Three Gorges Reservoir Mainstream Research Articles

Sr-Nd isotope and REE compositions of surface sediments from the three Gorges Reservoir: Implications for source identification and apportionment

Sediment deposition has already been considered to be a potential risk to the general running of the Three Gorges Reservoir (TGR). Identifying the sediment sources and their contributions to the TGR is of great significance for the sustainable management of reservoir operation. In this study, surface sediments of the TGR and its main tributaries were sampled in the flood (June to September), dry (November to next February) and even (March to May, October) seasons, 2017–2018, together with surrounding upland soils, and analyzed for Sr-Nd isotopes and rare earth elements (REE). The geochemical results showed that εNd(0) values and 87Sr/86Sr ratios of surface sediments from the TGR mainstream varied from − 12.21 to − 9.75 and from 0.720241 to 0.726655, respectively. All sediment and upland soil samples generally showed the LREE enrichment and significant Eu depletions. Three potential sources were identified for the TGR sediments and ranked in the order: regional source (adjacent tributaries) > distal source (upstream tributaries) > proximal source (upland soil), based on the Sr-Nd isotopes and REE parameters. Isotopic mixing model coupled with Monte-Carlo simulations was firstly applied to quantify source contributions to the TGR sediments among different seasons. Through Monte-Carlo model simulations, the regional source contributed approximately 40% to the TGR sediments during the flood and even seasons but approximately 50% during the dry season. The contribution from distal sources arrived at about 30% in the three seasons, and 30% from proximal sources during flood and even seasons and 20% in the dry season. Dam construction was the primary factor affecting the sediment deposition in the TGR, and it would further reduce sediment supply from the distal source with the operation of new increasing dams in the upper Yangtze River Basin. Hence, more attention should be paid to the regional sediment supply of the TGR in the future.

Geochemistry of phosphorus release along transect of sediments from a tributary backwater zone in the Three Gorges Reservoir

As an important phosphorus (P) source in water, sediment P release can influence water quality in the tributary backwater zone of “river-type” reservoir. Chemical sequential extraction and diffusive gradients in thin films technique (DGT) were applied to study sediment P mobility in the Daning River backwater zone (DNB) in the Three Gorges Reservoir (TGR). Influenced by the TGR backwater intrusion, both of water P forms and sediment P forms in the DNB zone exhibited increased spatial distributions from upper reach to lower reach. Sediment particles in the upper, middle, lower reaches were mainly derived from the settled particles (with high Ca contents) carried by the Daning River outflow, the former and sedimentary dead algal, the settled particles carried by the TGR backwater, respectively. According to P adsorption/desorption experiment results and concentration gradients of DGT-labile P at the sediment-overlying interface (SWI), sediments in the DNB zone acted as P source for overlying water during the sampling period. In the lower reach of the DNB zone and the adjacent TGR mainstream, iron oxides reduction in the anoxic or anaerobic sediments drove the synchronous release of labile P and Fe(II). Whereas, algal decomposition directly produced labile P and S(-II) in the top 0 to −10 mm sediment layer in the middle reach, and then the downward increases of labile P and Fe(II) in the depth of −30 to −80 mm were caused by iron oxides reduction. In the upper reach, the released labile P in sediments might be re-trapped by precipitating with Ca(II) or being adsorbed onto the newly formed iron oxides near the SWI. Sediment P release in DNB zone had a limited influence on water quality, but the influence would be significant in the future time after the external P source is reduced efficiently.

The spatial and vertical distribution of heavy metal contamination in sediments of the Three Gorges Reservoir determined by anti-seasonal flow regulation

The contamination of heavy metals in sediments of the Three Gorges Reservoir (TGR) is increasingly concerned as a major issue for water quality since the full impoundment. In this study, the sediment profiles in the riparian and submerged areas from Fuling to Zigui in the middle-low TGR mainstream were collected to investigate the stratified distribution of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) and assess their contamination and potential eco-risk. The results showed that the concentrations of these metals in the riparian sediments did not present a marked spatial trend except the concentrations of Cd that increased towards the dam. However, the metal concentrations (except Cd) in the submerged sediments were generally higher near the dam. The concentrations of heavy metals in the riparian sediments did not show marked vertical variation, while in the submerged sediments they fluctuated dramatically with depth, indicating the metal accumulation processes in last few years. Sediment grain sizes as an indicator of hydrodynamic regimes dominated the vertical distribution of heavy metals over organic matters and Fe/Mn oxide/hydroxides. The sediments from both riparian and submerged areas of the TGR were contaminated by anthropogenic metals of Cd, Cu and Pb that were mainly from the ore mining, fossil fuel combustion, agricultural pollution and atmospheric deposition. Cadmium was a major metal pollutant in the sediments with a high contamination and potential eco-risk level. The results of this study indicate that the sorting of sediments with the anti-seasonal flow regulation determines the spatial and temporal distribution of heavy metal contamination in the sediments, and the impoundment stages of the TGR in history regulate the accumulation processes of the metals.

Spatial variation of heavy metal contamination in the riparian sediments after two-year flow regulation in the Three Gorges Reservoir, China

Regular impoundment of the Three Gorges Reservoir (TGR) with intensified human activities in the watershed imparts a significant effect on the environmental changes in the riparian zone. In this study, six heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in the riparian sediments of the entire TGR mainstream were investigated in 2014 and 2016 to identify their contamination and risk characteristics and decipher the main factors for the variation of the metal contamination. The results showed that the concentrations of the heavy metals in the sediments did not vary significantly between 2014 and 2016, and their contamination degrees decreased in the order of Cd> > Cu ≈ Zn > Pb > Cr ≈ Ni in 2014 and Cd> > Zn > Cu ≈ Pb > Cr ≈ Ni in 2016. The potential eco-risk of Cd was extremely high in the two years, while the eco-risk of other metals was very low. The sediments showed a moderate to high contamination level, a high potential eco-risk but a low toxic risk to aquatic biota in the two years. Spatially, the contamination and risk levels of heavy metals were relatively higher in the downstream TGR region in 2014 except for the sites close to the urban areas but in the upper-middle TGR region in 2016. Increasing anthropogenic influence contributed to the high contamination and risk levels of Cd, Cu, Pb and Zn in the upper-middle region in 2016. The results indicated that the Cd contamination in the riparian sediments of the TGR was still a vital environmental issue, and the decreased sediment inputs from the upstream major tributaries, the periodic and anti-seasonal flow regulation, local geomorphological characteristics and anthropogenic activities determined the contamination distribution of heavy metals in the riparian sediments.

Water quality trends in the Three Gorges Reservoir region before and after impoundment (1992–2016)

Reservoirs are essential for the wellbeing of human societies, but can also be subject to negative ecological impacts. The Three Gorges Reservoir (TGR) in the upper Yangtze River is remarkable for its size and engineering; however, its effects on water quality are poorly understood. To the best of our knowledge, this study is the first to describe long-term (1992–2016) monitoring. It showed that the chemical oxygen demand (COD, via the potassium permanganate index) and total phosphorus (TP) have decreased 40.9% ± 9.9% and 22.2% ± 9.7% respectively in the TGR mainstream between impoundment in June 2003 and 2016, while total nitrogen (TN) and ammonium (NH4-N) have increased 1.3% ± 2.4% and 8.2% ± 2.6%. In addition, phytoplankton biomass has increased by a factor of 2.7 (1.1–4.8) over pre-impoundment levels in the mainstream, and tributary algal blooms have increased in frequency since 2004. The reductions in COD and TP were caused primarily by decreases in water flow speed, which lead to sediment settlement. The anti-seasonal operation pattern and water volume increased TGR may also increases the dilution capacity. TN and ammonium are less affected by sediment deposition and have increased slightly under intensified human activities. Decreased water flow speeds and nutrient enrichment have promoted increases in algal biomass, leading to blooms in tributary backwater zones. In situ experiments indicate that phytoplankton growth in the TGR is phosphorus limited during all seasons. Therefore, controlling phosphorus will reduce the short-term eutrophication potential in the reservoir. However, concurrent control of nitrogen and phosphorus inputs are necessary in the long term.

CO2 oversaturation and degassing using chambers and a new gas transfer velocity model from the Three Gorges Reservoir surface

Reservoirs are considered as important carbon source of the atmosphere, whilst, regional and global reservoir CO2 quantification is hampered by data limitation and bias in spatial and temporal sampling. By deploying chamber measurements and employing the newly developed model of gas transfer velocity, CO2 partial pressure (pCO2) and evasion in the main stem of the Three Gorges Reservoir (TGR) were investigated. The pCO2 ranged from 429 to 8668 μatm with an average of 2511.6 ± 1721.3 μatm, 6.1-fold higher than the ambient air pCO2 (mean: 410 μatm). All the samples were net CO2 sources via water-air interface, displaying pronounced spatial and monthly variability. The CO2 areal flux averaged 212.5 ± 120.1 mmol/m2/d in June, 123.3 ± 78.5 mmol/m2/d in July in the lotic TGR main stream, much higher than its lentic system, i.e., 79.6 ± 41.3 mmol/m2/d in November, and 76.3 ± 88.1 mmol/m2/d in March. Much lower k levels in the lentic reservoir surface resulted in lower CO2 evasion rates. Furthermore, dam impoundment considerably altered the riverine carbon cycle, as reflected by the changing magnitude of CO2 efflux and environmental controls of dissolved CO2. Precipitation and concurrent soil CO2 influx exhibited a central role in controlling riverine pCO2, and respiration of allochthonous organic carbon was a secondary factor in the TGR lotic system, whilst, both in-stream metabolism and terrestrial inputs played crucial roles in controlling aqueous CO2 in the TGR lentic system. In comparison, we provided key findings of k model and more reliable CO2 quantification with a consideration of water level shifts and a complete coverage of spatial sampling. Our higher CO2 emission (1.47 (1.16–2.13) Tg CO2/y) than previous studies called more field measurements to assess the resulting changes in CO2 flux owing to dam operation and changing environment, and their implications for regional carbon budgets should be warranted.

Heat budget contribute rate in the Three Gorges Reservoir tributary bay between mainstream and tributary using stable isotope analysis

Abstract Understanding the interaction processes between the mainstream and its tributaries and detailing the rates of contribution of water and heat from two different water bodies in the tributary bay are essential for water management in the Three Gorges Reservoir (TGR). The stable isotope ratios of hydrogen (δD) and oxygen (δ18O) were applied to explore the interactions between the TGR mainstream and the tributary, Zhuyi River. A heat budget of the TGR tributary bay was constructed for the year 2014 using data for water temperature, meteorological data and water mixing ratio. The results showed that approximately 91% of the water in the tributary bay was from the TGR mainstream. And the heat budget reveals that the tributary bay net heat flux is dominated by the mainstream advective heat, and the average contribution rate is 87% during the whole year, while the tributary advective heat and atmosphere make a minor contribution to the tributary bay heat budget.

Spatio-Temporal Variations of the Stable H-O Isotopes and Characterization of Mixing Processes between the Mainstream and Tributary of the Three Gorges Reservoir

Understanding the runoff characteristics and interaction processes between the mainstream and its tributaries are an essential issue in watershed and water management. In this paper, hydrogen (δD) and oxygen (δ18O) isotope techniques were used in the mainstream and Zhuyi Bay (ZYB) of the Three Gorges Reservoir (TGR) during the wet and dry seasons in 2015. It revealed that (1) Precipitation was the main source of stream flow compared to the TGR water line with meteoric water line of the Yangtse River basin; (2) The δD and δ18O values exhibited a ‘toward lighter-heavier’ trend along mainstream due to the continuous evaporation effect in the runoff direction, and the fluctuations reflected incoming water from the nearest tributaries. The general trend of d-excess increased with increasing distance from the Three Gorges Dam, which indicated that kinetic fractionation was an important process affecting the isotopic composition. The enrichment effect of isotopes was found in the downstream of TGR; (3) Water mass from the TGR mainstream flowed backward to the confluence zone of ZYB via the middle and bottom layers in the dry season, whereas in the wet season, water reversed through the upper-middle layers due to thermal density flows. This study described and demonstrated that the water cycle of TGR was driven by natural environmental variability and operational system, which will provide valuable information for the water resource management and for controlling the algal blooms in the future.

Impact of upstream river inputs and reservoir operation on phosphorus fractions in water-particulate phases in the Three Gorges Reservoir

The impoundment of the Three Gorges Reservoir (TGR) has changed water-sand transport regime, with inevitable effects on phosphorus transport behavior in the TGR. In this study, we measured phosphorus fractions in water and suspended particles transported from upstream rivers of the TGR (the Yangtze River, the Jialing River and the Wu River) to reservoir inner region over the full operation schedule of the TGR. The aim was to determine how phosphorus fractions in water and particulate phases varied in response to natural hydrological processes and reservoir operations. The results showed that total phosphorus concentration (TP) in water in the TGR inner region was 0.17±0.05mg/L, which was lower than that in the Yangtze River (0.21±0.04mg/L) and the Wu River (0.23±0.03mg/L), but higher than that in the Jialing River (0.12±0.07mg/L). In the TGR inner region, there was no clear trend of total dissolved phosphorus (TDP), but total particulate phosphorus (TPP) showed a decreasing trend from tail area to head area because of particle deposition along the TGR mainstream. In addition, the concentrations of TPP in water and particulate phosphorus in a unit mass of suspended particles (PP) in the TGR inner region were higher in October 2014 and January 2015 (the impounding period and high water level period) than that in July 2015 (the low water level period). The temporal variations of PP and TPP concentrations in the TGR may be linked to the change of particle size distribution of suspended particles in the TGR. The particle size tended to be finer due to large-size particle deposition under stable hydrodynamic conditions in the process of TGR impoundment, resulting in high adsorption capacities of phosphorus in suspended particles. The results implied that phosphorus temporal variations in the TGR could exert different impacts on water quality in the TGR tributaries.

Distribution and potential eco-risk of chromium and nickel in sediments after impoundment of Three Gorges Reservoir, China

ABSTRACTThe impoundment of the Three Gorges Reservoir (TGR) altered the hydrodynamic conditions and modulated the accumulation and behaviors of heavy metals in the sediments. In this study, the sediments from the riparian and submerged areas of the entire TGR mainstream were collected in 2014 to investigate the spatial distribution of chromium (Cr) and nickel (Ni), and to assess their contamination state and potential eco-risk to aquatic environment by multiple indices. Results showed that the concentrations of Cr and Ni in the sediments increased toward the dam. The concentrations of Cr were significantly higher in the riparian sediments than in the submerged sediments, whereas the Ni concentrations were comparable. The relatively high storages of Cr and Ni in the sediments existed near the dam. The physiochemical properties of sediments and local human activities controlled the spatial variations of Cr and Ni in the sediments. Currently, the TGR sediments are not heavily contaminated by Cr and Ni and show low eco-risk. Nevertheless, considering the decrease in sediment loads from the upper Yangtze River and the rapid urbanization in the TGR catchment, much more attentions should be paid to the geochemical behaviors and the eco-risk of Cr and Ni in the sediments.

Pollution Characteristics and Ecological Risk Assessment of Vanadium in Sediments of the Three Gorges Reservoir (Chongqing-Yichang Section)

In order to investigate the pollution characteristics of vanadium in sediments of the Three Gorges Reservoir (TGR), 67 sediment samples were collected from the section between Chongqing to Yichang in TGR. The concentration and chemical speciation of vanadium were measured and the spatial distribution of vanadium was discussed. Current pollution level of vanadium in TGR sediments was assessed by index of geoaccumulation and Häkanson ecological risk index. The results showed that:1the range of vanadium concentrations in TGR was 89.4-175.2 mg·kg-1 and the average concentration was 123.7 mg·kg-1, which was slightly higher than the background value in sediments of Yangtze River. 2the spatial distribution of vanadium in TGR sediments showed that vanadium concentrations in the mainstream of TGR were higher than those in tributaries and the vanadium contents in the sediments in downstream were higher than those in upstream in term of the mainstream of TGR. 3highest vanadium concentration was found in the sediment in a developed county, indicating that this high level of vanadium concentration was attributed to anthropogenic input. 4chemical forms of vanadium in sediments were mainly in residual state (80%-89%, mass fraction), which had a significant positive correlation with total concentrations in sediments. 5vanadium contamination degree and potential environmental risk in TGR sediments were weak.

Current state, sources, and potential risk of heavy metals in sediments of Three Gorges Reservoir, China

Heavy metal (HM) contamination in sediments of Three Gorges Reservoir (TGR) is a particularly important issue for the safety of water quality due to the potential threats of metal toxicity to local and downstream human health. Surface sediments from riparian and submerged areas in the entire TGR mainstream were collected in 2014 to investigate the spatial distribution of HMs (Cd, Cu, Pb, and Zn), identify their possible sources, and assess their potential risk by multiple indices and metal fraction. Results showed that the concentrations of HMs in the sediments increased after the TGR operation, but were lower than those in other Chinese rivers of developed areas. The acid-soluble Cd accounted for more than 50% of total Cd in the sediments, whereas that of other HMs was very low. The Cd concentrations in the riparian sediments increased towards the dam; however, other metals in the riparian sediments and all HMs in the submerged sediments did not show any regular variation trend spatially. The stocks of HMs were significantly higher in the submerged sediments than in the riparian sediments. The high accumulation of HMs in the riparian sediments emerged between Fuling and Fengjie, and those in the submerged sediments existed in the near dam areas. Grain size and Fe/Mn oxides controlled the mobility and transfer of HMs in the sediments. Human activity in the catchment including industrial and agricultural production, shipping industry, mining, etc., increased inputs of HMs in the sediments, and altered their spatial distribution patterns. The sediments were moderately to highly contaminated by Cd, and slightly contaminated by other HMs. The results indicate the current priority of Cd contamination in the TGR, and will conduce to ecological protection in the TGR region.

Characterization of Mixing Processes in the Confluence Zone between the Three Gorges Reservoir Mainstream and the Daning River Using Stable Isotope Analysis.

Understanding the interaction processes between the mainstream and its tributaries and detailing the rates of contribution of water and nutrients from two different waterbodies in the confluence zone are essential for water management in the Three Gorges Reservoir (TGR). The stable isotope ratios of hydrogen (δD) and oxygen (δ(18)O) were applied to explore the interactions between the TGR mainstream and a typical tributary, the Daning River. The results of the model calculations showed that approximately 78.9% of the water and 88% of the nitrate in the confluence zone were from the TGR mainstream. The dynamic vertical distributions of the mixing ratios, major ion contributions, and flow velocities indicated that the water mass from the Yangtze River mainstream flowed backward from the confluence zone up to the tributary along the surface and upper-middle layers, whereas water from the tributary flowed into the mainstream through the lower-middle and bottom layers. This study demonstrates the value of hydrogen and oxygen isotope tracers in accurately describing water mass mixing processes and estimating the rates of contribution of different nutrient sources in the confluence zone, which will provide valuable information for controlling algal blooms in the future.

Assessment of metal toxicity and development of sediment quality guidelines using the equilibrium partitioning model for the Three Gorges Reservoir, China.

The impoundment of the Three Gorges Reservoir (TGR) in China influences the quality of the water supply. Surface sediment samples from the TGR mainstream and three tributaries were collected. Acid volatile sulfide (AVS), simultaneously extractable metals (SEMs), and the fraction of organic carbon (f(oc)) were used to assess the toxicity of heavy metals. Sediment quality guidelines (SQGs) were established using the equilibrium partitioning approach. The results showed that the surface sediments were found to be oxic or suboxic. AVS concentrations in sediments were relatively low, below SEM concentrations. The [SEM] - [AVS] model indicated that all sediments possibly have adverse effects on aquatic life. However, ([SEM] - [AVS])/f(oc) predicted no adverse biological effects in some areas of the Meixi and Caotang Rivers, while adverse effects to aquatic life were uncertain for the other sediments. The partitioning coefficients, water quality criteria, and residual metals in the sediments were the main factors influencing the SQGs for the TGR, while the metals bound to AVS had a negligible effect. The normalized TGR SQGs were all much higher than the existing standards except for cadmium and copper. The differences might be attributed to the approaches used for derivation of SQGs and the physical and chemical characteristics of the sediments.

Isotope analysis of the nutrient supply in Xiangxi Bay of the Three Gorges Reservoir

Nutrients are material basis of eutrophication and algal blooms in the water body. Xiangxi Bay (XXB), the largest tributary bay in the lower reach of the Three Gorges Reservoir (TGR), is subject to such eutrophication and algal blooms. In order to investigate the supply patterns of nutrients in XXB, stable isotopes of hydrogen (δD) and oxygen (δ18O), nutrients, and flow velocity were observed from September 2012 to December 2013. A two-compartment linear mixing model based on the tracer principle of δD and δ18O was employed to calculate the water and nutrients contributions to XXB from the mainstream of TGR and the upper tributaries of the XXB. The average nutrients concentrations of XXB were estimated and compared to the observed values in the field. The results suggested that there was obvious layered and bidirectional density current in XXB, which had a significant impact on nutrient supply pattern and spatial and temporal distribution of the isotopes and nutrients in XXB. The water contributions of the mainstream of TGR and the upper tributaries for XXB were 84% and 16%, respectively. Moreover, the nitrogen and phosphorus nutrients imported into XXB through the mainstream of TGR were over 80% and 60%, respectively, higher than those of the upper tributaries. The calibration of nutrients concentration in XXB showed that the error between the calculated and the observed was about within 20%, which confirmed further the reliability that the nutrients in XXB come mainly from the mainstream of TGR. The results address the attention that the pollution control should be implemented not only in the tributary watershed, but also the whole upstream region of the TGR for prevention of its tributaries algal blooms.

Longitudinal Hydrodynamic Characteristics in Reservoir Tributary Embayments and Effects on Algal Blooms

Three Gorges Reservoir (TGR) is one of the largest man-made lakes in the world. Since the impoundment in 2003, however, algal blooms have been often observed in the tributary embayments. To control the algal blooms, a thorough understanding of the hydrodynamics (e.g., flow regime, velocity gradient, and velocity magnitude and direction) in the tributary embayments is particularly important. Using a calibrated three-dimensional hydrodynamic model, we carried out a hydrodynamic analysis of a typical tributary embayment (i.e., Xiangxi Bay) with emphasis on the longitudinal patterns. The results show distinct longitudinal gradients of hydrodynamics in the study area, which can be generally characterized as four zones: riverine, intermediate, lacustrine, and mainstream influenced zones. Compared with the typical longitudinal zonation for a pure reservoir, there is an additional mainstream influenced zone near the mouth due to the strong effects of TGR mainstream. The blooms are prone to occur in the intermediate and lacustrine zones; however, the hydrodynamic conditions of riverine and mainstream influence zones are not propitious for the formation of algal blooms. This finding helps to diagnose the sensitive areas for algal bloom occurrence.

Sedimentary nutrients in the mainstream and its five tributary bays of a large subtropical reservoir (Three Gorges Reservoir, China)

In order to analyze the distribution pattern of the sediment nitrogen and phosphorus of the Three Gorges Reservoir (TGR), the mainstream of TGR and five tributary bays in Hubei province were observed from October 2005 to July 2006. The spatial and seasonal dynamics of the sediment total nitrogen (TNsed) and total phosphorus (TPsed), the relationship between the mainstream and tributary, and the sediment nutrients' effect to the surface water were analyzed. The results showed that, concentrations of TNsed ranged from 0.27 to 2.04 mg/g, and TPsed fluctuated between 0.17 and 1.21 mg/g. Both of the maximum of TNsed and TPsed appeared in the Xiangxi Bay (the longest bays studied), 1.09 and 0.90 mg/g respectively. No significant spatial differences were found among different sites in other bays (p > 0.05). For seasonal dynamics of TPsed, only the difference in the Yuanshui Bay between spring and summer was significant (p < 0.05). For TNsed, significant differences existed among different seasons in most bays, especially between autumn and other seasons. Cluster analysis based on the surface water nutrients and sediment nutrients indicated that the influence range of the mainstream to the surface water ranged from 10.7 to 15.1 km, while the length of the influence range to the sediment was about 5.34 km. Results of the correlation analysis showed that the internal loading in Hubei province of TGR has not caused a significant effect on the surface water in a large spatial scale, although the influence was significant in some shallow regions. (C) 2012 Elsevier Ltd and INQUA. All rights reserved.

Using temporal coherence to determine the responses of water clarity to hydrological processes in a giant subtropical canyon-shaped reservoir (China)

Investigating whether different locations of a large lake exhibit synchronous or asynchronous fluctuations in limnological characteristics is essential to lake management. If synchronous fluctuations (termed as “temporal coherence”) among sites are found to prevail, regional factors are likely to be the drivers; otherwise, local factors may be dominant. Here, this idea is tested at two spatial scales: the large scale being the mainstream of Three-Gorges reservoir (TGR, China) and the small scale being Xiangxi Bay of TGR. Data of transparency and chlorophyll a were gathered monthly at 26 sampling sites from July 2003 to July 2006 to evaluate temporal coherence for water clarity. High spatial and temporal variation in transparency (4–510 cm) and chlorophyll a (0.1–190.2 μ g/L) mark the system. For the mainstream of TGR, significant high temporal coherence of water clarity among sites (correlation analysis: 0.872 ≤ r ≤ 0.991, mean r = 0.960) was detected and the among-site correlations showed no significant relationship with geographical distance (Mantel test: r = −0.186, p > 0.05) or chlorophyll a (Mantel test: r = 0.249, p > 0.05). By contrast within Xiangxi Bay, lower synchronous fluctuations (correlation analysis: 0.396 ≤ r ≤ 0.971, mean r = 0.785) occurred, and the among-site correlations decreased with an increase in geographical distance (Mantel test: r = −0.715, p < 0.01) and a decrease in temporal coherence of chlorophyll a (Mantel test: r = 0.893, p < 0.01). Seasonal dynamics of water clarity were very well explained by inflow discharges (regression analysis: 0.771 ≤ R 2 ≤ 0.906) and water residence time (0.782 ≤ R 2 ≤ 0.918) in the mainstream, but not in Xiangxi Bay. The high coherency of the dynamics of water clarity in TGR mainstream is likely caused by regional hydrological processes driven by subtropical monsoon climate. For Xiangxi Bay, local processes such as phytoplankton dynamics may override the regional effects of hydrological processes, because the water clarity of this bay is strongly determined by phytoplankton bloom dynamics.