the Three Gorges Dam(TGD), are assessed based on remote sensing images between 2000 and 2016. It can be found that the entire area of sandbars reduces drastically by 19.23% from 149.04 km~2 in 2003 to 120.38 km~2 in 2016, accompanied with an increase in water surface width. Owing to differences in sediment grain size and anti-erosion capacity, sandbar area in the upstream sandy gravel reach(Yichang-Dabujie) and downstream sandy reach(Dabujie-Chenglingji) respectively decreases by 45.94%(from 20.79 km~2 to 11.24 km~2) and 14.93%(from 128.30 km~2 to 109.14 km~2). Furtherly, morphological evolutions of sandbars are affected by channel type: in straight-microbend channel, mid-channel sandbars exhibit downstream moving while maintaining the basic profile; in meandering channel, point sandbars show erosion and deposition in convex and concave bank respectively, with mid-channel sandbars distributing sporadically; in bending-branching channel, point sandbars experience erosion and move downstream while mid-channel sandbars show erosion in the head part along with retreating outline. We document that the primary mechanism of sandbars shrinkages along the Yichang-Chenglingji Reach can be attributed to TGD induced suspended sediment concentration decreasing and increasing in unsaturation of sediment carrying capacity. Additionally, channel type can affect the morphological evolution of sandbars. Along the Yichang-Chenglingji Reach, sandbars in straight-microbend channel are more affected by water flow than that in bending-branching channel.

The Three Gorges Dam:Does it accelerate or delay the progress towards eliminating transmission of schistosomiasis in China?

The Three Gorges Dam,located in the largest endemic area of schistosomiasis in China,is one of the world’s largest hydroelectric projects to date.Some large-scale hydro projects have resulted in schistosomiasis emergence or re-emergence.Therefore,the dam’s potential impact on the transmission of Schistosoma japonicum has raised concerns from medical researchers worldwide.A systematic literature review,coupled with an analysis of data on the water level and snail density in the Yangtze River was conducted to assess the impact of the dam on schistosomiasis transmission after more than 10 years of operation.The dam has significantly altered the water levels in the Yangtze River according to different seasons.These changes directly impact the ecology of the schistosome snail host.Due to the dam,there has been a reduction in the density of Oncomelania snails and/or changes in the distribution of snails.The prevalence of infection with S.japonicum has decreased in the downstream areas of the dam,including in the Dongting and Poyang Lakes.The prevalence of infection with S.japonicum in humans has decreased from 6.80%in 2002(before the dam began operating)to 0.50%in 2012,and the number of people infected with S.japonicum have decreased from 94208 in 2002 to 59200 in 2011 in the Poyang Lake region.The presence of the dam does not seem to affect snail breeding or the prevalence of schistosomiasis in the Three Gorges Reservoir.Overall,the Three Gorges Dam has significantly contributed to changes in hydrology after more than 10 years of the dam operating.The changes caused by the dam,together with integrated control of schistosomiasis,might be accelerating the progress towards eliminating the transmission of S.japonicum in the middle and lower reaches of the Yangtze River.Despite the positive effect the dam is having in controlling S.japonicum transmission,continued surveillance is required to monitor the future ecological impacts of the dam over the long term.

FerryBox: Using Automated Water Measurement Systems to Monitor Water Quality: Perspectives for the Yangtze River and Three Gorges Dam

The building of the Three Gorges Dam （Hubei （湖北） Province, China） has transformed a region with an economy based on sustainable agriculture for millennia into an entirely different environment within an exceptionally short time. This disrupts the natural biogeochemieal cycles of carbon, nutrients, and metals and possibly will affect the whole catchment including downstream ecosystems, such as wetlands, estuaries, deltas, and adjacent sea areas. Starting from changes that have already been documented, this article concentrates on the possible use of a ＂FerryBox＂, which is an automated water quality measurement system on board a ship or on shore, to monitor the short and long term development of the quality of the river water in the backwater area and downstream of the dam. While there are already research programmes running to monitor the water quality of the river and the backwater area, these programs are limited to ship campaigns with sampling and laboratory analysis. The spatial and temporal resolution of such measurements is not sufficient for an overall assessment of the water quality and for prognoses in the context of anthropogenic and climate change. Therefore, a concept of applying regular automated observations by a FerryBox is presented. It is shown that such systems are very well suited to give feedback for the assessment of measures to improve the water quality.

Change patterns of oncomelanid snail burden in areas within the Yangtze River drainage after the three gorges dam operated

Background:An"integrated control"strategy has been implemented within seven provinces at highest risk for schistosomiasis along Yangtze River in Peoples'Republic of China(P.R.China)since 2004.Since O ncom elania hupensis is the only intermediate host of the blood fluke(Schistosom a japonicum),controlling the distribution of snails is considered an essential and effeaive way to reduce the risk of schistosomiasis infection.The study aimed to determine the snail area burden and annual trend among provinces with potential risk for schistosomiasis along the Yangtze River,above and below the Three Gorges Dam(TGD).Methods:This retrospective study utilized data previously collected from the National Parasitic Diseases Control Information Management System(NPDCIMS)on annual snail surveys from 2009 to 2017.Descriptive statistics were performed for analyzing the snail burden by provinces,counties,type of environmental location and year,and mapping was conducted to present the snails distribution.Results:From 2009 to 2017,the total snail infested area decreased by 4.22%,from 372253 hm^2 to 356553 hm^2 within the seven high risk provinces.The majority of snails were found in the marshland and lake regions,outside of control embankments.The total snail burden trend remained relatively stable in upstream regions above the TGD from 2010 to 2015,while the trend decreased within downstream regions during this period.In 2016 and 2017,the total snail burden trend increased in both upstream and downstream provinces,however,upstream saw a larger increase.From 2009 to 2017,there were a total of 5990 hm^2 of newly developed snail areas in the seven study provinces and the majority were concentrated in regions below the TGD,accounting for 5610 hm^2(93.70%).Conclusions:There has been a decline in total snail counts from 2009 to 2017.Meanwhile,new snail breeding areas were formed mainly within provinces downstream the TGD due to spread of snails,indicated that the oncomelanid snail would be difficult to completely eliminate.We suggest that the national schistosomiasis integrated control strategy,including mollusciding and environmental modification,will need to be enhanced significantly going forward to achieve a greater reduaion in snail burden and ultimately to achieve elimination.