Yangtze River

Japanese: 長江 - ちょうこう

China's longest river, originating in the northeastern Tibetan Plateau and flowing into the East China Sea. Its abbreviation is Jiang. In Japan, the name Yangtze River is merely the local name for the area downstream near Yangzhou, Jiangsu Province, but in Japan and other countries, the entire Yangtze River has traditionally been called the Yangtze River. It has a total length of 6,300 km and a drainage area of 1,808,500 square km. The drop from the source to the river mouth is 5,100 meters, and the annual flow rate is about 1 trillion tons, accounting for about 40% of China's total hydroelectric potential.

[Kono Michihiro]

Upstream

The source of the river is the northern slope of Mt. Gratangtong (6,621 meters), the main peak of the Tanggula Mountains on the Tibetan Plateau, where glacial meltwater flows out. The source is called the Toto River, which flows east at the southern tip of Qinghai Province, and becomes the Tongtian River together with the Chumar River and other tributaries, and after passing Yushu it becomes the Jinsha River. The Tongtian River flows through the Tibetan Plateau at an altitude of about 4,500 meters, with a gentle slope, a relatively wide river width, and a slow flow. The coast is a natural pasture with lakes and grasslands. The Jinsha River was formerly called Lishui, and refers to the 1,918 km section from Yushu to Yibin in Sichuan Province. From Yushu it flows south, forming a deep gorge along the border between Sichuan Province and the Tibet Autonomous Region. In Yunnan, the river meanders through steep gorges, with an elevation difference of over 3,000 meters from the summit to the valley floor, and in the Tiger Leaping Gorge, the river is less than 30 meters wide. This section is where the river runs through the Hengduan Mountains. It then enters the northern Yunnan-Guizhou Plateau, merges with major tributaries such as the Yalong River, and flows northeast, entering the Sichuan Basin and reaching Yibin, where it is called the Yangtze River.

In the Sichuan Basin, the Yangtze River flows eastward, joining the Min River, Jialing River, and Wujiang River. This section is called the Sen River. It passes through Chongqing, Wanxian, and crosses the Wushan Mountains before forming a deep gorge again. This is the Three Gorges. After the Three Gorges, it reaches Yichang in Hubei Province, where the Gezhouba Dam was completed, and the Three Gorges Dam was also built upstream at Sandouping. The upper reaches of the Yangtze River stretch from its source to Yichang, a length of 4,500 kilometers, with the Jinsha River in particular forming a large canyon, with large water volume and hydroelectric potential. With the completion of the Three Gorges Dam, the Three Gorges was transformed into a large reservoir lake stretching all the way to the area around Wanxian.

[Kono Michihiro]

Midstream

The middle reaches of the Yangtze River run for about 1,000 km from Yichang to Hukou in Jiangxi Province (the northern end of Poyang Lake), and flow through the Huguang Plain (also called the Huguang Lowlands or the Lianghu Plain). The river is wide, has a gentle gradient, and flows at only 1 meter per second. The river meanders violently, and the section from Zhijiang in Hubei Province to Chenglingji in Yueyang, Hunan Province (historically called the Jingjiang River) is described as a "nine-bend ileum." Although the river is 80 km long, the length of the river is 420 km. For this reason, this area has been known as a place prone to flooding since ancient times. After liberation, large levees were built on both banks, and a large reservoir called the Jingjiang River Flood Diversion Area was built near Gong'an, making it possible to store water during flood seasons. In addition, part of the meandering river channel has been straightened to prevent flooding.

In the middle reaches, many tributaries flow into the river from the north and south, including the Han River, the Xiang River, the Yuan River, the Zishui River, the Li River, the Gan River, and the Xinjiang River, and there are numerous lakes and marshes along the river course. In particular, tributaries from the south flow into Dongting Lake in Hunan Province and Poyang Lake in Jiangxi Province, before discharging back into the Yangtze River. The catchment area of these tributaries in the middle reaches accounts for 40% of the total basin area. In addition to the large number of tributaries, the Huguang Plain and the Poyang Plain around Poyang Lake are both low-lying basins, forming a water town with a network of waterways connecting countless lakes and marshes, making this an important grain-producing region of China. However, from spring to summer, the frontal zone stretching from east to west moves north from the Nanling area, and the rainy season begins in June in this region. Therefore, when both the main stream and the tributaries rise at the same time due to rainfall, they cause major flooding, and the lower reaches of the Han River, along with the Jingjiang River, have suffered frequent flood damage since ancient times. For this reason, a large reservoir was built near Tokadai and the Danjiangkou Dam was constructed upstream to regulate the water volume.

[Kono Michihiro]

downstream

The lower reaches of the Yangtze River stretch for over 800 km from its mouth to its river mouth, passing through Nanjing, Zhenjiang, and Nantong before flowing out into the East China Sea in the Shanghai metropolitan area. In this section, the river is wide and deep, with only short tributaries. The main tributaries are the Qingyi River, Chaohu Lake, and the Taihu Lake system, and unlike the midstream, they have little impact on the mainstream. Downstream from Zhenjiang, the river heads southeast and enters a vast delta region. The terrain becomes even flatter, with a dense network of waterways and many lakes and marshes, and the area has long been known as the "Land of Mountains." In particular, the area from the south bank to the Taihu Lake area is a typical water town known as the Jiangnan Delta, and is a grain-producing region. In addition to rice cultivation, cotton is also cultivated extensively on the Jianghuai Plain on the north bank. The river is 18 kilometers wide near Nantong, but at its mouth it expands to 80-90 kilometers, making it difficult to tell whether it is a river or sea. Near the mouth of the river is Chongming Island, an island that has gradually expanded since the Tang and Song dynasties.

[Kono Michihiro]

Water Transportation

Because the Yangtze is such a long river, it is heavily used for water transportation, and in the mainstream, 10,000-ton steamships can travel up to Nanjing, 3,000-ton ships can travel up to Wuhan, 1,000-ton ships can travel up to Chongqing, and 500-ton ships can travel up to Yibin. During periods of high water, it is even possible for 10,000-ton ships to travel up to Wuhan. Gezhouba Dam has locks, and Danjiangkou Dam has an elevator, allowing ships to travel upstream. Locks were also installed at the Three Gorges Dam, which, combined with the rise in the water level of the Three Gorges due to the construction of the dam, has made navigation easier. While the mainstream is the main route of east-west transportation, the tributaries are used as waterways connecting the north and south. In particular, the Xiang River was once connected to the Pearl River system by the Lingqu Canal built upstream (the present-day Lingqu Canal is called the Xing'an Canal and is used primarily as an irrigation canal).

The Grand Canal also crosses the Yangtze River from north to south, and ships can travel from Zhenjiang to Hangzhou in the south, and 3,000-ton ships can travel from Yangzhou to Huaiyin in the north. There are plans to extend the route to Dongping in Shandong Province in the near future. There are also plans to transport water from the Yangtze River to Beijing and Tianjin, known as the "South-to-North Water Diversion."

[Kono Michihiro]

"The Yangtze River - A Photo Collection of Tokumitsu Iwago" by Tokumitsu Iwago (1981, Sanseido)" ▽ "The Yangtze River" by Hiroshi Koide (1987, Tsukiji Shokan) ▽ "The Yangtze River" by Shunshin Chin and Tsuneo Masui (Chuko Shinsho)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

チベット高原北東部に源を発し東シナ海に注ぐ、中国最長の河川。略称は江(チヤン)。日本でいう揚子江(ようすこう)という名称は下流部の江蘇(こうそ／チヤンスー)省揚州(ようしゅう／ヤンチョウ)付近でよばれる地方名にすぎないが、日本はじめ外国では長江全域を揚子江と呼び習わしてきた。全長6300キロメートル、流域面積180万8500平方キロメートル。水源から河口までの落差は5100メートル、年流水量は約1兆トンで、中国全土の包蔵水力の約40％を占める。

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上流

水源はチベット高原上にあるタングラ山脈の主峰グラタントン山（6621メートル）の北斜面で、氷河の融水が源流となって流出する。源流はトト河とよばれ、青海省南端を東に流れ、チュマル河ほかの支流をあわせ通天(つうてん)河となり、玉樹(ぎょくじゅ)を過ぎると金沙(きんさ)江という。通天河は標高4500メートル前後のチベット高原を流れ、傾斜は緩やかで川幅も比較的広く流速は緩慢である。沿岸は湖沼と草原の広がる天然牧場である。金沙江は古くは麗水(れいすい)といい、玉樹から四川(しせん／スーチョワン)省宜賓(ぎひん／イーピン)までの1918キロメートルの区間をさす。玉樹からは四川省とチベット自治区の境界を深い峡谷をつくって南下する。雲南省に入ると、さらに険しい峡谷の中を蛇行し、山頂から谷底までの標高差は3000メートルを超え、とくに虎跳(こちょう)峡では川幅は30メートル以下となる。この区間が横断山脈を貫流する部分にあたる。ついで雲貴(うんき)高原北部に入り、雅礱(がろう)江などの大支流と合流して北東に流れ、四川盆地に入って宜賓に達し長江とよばれるようになる。

　四川盆地では、岷(びん)江、嘉陵(かりょう)江、烏(う)江などをあわせながら東流する。この区間を川(せん)江とよぶ。重慶(じゅうけい／チョンチン)を経て万県を通り巫山(ふざん)山脈を横断してふたたび深い峡谷をつくる。これが三峡である。三峡を過ぎると湖北省の宜昌(ぎしょう／イーチャン)に達するが、ここには葛洲壩(かっしゅうは)ダムが完成し、その上流三斗坪(さんとうへい)に三峡ダムも建設された。源流から宜昌までが長江の上流部で、長さ4500キロメートル、とくに金沙江では大峡谷をつくり、水量と包蔵水力が大きい。三峡は三峡ダムの完成により、万県付近まで連なる大ダム湖に変身した。

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中流

長江中流部は宜昌から江西省の湖口(ここう)（鄱陽湖(はようこ)北端）まで約1000キロメートルの区間で、湖広平野（湖広低地、両湖平原ともいう）を流れる。川幅は広く勾配(こうばい)は緩やかで、流速は毎秒1メートルにすぎない。河道は激しく蛇行し、なかでも湖北省枝江(しこう)から湖南省岳陽の城陵磯(じょうりょうき)までの区間（古称を荊(けい)江という）は「九曲回腸」と形容され、直線距離は80キロメートルであるが河道の長さは420キロメートルに達する。そのため、この地域は古くから水害の多い地として知られた。解放後は両岸に大堤防を築いたほか、公安付近には荊江分洪区とよぶ大遊水池を設け、増水期の水を収容することが可能となった。また蛇行する河道の一部を直線に変え、水害の防止を図っている。

　中流部では漢水をはじめ湘江(しょうこう)、沅江(げんこう)、資水(しすい)、澧水(れいすい)、贛江(かんこう)、信江など多くの支流が南北から流入し、河道沿いにきわめて多くの湖沼が存在する。なかでも南からの支流は、湖南省では洞庭(どうてい)湖、江西省では鄱陽湖に流入したのち、ふたたび長江に排出されている。これら中流部諸支流の集水面積は全流域面積の40％を占める。このように多くの支流が集まるうえに湖広平野も鄱陽湖周辺の鄱陽平原も低平な盆地なので、無数の湖沼をつなぐ水路が網の目のように発達する水郷が形成され、中国の重要な穀倉地帯となっている。しかし、春から夏にかけては東西に延びる前線帯が南嶺(なんれい)付近から北上し、この地方では6月に梅雨となる。したがって本流と支流がともに降雨によって同時に増水すると大氾濫(はんらん)が生じ、とくに漢水下流部は荊江とともに古くから水害の被害が多かった。そのため杜家台(とかだい)付近に大遊水池を設けたり、上流に丹江口(たんこうこう)ダムを建設して水量調節を行っている。

［河野通博］

下流

長江下流部は湖口から河口までの800キロメートル余りで、南京(ナンキン)、鎮江(ちんこう／チェンチヤン)、南通を経て上海(シャンハイ)市域で東シナ海へ流出する。この区間は川幅が広く、水深も大きく、また支流も短小なものばかりである。おもな支流には青弋(せいよく)江と巣(そう)湖および太湖水系があげられるのみで、中流部と異なり本流に与える影響は小さい。鎮江より下流では、河道は南東に向かい広大なデルタ地帯に入る。地勢がさらに平坦(へいたん)となり、水路網が稠密(ちゅうみつ)で湖沼も多く、古くから「沢国(たくこく)」とよばれてきた。とくに南岸から太湖周辺にかけての地域は、江南デルタの名で知られる典型的な水郷で穀倉地帯となっている。また北岸の江淮(こうわい)平原は米作のほかに綿花の栽培が盛んである。南通付近では川幅は18キロメートルに達するが、河口部では80～90キロメートルに広がり、海か川かわからないほどになる。河口近くに唐・宋(そう)代からしだいに拡大してきた中州の崇明(すうめい)島がある。

［河野通博］

水運

長江はこのように長大な河川なので、水運に盛んに利用され、本流では1万トン級の汽船が南京まで、3000トン級が武漢まで、1000トン級が重慶まで、500トン級が宜賓まで遡航(そこう)できる。増水期には武漢まで1万トン級の船が遡航することも可能である。葛洲壩ダムは閘門(こうもん)、丹江口ダムには昇降用エレベーターがあって、船を上流に遡航(そこう)させている。三峡ダムにも閘門が設けられ、ダム化による三峡の水位上昇と相まって航行が容易になった。本流が東西方向の交通の幹線であるのに対して、支流は南北を結ぶ水路として利用される。とくに湘江は上流に設けられた霊渠(れいきょ)によって珠江(しゅこう／チューチヤン)水系とつながっていたこともある（現在の霊渠は興安運河とよび、おもに灌漑(かんがい)用水路として利用されている）。

　また大運河は長江を横切って南北に通じており、南方へは鎮江から杭州まで船が通じ、揚州から北方の淮陰(わいいん)まで3000トン級の船が航行でき、近い将来には山東省の東平まで航路を延長する計画がある。また、長江の水を北京(ペキン)や天津(てんしん／ティエンチン)まで送る「南水北調」と称する計画もたてられている。

［河野通博］

『岩合徳光著『長江――岩合徳光写真集』（1981・三省堂）』▽『小出博著『長江』（1987・築地書館）』▽『陳舜臣・増井経夫著『揚子江』（中公新書）』

[参照項目] | 漢水 | 金沙江 | 沅江 | 三峡ダム | 湘江 | 南水北調

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