Zhongyuan Chen

Yangtze River of China : historical analysis of discharge variability and sediment flux

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 being derived from the upper basin. A dramatic reduction in sediment load (by ∼0.8×108 tons/year) 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. 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.

Landscape variability and the response of Asian megadeltas to environmental change

Deltas, occurring at the mouths of river systems that deposit sediments as they enter the sea, are some of the most dynamic sedimentary environments. They contain a long, and often economically significant, sedimentary record of their response to past episodes of climate and sea-level change. Geological investigation of these deposits, and the processes controlling sedimentation, provide insights into the response of deltas to environmental change, which in turn may offer rational and cost-effective strategies for the sustainable management of natural resources and land use in these dynamic systems in the face of future environmental change.

Neolithic settlement distributions as a function of sea level–controlled topography in the Yangtze delta, China

Combined geological and archaeological data sets indicate that sea level–controlled topography best explains the timing of Neolithic settlement onto the southern Yangtze delta plain, almost 1500 yr later than inland China. Information on settlement patterns of the three major Neolithic cultures (Ma-Jia-Bang, Song-Ze, Liang-Zhu), dated from ca. 5500 to 2200 B.C., is provided by petrologic study of habitat bases of sites and of sediment cores recovered near the sites. In the early Holocene, rising sea level induced decreased relief and an aggrading silt mantle on the low-lying delta surface. Changes of sea level and climate from early to mid-Holocene initiated a fertile delta plain at ca. 6000–5500 B.C., and settlement and cultivation, including rice, began within only 500 yr of delta formation. Rate of sea-level rise decelerated by mid-Holocene time, resulting in rising ground-water level and poor drainage, and a reduced delta plain area suitable for human occupation and agriculture. As a consequence, Neolithic settlements shifted progressively eastward toward higher, more restricted areas of the Yangtze delta chenier plain.

Implications of flow control by the Three Gorges Dam on sediment and channel dynamics of the middle Yangtze (Changjiang) River, China

The impacts of a dam on the river downstream in terms of hydrology and morphology are determined by a complex mix of variables that includes the patterns of release of water through the dam and the characteristics of the downstream channel. Scour of the downstream channel is a common response because large dams cause a significant interruption to sediment continuity. Here we show that in the case of China9s Three Gorges Dam on the Yangtze River the outcome is complicated, as is commonly the case in large rivers. The downstream channel and floodplain system compose an area of long-term sediment accumulation and unstable channels with seasonally contrasting erosion and deposition patterns related to the migrating seasonal monsoon rainfall zones. In achieving one of the main purposes of this dam, that of flood control in the middle and lower basins, the pattern of flows released from the dam will closely resemble those seasonal flows that are responsible for channel instability in the middle catchment, thus effectively making erosive conditions the most common during a year. There is obviously concern about the ultimate impact of sediment storage in the dam on the dynamics of the delta and adjacent coast, and we show that this depends on the trajectory and duration of the erosive responses in the middle Yangtze basin. In this particular case, the outcome is of great significance to the well being of the densely populated riparian areas of the river.

Early Holocene mud-ridge formation in the Yangtze offshore, China: a tidal-controlled estuarine pattern and sea-level implications

Abstract The purpose of the present study is to examine the topographic evolution and sedimentary characteristics of a mud-ridge system in the Yangtze offshore region. As revealed by seismic profiling, 2–3-km-wide, 5–8-km-long, up-to-15-m-thick mud ridges, situated 5–10 km apart, extend here in a southeasterly direction, approximately parallel to the direction of the ebb and flood flow of the modern tide. Directly resting upon the latest Pleistocene terrestrial sediments, some mud ridges are exposed on the present seafloor, while others are buried by landward-thickening Holocene deltaic sediments. An 11-m-long vibrocore recovered from one mud ridge reveals homogenous and mottled silty clay and clayey silt, rich in foraminifera, suggesting an estuarine environment of deposition. Chronostratigraphic correlation in association with acoustic facies indicate that the mud ridge was formed primarily during ∼10–7 ka, corresponding to a period of global decelerating rise in sea level. This mud ridge is underlain by thick, late Pleistocene fluvial to coastal fine to medium sands with an unconformity surface on top, and it is overlain by modern deltaic sediments with an erosional surface in between. We propose that the mud ridges were deposited under tidal-controlled estuarine conditions during the early Holocene deceleration of the rise in sea level. The Holocene transgression in the study area was nearly coeval with the mud ridge deposition, and the deltaic sedimentation occurred after the mud ridges formed. The ridges were subsequently reworked, largely by strong tidal currents.

Magnetostratigraphy and sedimentology of the Jingchuan red clay section and correlation of the Tertiary eolian red clay sediments of the Chinese Loess Plateau

A 325-m-thick loess and red clay sequence was recently discovered at Jingchuan, which is situated about 45 km north of the previously studied Lingtai section in the middle part of the Chinese Loess Plateau. The Jingchuan section is composed of 199 m of Pleistocene loess and 126 m of Tertiary red clay deposits. Paleomagnetic studies show that the Jingchuan red clay has a basal age of at least 7.7 Ma, which represents the oldest red clay deposits presently known in the Loess Plateau. Field observations and grain size analyses both suggest an eolian origin of the red clay, thus extending available records of the eolian deposits in the Loess Plateau from about 7.0 Ma to 7.7 Ma. Correlation of both pedostratigraphy and magnetic susceptibility records suggests that the late Cenozoic red clay records in the Loess Plateau have a general continuous nature, thereby providing potential for deriving long-term climatic evolution signals from this eolian loess and red clay sequence.

Yangtze delta, eastern China: 2. Late Quaternary subsidence and deformation

Abstract Late Quaternary deposits, examined on high-resolution seismic profiles, provide evidence of Recent subsidence of the outer Yangtze delta in the East China Sea. Deformed late Pleistocene strata and mud diapirs, tilted bedding, and gas-deformed structures in Holocene sequences record remobilization of underconsolidated sediment within and adjacent to the Yangtze depocenter. Deformation types and their specific position relative to the depocenter record displacement of sediment as a result of Quaternary overburden and probable compaction and deep-seated tectonic motion. The configuration of depositional sequences demonstrates that subsidence in the outer Yangtze delta has continued from the late Pleistocene to the present.

Yangtze delta, eastern China: 1. Geometry and subsidence of Holocene depocenter

Abstract Stratigraphic analysis of 59 borings serves to define the subsurface configuration of late Quaternary deposits of the outer Yangtze delta, including both subaerial lower plain and its contiguous submerged sector. Isopachous maps of the Holocene sediment section reveal a linear thickened trend parallel to the lower Yangtze estuary—Chongming island, which extends seaward (southeast) to the mid-shelf. Maps depicting depths of the Holocene to late Pleistocene contact also highlight this thickened northwest to southeast trend, and a deep downward-bowing depression formed by the sedimentary sequence seaward of the Yangtze mouth. Radiocarbon-dated facies in the cores serve to calculate long-term averaged subsidence rates for the Holocene section: 0.1 to 3.0 mm/year on land, and 1.6 to 4.4 mm/year at sea. This preliminary information is vital for planning coastal protection (erosion, salt water intrusion) in this densely populated, low-lying region vulnerable to marine inundation.

Holocene environmental change and Neolithic rice agriculture in the lower Yangtze region of China: A review

In this paper, we summarize the Holocene environmental history of the lower Yangtze region, east China, based on the sedimentary records and microfossil diatom, pollen, fungal and charcoal data that were published in the past two decades. We then examine the linkage between changes in the coastal environment and the development of rice agriculture in the region, with reference to the available archaeological evidence and historical archives. Based on the sedimentary and archaeological evidence, we conclude that during the early Holocene sea-level movements and sedimentary processes significantly changed the region’s landscape from an open, brackish water environment to a largely enclosed, wetland system. This newly established freshwater marsh environment provided a habitat favourable to rice agriculture. The early Neolithic farmers took the opportunity presented to them and started rice cultivation in locations where freshwater wetland systems were established. During the middle Holocene, environmental conditions were largely stable, and the coastal wetlands evolved slowly. Environmental change was only a supportive player in the Neolithic cultural processes, because the Neolithic people were able to adapt to these changes and took advantages of the newly emerging marsh land for food collection and production. Around 4200 years ago, the prosperous Liangzhu society fell, but there is no evidence to suggest the fall was related to a significant environmental change. The coastal environment continued to evolve slowly during the late Holocene. But this period saw rapid technological development in irrigation and flood protection, and the environmental factor was reduced to background noise. Throughout the Holocene, the main strategy taken by the Neolithic people to cope with environmental change was migration to find better food sources. Along with this strategy was the development of technology in landscape management to ensure a more reliable food production in addition to food collection through hunting, gathering and fishing.

The drivers of risk to water security in Shanghai

Big cities are often said to have big water problems, and Shanghai is no exception. In this paper, we examine and compare the influence of the major factors that give rise to the risk of water insecurity in Shanghai. There is an extensive and diverse literature on these issues, dealt with in isolation, and here, we provide a synthesis of the literature, together with our own assessments and calculations, to assess what are the risks to Shanghai’s water supply and what is our degree of confidence in this assessment. We describe the systems that supply water to the city, and past and future changes in the systems, including changes in the glaciers that supply some water to the river, changes in climate, changes in land use, the construction of dams, and water diversions. We show how, at the same time as Shanghai is increasing its dependence on the Yangtze river, water diversions and sea level rise are increasing the risk that this water will be too saline to consume at certain times of the year. This analysis suggests that most of the major drivers of the risk to water security in Shanghai are within the power of environmental managers to control.