Operation Of Three Gorges Dam Research Articles

Different effects of natural siltation, damming and pollution on two adjacent lakes of Dongting Lake Wetland: Evidence from paleolimnology

Evaluating the influences of hydrology on lotic floodplain lakes has been difficult due to concurrent effects of different human activities in the catchment. This study aims to explore the responses of floodplain lakes to natural siltation and damming by studying grain size and geochemical elements in 210Pb-dated sediment cores from two adjacent lakes with different hydrological conditions in the same catchment, including a hydrologically-isolated lake (Donghu Lake) and a hydrologically-connected lake (East Dongting Lake) in the middle Yangtze floodplain. The results showed that the riverine sediment supply declined in Donghu Lake after its isolation from the Yangtze River due to natural siltation in the 1910s. In East Dongting Lake, the riverine sediment supply remained relatively stable before the operation of the Three Gorges Dam (TGD) in 2003 CE, and then declined rapidly in response to a decrease in riverine sediment supply. Total phosphorus and total organic carbon in Donghu Lake increased after the “reform and opening-up” in the late 1970s, whereas they remained at low concentrations in East Dongting Lake before the TGD operation in 2003 CE. Hydrological isolation from the Yangtze River reduced the influxes of riverine sediments, but promoted nutrient enrichment and primary production in East Dongting Lake through extending water residence times. Stable element composition in Donghu Lake after 2003 CE suggested that the TGD operation had minor effects on hydrologically-isolated lakes. The comparison between hydrologically-closed and hydrologically-free lakes within the same catchment provides an effective method to identify different effects of natural siltation, damming and environmental pollution.

Long-distance recovery of nonuniform suspended load in the Middle Yangtze River owing to upstream damming

The amount of suspended sediment entering the Middle Yangtze River (MYR) was greatly reduced owing to the operation of the Three Gorges Dam (TGD) and other upstream dams, which has caused the process of sediment transport in the MYR at a severe non-equilibrium state. A detailed investigation was conducted into the transport of nonuniform suspended load in the MYR using about 30-year observed data: (i) firstly, the spatiotemporal variations in fractional sediment fluxes were investigated during the pre- and post-TGD periods. The total sediment flux decreased after the TGD operation, and the magnitude for the coarse fraction (d > 0.125 mm) was smaller than those for the fine (d < 0.031 mm) and medium (d = 0.031–0.125 mm) fractions; moreover, the sediment flux for each fraction continuously recovered along the 955-km-long reach, with the recovery distance for the coarse fraction being much shorter than the other two fractions; it was mainly attributed to the fact that the riverbed could supply abundant coarse sediment to the flow, and this fraction experienced a considerably higher recovery degree than the other two fractions; (ii) secondly, the sediment budgets for the fine, medium and coarse fractions were calculated to assess the contributions of different sediment sources to the recovery of graded sediments in the MYR. About 50 %, 29 % and 14 % of the increased fractional sediment amounts for fine, medium and coarse fractions were originated from the tributary confluence, while the remaining sediment was supplied from the riverbed during the process of channel erosion. Therefore, the recovery of suspended load was largely attributed to the supply of sediment from the riverbed, especially for the coarse fraction; (iii) finally, the impacts of the altered flow-sediment regime and the bed-material composition on sediment recovery efficiency were analysed. The sediment recovery efficiency greatly increased after the TGD operation, which was caused by the reduced sediment concentration; and the sediment recovery efficiency for the coarse fraction was much larger than those for the fine and medium sediment fractions. It is attributed to the fact that the channel can provide abundant coarse sediment to the flows in the MYR, while the sediment recovery efficiencies of fine and medium fractions were restricted by the low degree of sediment supply from the riverbed.

Three Gorges Dam Operation Altered Networks of Social–Economic–Ecological System in the Yangtze River Basin, China

It is necessary to review changes in the interactions of indicators following the construction of the Three Gorges Dam (TGD) in order to explore the impact of the dam on ecology. Research on changes in interactions among indicators of the comprehensive social–economic–ecology system in the Yangtze River Basin is limited, and the objective of this study was to investigate how this system changed after the operation of the TGD, as well as how the indicators contributed to this change. Here, the correlational network approach using 38 data point indicators from 1949 to 2018 of the Yangtze River Basin was applied to analyze the changes in indicator interactions before and after the TGD operation. The TGD impoundment altered networks of the social–economic–ecological system in the Yangtze River Basin. Indicators are both less positively and less negatively connected. The number of synergy and trade-off networks clusters changed from two (Modularity = 0.33) to -six (Modularity = 0.23) and from two (Modularity = 0.015) to four (Modularity = 0.34) after the TGD operation, indicating that the sustainable development of the Yangtze River Basin might be at a middle level after the TGD operation. Further analysis revealed that the mean annual discharge, downstream fry runoff, and downstream counts of the eggs and larvae of four carp, diatom abundance index, breeding population of Chinese sturgeon, and annual precipitation contributed more to the changes in the networks after 2003.

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.

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.

Impacts of hydrological alteration on ecosystem services changes of a large river-connected lake (Poyang Lake), China

Large lakes provide various types of ecosystem services (ESs), of which stocks and variations induced by hydrological alterations are largely unquantified. The present study investigates the long-term changes of five key ESs (i.e., flood regulation, water supply, fish production, nutrient retention and biodiversity conservation) in a large river connected lake (Poyang Lake), with special attention to impacts of hydrological alteration induced by the Three Gorges Dam (TGD). Hydrological data series, hydrodynamic model, the nutrient balance, fishery statistics, and wetland winter waterbird survey data from 1980 to 2016 were employed. Results showed that Poyang Lake provide significant ESs, with long-term average flood regulation, water supply and nutrient retention services of 167.7 × 108 m3, 31.53 × 108 m3, and 15.12% of total phosphorus load, respectively. The fish production service ranged from 1.74 × 104t to 7.19 × 104t, with an average value of 3.12 × 104t. All five key ESs exhibited a downward trend since the 2000s, especially for water supply, fish production and nutrient retention services (p < 0.05), which might be largely attributed to the hydrological condition changes induced by TGD operation. Nevertheless, more detailed monitoring data and biophysical models are required to further acknowledge the changes in biodiversity conservation and fish production services and their linkages with the TGD. The present study sheds light on long-term ES changes in large lakes and their possible linkages with human influences through hydropower projects.

Effects of the Three Gorges Dam on the downstream streamflow based on a large-scale hydrological and hydrodynamics coupled model

Study regionThe Yangtze River basin, China. Study focusIn this study, a large-scale coupled model of hydrological and hydrodynamic processes was developed via the Distributed Time Variant Gain Model and the hydrodynamics-operation coupled model. Two scenarios including operated by the Three Gorges Dam (TGD) and natural condition hypothesis without the reservoir construction, were analyzed based on the coupled model to investigate the effects of TGD operation on the streamflow from 1970 to 2018. New hydrological insightsResults exhibited that the annual average relative deviation of flow operated by the TGD was approximately 24% compared with the natural state at Yichang, while the value of Chenglingji, Hankou, Hukou and Datong were 8.5~12.8%. During water supply period, the TGD operation increase the streamflow by 33% at Yichang, and the streamflow in downstream can gain about 6–17%. For impounding process, except Hukou, the others showed a decreasing trend, and the reduction ranged from about 7–20%. For flood season, the dam operation mainly intercepts high flood peaks while the average streamflow changes a little. The high flood peaks at Yichang can be reduced by 20%, and the changes of high flood peaks in downstream reduce to no more than 10% due to the influence of interval inflow. The deviations during drawdown period are almost below 10%.

Centurial changes in sedimentary phosphorus forms and trace elements in response to damming and anthropogenic pollution in a floodplain lake, central China.

A massive increase in dam construction has decreased fluvial sediment discharge at a global scale. In order to explore potential effects of the Three Gorges Dam (TGD) on floodplain lakes in the middle Yangtze reaches (central China), this study investigated phosphorus forms (i.e., Ca-bound phosphorus, Fe/Al-bound phosphorus, and organic phosphorus) and trace elements (i.e., Sc, Ba, Be, Pb, and Zn) in a 210Pb-dated sediment core collected from East Dongting Lake, a hydrologically open lake proximal to the TGD. Sedimentary records revealed that the fluxes of phosphorus in different forms and trace elements were high before 2005. Thereafter, the fluxes of Ca-bound phosphorus, Sc, Ba, and Be declined sharply, probably due to declining supply of riverine detritus from the upstream after the TGD operation. In contrast, the fluxes of Fe/Al-bound phosphorus and heavy metals remained high after 2005, indicating the impacts of industrial sewage inputs. Our results underscore that river damming and anthropogenic pollution have altered sedimentary geochemical composition in East Dongting Lake. This phenomenon might be widespread in similar floodplain lakes due to increasing human disturbance during recent decades.

Drought propagation modification after the construction of the Three Gorges Dam in the Yangtze River Basin

The construction of Three Gorges Dam (TGD) has exerted substantial effects on local hydrological regime and eco-system since its operation in 2003. Due to this significant anthropogenic activity, drought propagation processes from Meteorological drought to Hydrological (M-H) and Soil moisture (M-S) drought are inevitably modified. However, current studies are still insufficient to answer whether the TGD operation mitigated or exacerbated the drought propagation process due to its complex impacts. To explore this key issue, this study proposed an integrated framework to quantify the impact of TGD construction on drought propagation process in the Yangtze River Basin (YRB). The drought characteristics propagation models and propagation time were calculated to identify the modification of drought propagation process between different TGD operation scenarios. Trend and attribution analyses were also utilized to explore potential influence factors. Results demonstrated that TGD construction did significantly aggravate the drought characteristics propagation. Specifically, the slopes of hydrological drought duration and severity in linear propagation models increased 23.98% and 29.23% in average. While for M-S propagation, the slopes of duration and severity models increased 28.72% and 21.91%, respectively. This research also found that, after TGD operation, the M-H propagation speed had been slowed down but the M-S lag time became shorter. Additionally, the variations and trend patterns of six potential influence factors at four separate periods were full analyzed to provide implications for the drought propagation modifications. And TGD operation was confirmed to become a major factor inducing the variations of annual runoff, which accounted for 56.75% for the entire YRB. The above quantitative findings are expected to reveal the potential changes of drought propagation process caused by the construction of TGD in YRB. The proposed integrated framework is also beneficial to the drought early warning system and understanding of drought development not only in YRB, but also in other basins at the global scale.

Responses of spawning thermal suitability to climate change and hydropower operation for typical fishes below the Three Gorges Dam

River thermal regime is a critical factor affecting the initiation of spawning and breeding success in freshwater fish species. The four main domestic carps (FMDCs) and Chinese sturgeon are representative warm-water and cool-water fishes with different spawning thermal requirements in the Yangtze River Basin, China. This study focused on these two types of fish species below the Three Gorges Dam (TGD), and thermal response curves of fish spawning were constructed. The daily thermal suitability index (TSI) was simulated based on scenario analysis of climate-induced water warming and cooling. The variation in four TSI-based spawning indicators was then analyzed to evaluate the response of fish spawning to climate change and hydropower operation. The results showed that the spawning time is likely to be advanced by 1.3 days on average for the FMDCs and delayed by 2.1 days on average for the Chinese sturgeon with a 0.2 °C increase in the water temperature. For the FMDCs, climate warming would narrow the spawning window, although it may advance the spawning time that has been postponded by TGD operation, and climate cooling would widen the spawning window but further postpone spawning. For the Chinese sturgeon, climate warming would aggravate the negative impacts of TGD operation by further narrowing the spawning window and delaying spawning, while climate cooling would likely offset the negative impacts of TGD operation on fish spawning. We suggest that adjusting hydropower operation rules, e.g., elevating the water temperature in the early spawning period of the FMDCs to expand the spawning window and reducing the water temperature by at least 1.2 °C during the spawning period of the Chinese sturgeon, is essential for maintaining and restoring the natural spawning process and populations. Our study is also beneficial for inferring the cumulative impacts of cascading hydropower operations and provides insights for the management and conservation of fish species with different thermal tolerances in other hotspot regions.

A review on the driving forces of water decline and its impacts on the environment in Poyang Lake, China

Abstract The recession of water levels of natural lakes and their associated impacts on wetland ecosystems is a serious issue worldwide. Poyang Lake (the largest freshwater lake in China) has experienced a heightened and prolonged water decline since the year 2000, which causes concern for associated ecological impacts. In particular, climate change, operation of the Three Gorges Dam (TGD), and high magnitude sand mining appear to be well-correlated with the occurrence of water decline in Poyang Lake. Though the above factors have been analyzed in previous studies, a comprehensive summary has never been compiled. This paper provides a detailed literary review highlighting the driving forces and possible impacts of the consistent water decline in Poyang Lake. We conclude here that the operation of TGD is a fundamental cause for the lake water decline, aggravated by climate change and sand mining. The water decline has caused a deterioration of water quality, as well as having given rise to a potential threat to the habitat of migratory birds and Yangtze finless porpoises. The paper intends to offer constructive references that can be used in decision-making for effective protection of water resources and lake ecosystems.

Spatiotemporal water quality variations and their relationship with hydrological conditions in Dongting Lake after the operation of the Three Gorges Dam, China

Identifying the spatiotemporal characteristics of water quality and related driving factors is essential for lake water quality management. To further understand the unique relationship between hydrological conditions and water quality in river-connected lakes, we analysed the spatiotemporal variations of water quality and hydrological factors in Dongting Lake from 1991 to 2018 and identified the effects of the operation of the Three Gorges Dam (TGD) on lake water quality. The total nitrogen (TN) and total phosphorus (TP) concentrations showed general increasing trends in the pre-TGD period, in contrast to a significant increasing TN trend and a decreasing TP trend in the post-TGD period, the latter of which was closely linked to a prominent decrease of water and suspended sediment discharge after the initiation of TGD operation. TN concentrations showed significant spatial variations during the study period, with the highest concentration in East Dongting Lake and the lowest concentration in West Dongting Lake. Nutrient inputs were the dominant pollution source determining the water quality of Dongting Lake; water discharge and water level were also important factors controlling the spatiotemporal variability of water quality. The contribution of the four tributaries to lake water quality was much greater than that of the three channels, accounting for 89.94–92.25% of N inputs and 74.10–75.85% of P inputs. Moreover, the contribution of the three channels to lake water quality decreased after TGD operation, mainly because of reduced nutrient input, water discharge, and suspended sediment discharge. This study indicates that controlling pollutant effluents in tributaries and monitoring hydrological conditions within the lake are the key factors necessary to improve the water quality in Dongting Lake and may provide a practical reference for water quality management in river-connected lakes worldwide.

Hydrological Drought in Dongting Lake Area (China) after the Running of Three Gorges Dam and a Possible Solution

Dongting Lake is located at the downstream of Three Gorges Dam (TGD) and the hydrological drought is intensified after the impoundment of TGD as the dry period has been extended from 123 days/year before the operation of TGD (1981–2002) to 141 days/year (2003–2016) on average. Particularly, the Dongting Lake’s water shortage becomes very severe. To solve the problem caused by upstream dams, an innovative flood control scheme (IFCS) was introduced, and its feasibility of application in Dongting Lake is studied using the hydrodynamic module of Mike 21. The results show the IFCS can effectively convert the peak discharge of floodwater in wet seasons into water resources in dry seasons as the IFCS could significantly increase the usable water storage of the lake. For example, the usable water storage could increase to 2.85 billion m3 and 1.81 billion m3 in the extreme drought year of 2006 and 2011, respectively. The average increment of the water level would be about 0.4 m, 0.6 m, and 0.5 m in the West Dongting Lake (WDL), South Dongting Lake (SDL), and the East Dongting Lake (EDL), respectively, if the water stored in the inner lake was discharged uniformly in 30 days (27 November to 27 December 2006) with the application of IFCS. This study may provide an innovative method to alleviate the water shortage problem in Dongting Lake and other similar lakes.

A thirty-year Landsat study reveals changes to a river-lake junction ecosystem after implementation of the three Gorges dam

Ecosystems at the junctions of large lakes and rivers are a significant resource which are yet susceptible to changes in the various water bodies. One of the most crucial river-lake junction points in China is the junction of Poyang Lake and the Yangtze River (JPLYR), with the Three Gorges Dam (TGD) upstream. Based on remote sensing images of the dry season from 1987 to 2018, landcover change of JPLYR was obtained using the random forest classification method. Linear regression, principal component analysis (PCA), and classification and regression tree (CART) were used to determine relationships between 16 hydrological variables regarding magnitude, duration, timing, variation, and vegetation area throughout different operational stages of the TGD. Results showed that the operation of the TGD has anticipated and elongated the dry seasons, with a consequential shortening of the flood seasons that now occur later in the year. The peak water level was significantly reduced (p < 0.05), while the minimum water level remained unchanged throughout the study period. There was a 344% increase in vegetation coverage across the JPLYR from 1987 (0.677 km2) to 2018 (3.005 km2). The earlier beginning of dry season (BDS) was found to be most significant in affecting the increased vegetation area before and after the operation of TGD. The results provide an essential reference for assessing the potential influence of newly proposed dams in river-lake junctions and for optimizing future operations.

Enhanced hysteresis of suspended sediment transport in response to upstream damming: An example of the middle Yangtze River downstream of the Three Gorges Dam

AbstractThe peak in sediment transport in alluvial rivers generally lags behind the peak in discharge. It is thus not clear how the hysteresis in the sediment/discharge relationship may be impacted by damming, which can fundamentally alter the water and sediment regimes in the downstream reaches of the river. In this study, a total of 500 flood events in the Yichang–Chenglingji Reach (YCR) of the Middle Yangtze River immediately downstream of the Three Gorges Dam (TGD) are analysed to study the impacts of dam operations on the hysteresis of suspended sediment transport. Sediment rating curves, hysteresis patterns, as well as lag times, are investigated to determine the relationship between suspended sediment concentration (SSC) and flow discharge (Q) at different temporal scales, from inter‐annual to individual flood events, for the pre‐ and post‐TGD period from 1992 to 2002 and from 2003 to 2017, respectively. The results showed that the TGD operation decreased the frequency and magnitude of floods. The decrease in peak flow and increase in base flow weakened the flood contribution to the annual discharge by nearly 20%. However, the relative suspended sediment load contribution during flood events was much higher than the discharge contribution, and was little impacted by the dam. At seasonal and monthly scales, more than 80% of the suspended sediment was transported by ~65% of the water discharge in the summer and early autumn. The monthly SSC–Q relationship changed from a figure‐eight to an anti‐clockwise pattern after the construction of the TGD. For single flood events, the TGD operations significantly modified the downstream SSC–Q hysteresis patterns, increasing the frequency of anti‐clockwise loops and the lag time between peak Q and peak SSC. These adjustments were mainly caused by differences in the propagation velocities of flood and sediment waves and the sediment ‘storage–mobilization–depletion’ process, whereas the influence of lateral diversions was small. © 2020 John Wiley &amp; Sons, Ltd.

Variability in water chemistry of the Three Gorges Reservoir, China

The environmental influence of the Three Gorges Reservoir (TGR) on the Changjiang River has been widely studied since the Three Gorges Dam (TGD) began operation in 2003. However, the changes in water chemistry in the reservoir in response to damming effect variations are poorly documented in the area of this large reservoir. The results suggest that in comparison to the water chemistry before the TGR operation, the inflow concentrations of Mg2+, K+, Na+ and Cl− increased in the TGR, and the abundance of Ca2+ and HCO3- decreased in the inflow in the period after the TGR filling as a result of climate change and human activities in the Changjiang River basin. The ionic composition in the TGR is primarily controlled by contributions from the upstream region of the Changjiang River but was modified by the interaction between water and rocks within the TGR. The concentrations of most major ions as well as the equivalent ratios of the major ions increased in the TGR after the operation of TGD. This change yielded a 6% increase in the major ion loading downstream of the TGD. The Three Gorges area strongly contributes to the increase in ion loading in the TGR due to enhanced water and rock interactions in comparison with the period before TGD operation.

Remotely Sensed Mid-Channel Bar Dynamics in Downstream of the Three Gorges Dam, China

The downstream reach of the Three Gorges Dam (TGD) along the Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). MCBs dynamics are crucial to the river’s hydrological processes and local ecological function. However, a systematic understanding of such dynamics and their linkage to TGD remains largely unknown. Using Landsat-image-extracted MCBs and several spatial-temporal analysis methods, this study presents a comprehensive understanding of MCB dynamics in terms of number, area, and shape, over downstream of TGD during the period 1985–2018. On average, a total of 140 MCBs were detected and grouped into four types representing small (&lt;2 km2), middle (2 km2 – 7 km2), large (7 km2 – 33 km2) and extra-large size (&gt;33 km2) MCBs, respectively. MCBs number decreased after TGD closure but most of these happened in the lower reach. The area of total MCBs experienced an increasing trend (2.77 km2/yr, p-value &lt; 0.01) over the last three decades. The extra-large MCBs gained the largest area increasing rate than the other sizes of MCBs. Small MCBs tended to become relatively round, whereas the others became elongate in shape after TGD operation. Impacts of TGD operation generally diminished in the longitudinal direction from TGD to Hankou and from TGD to Jiujiang for shape and area dynamics, respectively. The quantified longitudinal and temporal dynamics of MCBs across the entire Yangtze River downstream of TGD provides a crucial monitoring basis for continuous investigation of the changing mechanisms affecting the morphology of the Yangtze River system.

Water-Exchange Response of Downstream River–Lake System to the Flow Regulation of the Three Gorges Reservoir, China

Hydrological regime changes in the river–lake system and their influences on the ecological environment downstream dams have attracted increasingly more attention all over the world. The Dongting lake downstream of the Three Gorges Dam (TGD) in the Yangtze River has been experiencing a series of hydraulic and hydrological changes over the last decade. The hydrological and ecological influences of the TGD flow regulation on the Dongting river–lake system and its functional mechanism during the impounding periods remain extremely unclear. This study examines the hydrological changes in the Dongting river–lake system based on a 1D/2D coupled hydrodynamic model. In particular, the inflow boundary of the model with and without the TGD was applied with the outflow and inflow of the TGD, respectively, during the same regulation periods. The results show that the diverted flow from the Yangtze River into the Dongting lake and outflow from the lake back to the river drastically decreased during the impounding periods, especially in October. The decreased water exchange between the Yangtze River and the Dongting lake impaired the water residence capacity to some extent in the lake. Stage decrease in the lake area resulted in a significant reduction in the water volume of the Dongting lake with the same time percentage. In addition, the obvious drainage effect in Dongting lake due to the increased stage difference and current speed after the TGD operation was the essential cause of hydrological changes in the lake area. These results provide an improvement in the understanding of impoundment influences on the large river–lake system and give some practical information for ecological environment management in similar river–lake systems.

Downstream geomorphic impact of the Three Gorges Dam: With special reference to the channel bars in the Middle Yangtze River

AbstractThe Three Gorges Dam (TGD) has altered downstream flow–sediment regimes and led to significant changes in the morphodynamic processes in the Middle Yangtze River (MYR). However, due to the complexity of this large river, the driving forces and implication of the morphodynamic processes remain insufficiently understood. This study selected two typical meandering and bar‐braided reaches, the Zhicheng (ZC) and Shashi (SS) reach, to examine their responses to the TGD operation. The results showed that in the post‐dam period significant channel erosion occurred with a higher erosion rate in the ZC reach (closer to the TGD) compared with the SS reach. The area of the Guanzhou mid‐channel bar (ZC reach) and the Sanba mid‐channel bar (SS reach) shrank by 30 and 90% from 2003 to 2015, respectively. The increased fluvial erosion intensity due to the reduction in suspended sediment concentration (SSC) drove the shrinkage of the mid‐channel bars, as demonstrated by empirical relationships between bar geometry and fluvial erosion intensity. An increase of 22 days per year in the frequency of post‐dam medium‐to‐high discharges (10 000–25 000 m3 s−1), and associated with the reduction in SSC, jointly led to the greater erosion at the convex (inner) banks than the concave (outer) banks, which has negatively affected the designed navigation channels at the concave banks by decreasing their discharge partitioning ratios. The post‐dam water level at a given high discharge (&gt;25 000 m3 s−1) showed no evident change, but the water level at a given low discharge (&lt;10 000 m3 s−1) decreased. The reduction in water levels at low flows can affect water supply and riverine ecosystems in the MYR. © 2019 John Wiley &amp; Sons, Ltd.

Quantitative Assessment of the Influences of Three Gorges Dam on the Water Level of Poyang Lake, China

Lakes are important for global ecological balance and provide rich biological and social resources. However, lake systems are sensitive to climate change and anthropogenic activities. Poyang Lake is an important wetland in the middle reach of the Yangtze River, China and has a complicated interaction with the Yangtze River. In recent years, the water level of Poyang Lake was altered dramatically, in particular showing a significant downward trend after the operation of the Three Gorges Dam (TGD) in 2003, thus seriously affecting the lake wetland ecosystem. The operation of the TGD changed both the hydrological regime and the deeper channel in the middle reach of the Yangtze River, and affected the river–lake system between the Yangtze River and Poyang Lake. This study analyzed the change in the water level of Poyang Lake and quantified the contributions of the TGD operation, from the perspectives of water storage and erosion of the deeper channel in the middle reach of the Yangtze River, through hydrodynamic model simulation. The erosion of the deeper channel indicated a significant decrease in annual water level. However, due to the water storage of the TGD in September and October, the discharge in the Yangtze River sharply decreased and the water level of Poyang Lake was largely affected. Especially in late September, early October, and mid-October, the contributions of water storage of the TGD to the decline in the water level of Poyang Lake respectively reached 68.85%, 59.04%, and 54.88%, indicating that the water storage of the TGD was the main factor in the decrease in water level. The erosion of the deeper channel accelerated the decline of the water level of Poyang Lake and led to about 10% to 20% of the decline of water level in September and October. Due to the combined operation of the TGD and more reservoirs under construction in the upper TGD, the long-term and irreversible influence of the TGD on Poyang Lake should be further explored in the future.