Severely Declining Suspended Sediment Concentration in the Heavily Dammed Changjiang Fluvial System

Global and Planetary Change, 2018

Changes in riverine suspended and riverbed sediments have environmental, ecological and social implications. Here, we provide a holistic review of water and sediment transport and examine the human impacts on the flux, concentration and size of sediment in the Yangtze River in recent decades. We find that most of the fluvial sediment has been trapped in reservoirs, except for the finest portion. Furthermore, soil-conservation since the 1990s has reduced sediment yield. From 1956-1968 (pre-dam period) to 2013-2015 (post-dams and soil-conservation), the sediment discharge from the sub-basins decreased by 91%; in the main river, the sediment flux decreased by 99% at Xiangjiaba (upper reach), 97% at Yichang (transition between upper and middle reaches), 83% at Hankou (middle reach), and 77% at Datong (tidal limit). Because the water discharge was minimally impacted, the suspended sediment concentration decreased to the same extent as the sediment flux. Active erosion of the riverbed and coarsening of surficial sediments were observed in the middle and lower reaches. Fining of suspended sediments was identified along the river, which was counteracted by downstream erosion. Along the 700-km-long Three Gorges Reservoir, which retained 80% of the sediment from upstream, the riverbed gravel or rock was buried by mud because of sedimentation after impoundment. Along with these temporal variations, the striking spatial patterns of riverine suspended and riverbed sediments that were previously exhibited in this large basin were destroyed or reversed. Therefore, we conclude that the human impacts on sediment in the Yangtze River are strong and systematic.

Hydrological Sciences Journal-journal Des Sciences Hydrologiques, 2011

In 2006, the suspended sediment discharge (SSD) into the Yangtze (Changjiang) Estuary, China, reached the historical low value of 85 × 106 t. One hypothesis is that this was caused by the second impoundment, i.e. the second stage of the water-level increase behind the Three Gorges Dam (TGD). However, coincidentally, a significant drought occurred in the same year. From our analysis of long-term data on discharge and SSD, we conclude that the SSD decrease in the upstream catchment area resulting from the extreme drought is primarily responsible for the historical low SSD into the Yangtze Estuary. We quantified the contributions of the extreme drought and the second impoundment to the reduction of SSD into the Yangtze Estuary in 2006 as 82% and 18%, respectively. Even though the TGD is the largest dam in the world, the results indicate that the extreme drought conditions had a greater impact than such a manmade river regulation. Citation Dai, Z. J., Chu, A., Stive, M, Du, J. Z. & Li, J. F. (2011) Is the Three Gorges Dam the cause behind the extremely low suspended sediment discharge into the Yangtze (Changjiang) Estuary of 2006? Hydrol. Sci. J. 56(7), 1280–1288.En 2006, le débit des sédiments en suspension (DSS) dans l'estuaire du Yangtze (Changjiang), en Chine, a atteint la valeur historiquement basse de 85 × 106 t. Une hypothèse est que cela est causé par la seconde mise en eau, i.e. la seconde étape de l’élévation du niveau de l'eau derrière le Barrage des Trois Gorges (BTG). Toutefois, une sécheresse importante s'est produite en coïncidence la même année. D'après notre analyse des données à long terme de débit et de DSS, nous concluons que la diminution du DSS dans le bassin versant amont résultant de l'extrême sécheresse est la cause principale du niveau historiquement bas du DSS dans l'estuaire du Yangtze. Nous avons quantifié les contributions des effets de la sécheresse extrême et de la seconde mise en eau sur la diminution de 2006 du DSS au niveau de l'estuaire du Yangtze respectivement à 82 et 18%. Même si le BTG est le plus grand barrage du monde, les résultats indiquent que les conditions de sécheresse extrême ont eu une influence plus grande que cette infrastructure anthropique de régulation fluviale.

Eos, Transactions American Geophysical Union, 2006

Journal of Hydrology, 2018

Knowledge of the transfer of sediment through river systems is essential for understanding the physical, chemical and biological processes on the Earth's surface. A holistic analysis of long-term records of water discharge, sediment transport, riverbed morphology and estuarine hydrology is here used to quantify spatial and temporal variations in fluvial sediment fluxes along the Changjiang River. We show that the establishment of the Three Gorges Dam (TGD) has directly changed the fluvial sediment-transport process by annually trapping 1.23 × 10 8 t of sediment. The upper Changjiang reach has switched from being the main sediment source before 2003 to a depositional sink of fluvial sediment subsequently. The major lakes, such as Dongting Lake and Poyang Lake, have shifted from being local sediment sinks before 2003 to sediment sources thereafter, such that they now provide sediment to the Changjiang River. Since the 2003 closure of the TGD the riverbed of the middle-lower Changjiang has become the major source of sediment being transmitted downstream, now providing almost 50% of the material entering the estuary. Shoals in the estuarine channels and landward sediment transport from the sea have become major sediment sources for the river estuary. We conclude that dams currently in preparation along the upper Changjiang reach and adjacent lakes may cause the cessation of sediment supply to downstream reaches. Rising sea levels and frequent storms may terminate landward sediment transport, increasing estuarine erosion and inducing seaward sediment transport. It can therefore be expected that substantial erosion could occur in the near future in the Changjiang estuary system.

Geomorphology, 2009

In 2000 and 2003 before the closure of 3-Gorges Dam, numerous sediment samples were taken from the middlelower Yangtze River channel to examine sediment transport processes and associated hydromorphological nature of the river. Analytical results show that the riverbeds consist mostly of medium to coarse sands and gravelly sands, and fine sand occurs locally, especially near the river coast. The results further indicate a downstream fining trend in riverbed sediment from Yichang to the river mouth, totaling about 1900 km long with 12 sediment zones (I-XII). These were identified as alternate coarse-and fine-grained sediment on the riverbed, although the zonation of I-III below Three-Gorges Dam site is weaker. The mode of sediment transport in the river is dominated by saltation (20-80%), followed by bed-load transport with 3-15%; transport by suspension is quite low. Grainsizes associated with hydrological parameters have greater values in the Jingjiang Reaches (from Yichang to Chenglingji; unit stream power: 5-18 N m − 1 s − 1 , boundary shear stress: 14 Nm − 2 and mean flow velocity: 2-3.2 ms − 1 ), whereas the values obtained from Chenglingji downstream are considerably low (b 5 N m − 1 s − 1 , 1-4 Nm − 2 and b 0.7-1.5 ms − 1 ). These values, when compared with on-site measured velocity of the ADP flow column, revealed the erosive riverbed sediment transport in the Jingjiang Reaches, and the accumulative riverbed transport downstream, from Wuhan to the river coast. Hydrological parameters together with distribution of grain-sizes indicate a coarsening riverbed in the Jingjiang river, largely because damming peaked since the last half-century. This corroborates the weakening sediment zonation in the Jingjiang Reaches, which is expected to extend further downstream towards the river coast in response to the potential impact of 3-Gorges Dam in the coming decades.

Quaternary International, 2011

The natural hydrologic regime of the Changjiang (Yangtze River) in China has been disturbed considerably by intensified human activities over the past five decades. In this study, sediment rating curves were analyzed based on monthly data of water discharge and suspended-sediment concentration at stations in the upper (Yichang station), middle (Hankou station) and lower (Datong station) reaches of the Changjiang in different periods from 1955 to 2007. Temporal and spatial variations of the sediment rating curves were analyzed with respect to the impact of human activities and watershed characteristics. Results indicate that human disturbances have had a substantial impact on sediment rating parameters, with the magnitude of the impact related to the scale of the river sections. As indicated by the sediment rating parameters, the sediment transport regimes between the upper and the mid-lower reaches differ significantly. In particular, the impoundment of the Three Gorges Dam in 2003 changed the sediment transport regimes in the upper and mid-lower reaches of Changjiang into similar regimes characterized by a decrease in the sediment transport capacity of high water discharge. Furthermore, the sediment rating curve at the Datong station in 1955-1968 was applied to estimate the potential sediment load (1969-2007) in the absence of human influences. The mean annual sediment deficit caused by human activities has increased from 80 Mt/yr (1969-1985) to 250 Mt/yr (2003-2007). This indicates that intensified human activities in the Changjiang basin, especially the construction of the Three Gorges Dam, have altered the natural sediment transport process, and have thus become a dominant force in Changjiang sediment delivery to the sea.

Quaternary International, 2008

Sediment loads from the Yangtze River into the sea experienced a major increase in the late Holocene when sand beaches along the deltaic coast changed into tidal-flats. Sediment transport averaged about 472 million tons a year between the 1950s and the mid-1980s and decreased by 124 million tons a year after the mid-1980s, followed by a dramatic drop since the closure of the Three Gorge Dam (TGD). This paper discusses the major processes for the decline of sediment discharge since the 1950s and discusses the method for evaluating its future development over the Post-TGD Period. Sediment loads in the Post-TGD Period will be mainly determined by the major processes that may significantly contribute to the following three components, i.e. (1) the sediment input from the upper basin into the Three-Gorge Reservoir (TGR); (2) the ratio of sediment export from the TGR relative to the input and (3) the amount of sediment recovery downstream the TGD restrained by available sediment sources. An integrated evaluation shows that the annual sediment loads over the Post-TGD Period will possibly vary from 1.12 to 1.32 million tons a year or less in ordinary years.

Quaternary International, 2015

Reservoir interception has significantly affected the fluvial sediment budget as well as the sedimentary processes of the entire Changjiang catchment. To evaluate the impact of reservoirs, we analyze the combined effects of 1037 large and medium-sized reservoirs on the fluvial flux in general, and more specifically on the sedimentary processes in the middle and lower reaches. Results indicate that reservoir emplacement in the Changjiang catchment currently reduces the sediment load towards the East China Sea by 453 Mt y À1. Estimates at Yichang station show that the sediment discharge would exceed 555 Mt y À1 , if there were no reservoirs involved. It is expected that in the near future, more dams will be constructed. The entire reach of the Changjiang River can be divided at Yichang station into two distinctly characterised reaches with regard to sedimentation, where the upper reach exhibits mostly siltation (over 589 Mt y À1 of sediment deposition), and the lower reach is affected by erosion (sediment loss, including sand extraction, exceeding 112 Mt y À1). As a consequence, the sediment flux to the sea will further decrease to 100 Mt y À1. Due to human interference, the upstream sediment load reduced and caused significant changes in the erosion/deposition pattern of the middle and lower reaches, which together altered the terrestrial sediment input to the sea. Before 2003, the upstream reaches were the dominant sediment source. After 2003, the sediment contribution of the middle and lower reaches became more important, and its sediment contribution will further increase to 78% of the total sediment load reaching the sea, after completion of the cascade reservoirs at the Jinsha Tributary. Hence, the middle and lower reaches are converting from a sediment sink to a major sediment source.

Hydrological Sciences Journal, 2008

Periodicity of the runoff and the sediment load, and possible impacts from human activities and climatic changes, in the Yangtze River basin during 1963-2004 are discussed based on the monthly sediment and runoff data, and using the wavelet approach. Research results indicated that: (a) Sediment load changes are severely impacted by the different types of human activity (e.g. construction of water reservoirs, deforestation/afforestation); and the runoff variability is the direct result of climatic changes, e.g. the precipitation changes. (b) The impacts of human activity and climatic changes on the sediment load and runoff changes are greater in smaller river basins (e.g. the Jialingjiang River basin) than in larger river basins. The response of sediment load and runoff changes to the impacts of human activities and climatic changes are prompt and prominent in the Jialingjiang River basin relative to those in the mainstem of the Yangtze River basin. (c) Construction of the Three Gorges Dam has already had obvious impacts on the sediment transport process in the middle and lower Yangtze River basin, but shows no obvious influence on the runoff changes. Construction of the Three Gorges Dam will result in further re-adjustment of the scouring/filling process within the river channel in the middle and lower Yangtze River basin, and have corresponding effects on the altered sediment load because of the Dam's operation for the river channel, ecology, sustainable social economy and even the development of the Yangtze Delta. This will be of concern to local governments and policy makers.

Large Rivers

Geomorphology, 2012

The geomorphic impacts of dams on downstream river channels are complex, not readily predictable for specific cases, but widely reported in the literature. For the Three Gorges Dam on the Yangtze (Changjiang) River in China, no studies of the impact of the changed flow and sediment conditions below the dam on the behaviour of the channel were included in the pre-dam feasibility report. We have assembled a database of flow and sediment data for the middle Yangtze River from Yichang to Hankou and used this to analyse changes following the closure of the dam. While total flow is little affected, the operating strategy for the dam that provides for storage of part of the summer high flows to maintain hydroelectric power generation in winter (the low flow season) is reflected in changes to the seasonal distribution of flow below the dam. We calculated potential sediment carrying capacity and compared it with measured sediment concentrations for both pre-and post-dam conditions. While channel sedimentation is indicated along the middle Yangtze for pre-dam conditions, scour is indicated for post-dam conditions, highest at Yichang immediately below the dam and decreasing downstream.

Geomorphology, 2007

This study examines the characteristics of sediment rating parameters recorded at various gauging stations in the Yangtze Basin in relation to their controls. Our findings indicate that the parameters are associated with river channel morphology of the selected reaches. High b-values (N 1.600) and low log(a) values (b −4.000) occur in the upper course of the steep rock-confined river, characterizing high unit stream power flows. Low b-values (b 0.900) and high log(a) values (N −1.000) occur in the middle and lower Yangtze River associated with meandering reaches over low gradients, and can be taken to imply aggradation in these reaches with low stream power. Higher b-values (0.900-1.600) and lower log(a)-values (−4.000 to −1.000) characterize the reaches between Yichang and Xinchang, immediately below the Three Gorges. These values indicate channel erosion and bed instability that result from changes in channel gradient from the upstream steep valley to downstream low slope flood plain settings. Differences in channel morphology accompany these changes. Confined, V-shaped valleys occur upstream and are replaced downstream by broad U-shaped channels. The middle and lower Yangtze shows an apparent increase in channel instability over the past 40 years. This inference is based on sediment rating parameters from various gauging stations that record increasing b-values against decreasing log(a)-values over that time. Analysis of the sediment load data also reveals a strong correlation between changes in sediment rating curve parameters and reduction of annual sediment budget (4.70 × 10 8 t to 3.50×10 8 t/year, from the 1950s to 1990s), largely due to the damming of the Yangtze and sediment load depletion through siltation in the Dongting Lake. Short-term deviations from the general trends in the sediment rating parameters are related to hydroclimatic events. Extreme low b-values and high log(a)-values signify the major flood years, while the reverse indicates drought events. When compared with rivers from other climate settings, it is evident that the wide range of values of the Yangtze rating parameters reflects the huge discharge driven by the monsoon precipitation regime of eastern China.

Journal of Hydrology, 2005

The present study focuses on simulating sediment flux for 1998 big-flood with 60-year recurrent period in the Yangtze River catchment. On the basis of close correlation between discharge and sediment load recorded on the daily base of the past decades at a series of hydrological gauging stations located in the Yangtze River, the sediment rating curve of 1987/1988 was selected to simulate the annual and flood season sediment fluxes of 1998, when measured discharge was available in the most gauging stations. The result indicates that enormous sediment load was delivered downstream and to the estuary during the flood year. The simulated annual sediment flux was about 930 million-tonnes in the upper drainage basin, about 520 million-tonnes in the middle catchment and 720 million-tonnes in the lower drainage basin. These loads, respectively, approximate almost 1.9, 1.2, and 1.8 times those of the multiyearly sediment flux in the upper, middle and lower Yangtze catchments for the past decades.

Journal of Geology & Geophysics, 2015

Change of River Regime in Post-TGD Study area Shashi Reach from Chenjiawan to Sanbatan Shoal is selected as a case study. It is located about 150 km downstream from the TGD and is about 20 km long [13].

Geomorphology, 2001

Ž. Hydrological records covering a 100-year period from the upper, middle and lower Yangtze River were collected to examine the temporal and spatial distribution of discharge and sediment load in the drainage basin. The Yangtze discharge, as expected, increases from the upper drainage basin downstream. Only an estimated 50% of the discharge is derived from the upper Yangtze, with the rest being derived from the numerous tributaries of the middle and lower course. However, the distribution of sediment load along the Yangtze is the reverse of that observed for discharge, with most of the sediment Ž 8. being derived from the upper basin. A dramatic reduction in sediment load by ; 0.8 = 10 tonsryear occurs in the middle Yangtze because of a marked decrease in slope and the change to a meandering pattern from the upper Yangtze rock sections. Considerable siltation also occurs in the middle Yangtze drainage basin as the river cuts through a large interior Dongting Lake system. Sediment load in the lower Yangtze, while significantly less than that of the upper river, is somewhat higher than the middle Yangtze because of additional load contributed by adjacent tributaries. A strong correlation exists between the discharge and sediment load along the Yangtze drainage basin during the dry season as lower flows carry lower sediment concentration. During the wet season, a strong correlation is also present in the upper Yangtze owing to the high flow velocity that suspends sand on the bed. However, a negative to poor correlation occurs in the middle and lower Yangtze because the flow velocity in these reaches is unable to keep sand in suspension, transporting only fine-grained particles downstream. Ž. Hydrological data are treated for 30 years 1950-1980 , when numerous dams were constructed in the upper Yangtze drainage basin. At Yichang and Hankou hydrological stations, records revealed a decreasing trend in annual sediment load, along with slightly reduced annual discharge at the same stations. This can be interpreted as the result of water diversion primarily for agriculture. Sediment load at Datong further downstream is quite stable, and not influenced by slightly reduced discharge. Furthermore, sediment concentration at the three hydrological stations increased, which can be attributed to sediment loss in association with intensifying human activity, especially in the upper drainage basin, such as deforestation and construction of numerous dams. Mean monthly sediment load of these 30 years pulses about 2 months behind discharge, implying dam-released sediment transport along the entire river basin during the high water stage.