Zuosheng Yang

The Huanghe (Yellow River) and Changjiang (Yangtze River) deltas: a review on their characteristics, evolution and sediment discharge during the Holocene

Abstract Today, Asian rivers play an important role in delivering sediment from the land to the oceans. However, past sediment discharge before human activities modified the landscape is not well known and is estimated to have been roughly one-third to one-half of the present level. Chinese rivers have been strongly influenced by human activities, and their sediment discharge has changed over the last 6000 years. For the study of the long-term human impact on rivers and catchment basins, Chinese rivers provide excellent examples because continuous historical records are available. Here, the characteristics, evolution and sediment discharge during the Holocene of the deltas of two large Chinese rivers, Huanghe (Yellow River) and Changjiang (Yangtze River) are summarized. The Huanghe and Changjiang deltas are different with respect to coastal environments, water and sediment discharge, the pattern of development of the delta lobe, thickness of Holocene sediments, and basal topography of the incised valleys formed during the last glacial period. The Huanghe Delta is characterized by a high concentration of sediment, huge sediment discharge, thin Holocene deltaic sediments, a lateral delta lobe shift, and a steep longitudinal profile in its lower reaches. The Changjiang Delta is characterized by a large water discharge, a large interseasonal water-level change, a tide-dominated coastal environment, continuous seaward progradation with isolated river-mouth sand bodies, thick Holocene sediments, and a deep-incised valley formed during the last glacial period. Based on the sediment volume of prograding deltaic sediments over the last 6000 years, the sediment discharge of Huanghe 1000 years ago is estimated to have been 1×10 8 tons/year, 1/10 of the present level. An abrupt increase occurred at ca. 1000 years ago as a consequence of human activities on the Loess Plateau, especially involving cultivation and deforestation. Moreover, the estimated sediment load of Huanghe at Sanmenxia station prior to 1000 years was 2.7–4.0×10 8 tons/year, or about 20% (15–25%) of the present level. Our recent study estimated that the sediment discharge of Changjiang before 2000 years BP was ca. 2.4×10 8 tons/year. Total sediment discharge of both rivers before 2000 years BP is estimated to have been 3.4×10 8 tons/year, or only about 20% of the present level. Moreover, the total sediment load to the lower reaches is estimated to have been 5–6×10 8 tons/year, or one-fourth the present level.

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.

Yangtze sediment decline partly from Three Gorges Dam

It could be argued that nowhere has the impact of dams on rivers been more important than in China, where since 1950 almost half of the worlds large dams (higher than 15 meters) have been built ]Fuggle and Smith, 2000]. Chinas Yangtze River (Changjiang)—the largest river in south Asia (1.8 million square kilometers) and whose basin is home to more than 400 million inhabitants—alone has more than 50,000 dams within its watershed, including the worlds largest, the Three Gorges Dam (TGD) (Figure 1a ). Water and sediment began being impounded behind the TGD in June 2003, and two years after impoundment, river sediment discharge downstream (at Datong Station) had decreased by nearly half of its 2002 load (Figure 1c). However, the decrease in Yangtze sediment load did not begin with TGD impoundment. Rather, the sediment load at Datong has declined continually since 1987 despite a slight increase in river discharge (Figure 1c ). The change in pre-TGD loads atYichang, just downstream from the TGD, has been even more extreme, decreasing by approximately 300 million tons in 1986–2002, before declining another 130 million tons after 2002 (Figure 1b). All of this suggests that collective changes on the Yangtze upstream (above Yichang) have been more important in decreasing the rivers sediment load than the TGD.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

Yellow River's water and sediment discharge decreasing steadily

The amount of water and sediment discharged by the Yellow River in northern China has been decreasing steadily over the past 20–25 years, such that in recent years it has contributed relatively little sediment to the Gulf of Bohai. This is quite at odds with the well-known story in which the Yellow River has been regarded as a primary contributor of fluvial sediment to the global ocean. If the trend of decreasing discharge continues, the environmental and economic impacts to the Yellow River delta and adjacent coastal waters could be dramatic. Already shrimp harvest from the brackish waters of the western Gulf of Bohai has decreased markedly, and there has been considerable coastal retreat along parts of the delta.

Active slope failure, sediment collapse, and silt flows on the modern subaqueous Huanghe (Yellow River) delta

Post-depositional slope instability and bottom mass-movement processes strongly modify the progradational subaqueous slopes of the modern Huanghe (Yellow River) Delta. Wide, shallow gullies dissect the submarine slopes with gradients of 0.3 to 0.4°. Lower delta-front sediments experiencein situ subsidence, forming numerous collapse depressions. These processes are pronounced over much of the delta, incising and redistributing the most recently deposited silt-rich sediment. Principal causative factors include low sediment strengths created by rapid deposition in the delta during annual peak discharges from the river and severe bottom perturbations by surface storm-generated waves.

The subaqueous delta of the modern Huanghe (Yellow River)

The subaqueous delta of the Huanghe (Yellow River) has been studied using high-resolution acoustic systems. There are many subtle variations in sea floor morphology and sediment geometries; smooth, featureless areas are rare. The main components of the subaqueous delta include broad, shallow channels; moderately disturbed areas with near-surface cut and fill structures; heavily disturbed areas with sea floor depressions, pits, and gullies; and a smooth, gently sloping distal delta apron or “rise.” These features are not directly related to sediment settling from dilute surface plumes but are due to gravity-driven hyperpycnal underflows, submarine mass movements, and silt flows.

Sources of polycyclic aromatic hydrocarbons to sediments of the Bohai and Yellow Seas in East Asia

[1] The coasts of Bohai Sea (BS) and Yellow Sea (YS) in China support almost one-quarter of its population and provide more than one-third of the national GDP. BS and YS are downwind of the Asian continental outflow in spring and winter as influenced by the East Asian monsoon. This makes the two seas important sinks of land-based pollutants associated with the Asian continental outflow. The sixteen U.S. EPA proposed priority polycyclic aromatic hydrocarbons (PAHs) in 130 surface sediment samples collected from BS and YS were measured. Combined with our previous PAH data of 90 PM2.5 samples from the upwind areas, the sources of the PAHs in BS and YS were apportioned using positive matrix factorization (PMF) modeling. Four sources were identified: petroleum residue, vehicular emissions, coal combustion and biomass burning. Petroleum residue was the dominant contributor of PAHs in the coast of the Bohai Bay probably due to Haihe River runoff, oil leakage from ships and offshore oil fields. The contribution of vehicular emissions in BS was higher than that in YS, and the reverse was true for coal combustion and biomass burning. This difference in the source patterns in the sediments of the two seas could be attributed to the different PAH emission features of the upwind area related to demographic and economic conditions, as well as the marine geography. The ratios of selected 4–6 ring PAHs in the sediments compared well with those of the PM2.5 of the upwind areas, implicating that the particle phase PAHs in the atmosphere play an important role in the source to sink process of the pyrogenic PAHs in the region.

Rapid shifts of the river plume pathway off the Huanghe （Yellow） River mouth in response to water-sediment regulation scheme in 2005

Freshwater plume represents a key passage for the river to deliver sediment to the coastal ocean. A water-sediment regulation scheme was implemented for the Huanghe River in 2005; in order to examine the effect of such an activity on the river plume dynamics, three cruises were carried out off the Huanghe River mouth. The preliminary results of the in-situ measurements suggested that the plume pathway shifted rapidly during the period of less than twenty days, which was confirmed by satellite remote sensing data in the same period. The rapid shifts were resulted from intensive interactions between river discharge and the varying bathymetry in the river mouth area. A large amount of river sediment was delivered to the coastal region during this period and mostly deposited in the mouth bar area due to jet-flow deceleration, forming a cluster of mouth bars across the river mouth, which caused the rapid shifts of the plume pathway.