Yangtze River
The Yangtze River, Asia's longest watercourse at about 6300 kilometers, has a basin that holds approximately one third of the population of China. The headwaters of the Yangtze, also termed the Changjiang River, originate in the snow covered Geladaindong Mountains of the Tibetan Plateau at an elevation of about 6000 meters; discharge is to the East China Sea near Shanghai.
The Chinese government has created the world's largest hydroelectric project at the Three Gorges Dam, an activity that has exacted a great toll in environmental impact to the Yangtze River and massive relocation impacts to more than 1.3 million Chinese people.
Contents
Hydrology
The drainage basin of the Yangtze River covers an area of approximately 1.81 million square kilometers.
The upper Yangtze basin is considered the part from the headwaters to the Three Gorges area, or a catchment area of approximately one million square kilometers; this upper basin is quite mountainous.
The middle Yangtze basin drains an area of approximately 680,000 square kilometers, as it meanders through the Dongting and Jianghan Lake basins terminating around Hukou.
The lower Yangtze basin runs from Hukou to the East China Sea and includes all of the estuarine portion of the river. The length of the lower Yangtze is about 900 kilometers, and the catchment area of the lower Yangtze basin is approximately 130,000 square kilometers.
After emergence from the steep Three Gorges reach, the Yangtze flow rate is on the order of one to two meters per second. At such flow rates considerable sediment deposition is occurring, well above historic rates. Thus channel bottom characteristics have been altered by silt accretion well upstream of the Three Gorges Dam.
Sediment loading of the middle Yangtze has risen sharply over the laste several decades, mainly due to anthropogenic activities in the steep watershed; these activities include dam related upstream projects, road grading on steep slopes and massive resettlement efforts in relocation of over 1.3 million native people. Prior to this construction the steep slopes bore dense vegetation, untouched for millennia, and limestone outcrops. Present sediment loading from the Three Gorges stretch amounts to roughly 70 million tons per annum of soil, evidenced by the deep brown colour of the river here. Sediment deposition at the Three Gorges Dam site is quite high, since this sediment load from the Three Gorges runoff is essentially trapped behind the dam; discussions with engineers at the Three Gorges Dam do not indicate confidence that the nominal lifetime of the project can be sustained, given the rate of sediment deposition.
A new reservoir is filling in central China. The Three Gorges Dam on the Yangtze River—the world’s largest dam—was completed in 2006, and the river is filling up its valley behind the dam to form a narrow reservoir extending more than 600 kilometers. This image from April 15, 2009, is one of the first images that astronauts on the International Space Station have been able to capture of the flooding behind the dam. The main objective for the dam is to supply water for the largest hydroelectric plant in the world and to help control the devastating floods that plague the lowlands downstream from the dam.
The epic scale of the dam project is matched by the level of controversy it continues to generate. Concerns about major environmental impacts, the relocation of 1.2 million people, and the flooding of 13 cities, more than 1300 villages, archeological locations, and hazardous waste dumps were razed throughout the planning and implementation. Environmental concerns include increased seismicity from the loading of the water, landslides, changed ecosystems, accumulated pollution, increased chances for waterborne diseases, and salinity changes in the Yangtze estuary. Credit: NASA. Astronaut photograph ISS019-E-7720 was acquired on April 15, 2009.
Water quality
Lower and middle basins of the Yangtze carry heavy pollutant loads. In the lower Yangtze basin nitrate levels are high, measuring at about 1000 tons per day at Datong; these levels accrue from high applications of chemical fertilizer applied and also considerable loadings of untreated sewage due to the large human population of the basin, with correspondingly little infrastructure for sewage treatment.
Heavy metal concentrations are also high in the lower Yangtze, with measurements of dissolved lead at 0.078 microgram/liter; cadmium (0.024 microgram/liter), chromium (0.57 microgram/liter), copper (1.9 microgram/liter), and nickel (0.50 microgram/liter). Levels of dissolved arsenic have been measured at 3.3 microgram/liter) and zinc at 1.5 microgram/liter), both notably higher by factors of 5.5 and 2.5 respectively than other typical large world rivers. In Yangtze River suspended sediment, arsenic comprises 31 microgram/gram, lead comprises 83 microgram/gram, and nickel comprises 52 micrograms/gram of sediment content; each of these fractions of suspended sediment are at the maximum levels, for example, recommended by the European Union (EU). Cadmium, chromium, zinc and copper percentages of sediment run at twice the EU standard, while mercury in suspended sediment exceeds the percentage recommended by the EU by a factor of four. Given the high flow rates of the Yangtze, the total heavy metal discharge to the East China Sea is massive with 2008 levels at 4600 kilograms (kg) per day of arsenic; 3000 kg per day of lead; 2000 kg per day of nickel. About 6000 tons per day of dissolved organic carbon is also discharged to the East China Sea. Such quantities pose severe ongoing adverse impacts to the East China Sea ecosystem. It should also be noted that these discharge rates and concentrations are on the increase, based upon the trend of the prior two decades.
On May 15, 2006, Chinese news media reported that engineers were pouring the last of the concrete to finish the construction of the massive Three Gorges Dam on the Yangtze River in central China. The dam is intended to provide flood control and hydroelectric power. When all the generators are operational, Three Gorges will be the largest hydroelectric project in the world.
This pair of images shows the dam in partial completion in July 2000 and again in May 2006. The Yangtze River flows from upper left toward upper right in the images. In 2000, construction along each riverbank had occurred, but sediment-filled water still flowed freely through a narrow channel near the river’s south bank (bottom left). A smaller passage closer to the north bank of the river, where the water appears calmer, is likely a system of temporary locks that allowed for boat passage. A second dam bypass appears to be under construction about one kilometer to the north.
By May 15, 2006, the dam spanned the entire river, and a large reservoir had filled behind it, to the northwest. The new reservoir is more than three kilometers across just upstream of the dam. White spray shoots through gates in the center portion of the dam. The former locks are much less prominent, and the new ones to the north appear as a linear arrangement of thin, blue rectangles. Credit: NASA image created by Jesse Allen, Earth Observatory, using ASTER data made available by NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team.
Geology
The upper Yangtze basin consists chiefly of Paleozoic limestone and terrigenous sedimentary rock, with some granitic material. The most downstream element of the upper Yangtze basin is often termed the Sichuan Basin; here the Yangtze cuts through Triassic and Permianmaterial before entering the Three Gorges. The Three Gorges area is a stretch of the Yangtze that runs approximately 660 kilometers, terminating at the site of the Three Gorges Dam. Prior to construction of the dam, the Three Gorges area was a site of exceptional natural beauty; after dam construction the gorge areas were filled with approximately 100 meters in depth of Yangtze water, and considerable amounts of the watershed were graded to achieve logistical support and relocation of over 1.3 people.
The lower Yangtze basin consists of anabranching river structures and Pleistocene coastal terraces. Prior to development of the Three Gorges Dam, the Yangtze Delta was replenished with a copious sediment load reaching the river mouth; however, the dam has now severely limited the natural flow and deposition of sediment to the delta region. Consequently, the integrity of the delta is been compromised, with scouring exceeding deposition, and the very stability of the delta is endangered.
Aquatic biota
The Yangtze River dolphin, now thought to be extinct, was endemic to the Yangtze River. No more than 100 of the species were thought to survive by the latter part of the twentieth century. Declining water quality exacerbated by the construction of the Three Gorges Dam are thought to have been the chief drivers of extinction.
There are a large number of native fish species in the Yangtze River, largely attributable to the diversity of riverine habitat types encountered in the river's long course. Several endemic benthopelagic species are found in the upper Yangtze, including: Liobagrus kingi, Anabarilius polylepis, Bangana rendahli, Pseudogyrinocheilus prochilus, 'Sinocyclocheilus grahami and Siniperca roulei. The demersal fish Silurus meridionalis also is found as a middle reach Yangtze River endemic species.
There are several large native demersal fish found in the Yangtze River, chiefly the 250 centimeter (cm) long endangered Yangtze sturgeon (Acipenser dabryanus), the 120 cm Chinese sturgeon (Acipenser sinensis), the 200 cm Giant mottled eel (Anguilla marmorata), the 122 cm black carp (Mylopharyngodon piceus), the 300 cm Chinese paddlefish (Psephurus gladius), and the 100 cm Silurus meridionalis. Furthermore, there are a few exceptionally large native benthopelagic fishes found in the Yangtze, namely, the 105 cm Silver carp (Hypophthalmichthys molitrix), the 200 cmWuchang bream (Megalobrama amblycephala), the 200 cm yellowcheek (Elopichthys bambusa), the 145 cm common carp (Cyprinus carpio carpio), the 122 cmMongolian redfin (Chanodichthys mongolicus), the 102 cm predatory carp (Chanodichthys erythropterus) and the 100 cm snakehead (Channa argus argus). All fish lengths given are the expected maximum mature adult length for the given species in the Yangtze Basin. All fish species known to exceed one meter in length are listed in the foregoing.
Terrestrial ecoregions
The most downstream reach of the Yangtze, the lower basin, flows through the Changjiang Plain evergreen forests. Middle elevations of the Yangtze basin are exemplified by the Sichuan Basin evergreen broadleaf forests, whose uncultivated areas include evergreen scrub and forests. The highest elevation ecoregion of the Yangtze Basin is the uppermost reach or headwaters catchment area of the Yangtze, known as the Tibetan Plateau alpine shrub and meadows.
Changjiang Plain evergreen forests
The Changjiang Plain evergreen forests extend along floodplains and low hills that prehistorically supported extensive evergreen oak forests (Cyclobalanopsis spp., Castanopsis spp.) associated with laurels (Phoebe spp., Cinnamomum spp., Persea spp.) as well as some tropical forest taxa. Reed swamps surrounded seasonally inundated lake basins. As of 2012, some of this habitat remains. Most, however, has been converted to paddy rice agriculture which produces massive methane emissions contributing to greenhouse gas buildup in the Earth's atmosphere. Hill areas such as the Dabie Shan once supported a climax forest of conifers (Pinus massoniana, Cunninghamia lanceolata) and deciduous broadleaf taxa like birch (Betula spp.) and maple (Acer spp.). Today most of these areas are reduced to shrublands.
Sichuan Basin evergreen broadleaf forests
Original vegetation in the [[Sichuan Basin evergreen broadleaf forests] ecoregion] likely consisted of a mixture of subtropical oak (Quercus, Castanopsis), laurels (Lauraceae), and Schima (Theaceae). Diverse laurel species are also native to the basin; Machilus, Lindera, Litsea, and Cinnamomum are some of the more important genera.
Uncultivated areas in the Sichuan Basin include deforested slopes that now support a scrub of Rhododendron, Vaccinium bracteatum and Myrica nana. Places disturbed less recently may support thin stands of the pine Pinus massoniana or Japanese cypress Cryptomeria japonica, many of which have been planted. Limestone areas support scrub and forest vegetation that is florally distinctive, consisting of dyetrees (Platycarya sp.), Sichuan pepper (Zanthoxylum planispinum) and rose (Rosa spp.)
Tibetan Plateau alpine shrub and meadows
Tibetan Plateau alpine shrub and meadows is the ecoregion occupying the headwaters of the Yangtze. As is characteristic of much of the Eastern Himalayan alpine region, aspect determines the physiognomy of the vegetation. South-facing slopes, frequently snow-free and exposed to cold winds during winter, support sedge meadow vegetation while sheltered, north-facing sites support evergreen shrubs such as juniper (Juniperus) and rhododendron (R. setosum and R. cephalanthus). Valley bottoms with stable, well-drained soils support deciduous shrubs, such as cinquefoil (Potentilla fruticosa), the legume (Caragana jubata), and willows (Salix spp). As in other parts of the Tibetan Plateau, local zonation of plant communities is observed on mountain slopes. Shrubs dominate the lower slopes, especially north-facing aspects. Kobresia meadows occur in the higher, drier locations, with cushion plants (Arenaria musciformis and Androsace tapete). Expanses of bare gravel are found at the sub-nival elevations above 5000 meters.
History
One of the earliest large public works projects in China was construction of the Grand Canal, a major link of which was completed in 483 BC, connecting the Yangtze River with the Huai River; this effort was overseen by King Fuchai of Wu as one of the accomplishments of the Spring and Autumn historical period of ancient China.
One of the notable early accomplishments referencing the Yangtze Basin is the Dujiangyan Irrigation System, constructed on the Min River tributary circa 256 BC. This large scale project carried out in the Iron Age allowed a sizable acceleration of sedentary agriculture in the basin. An accomplishment of the Qin Dynasty during the Warring States Period, the project, although modified over the ages, is still in use today.
By the time of the Song Dynasty, the lower Yangtze Basin had become recognized as the most economically productive and affluent regions of the Chinese people. Significant archaeological sites are found throughout the basin, reaching back to prehistoric as well as historic times. Particularly notable sites are found in the Three Gorges area, many of which have been destroyed by inundation via the Three Gorges Dam construction. One of the most curious site types are the hanging coffins , which actually consist of hundreds of individual prehistoric wooden coffins that are perched high on the limestone cliffs, along tributary gorges of the Yangtze. Many of these remarkable coffins are readily visible and survive inundation from the Three Gorges Dam, due to their extreme height. The Shen Nong River manifests an important tributary where such coffins are extant.
References
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