Taihu

Taihu

This entry is compiled and applied by "popular science China" Science Encyclopedia

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Taihu Lake is located in the southern margin of the Yangtze River Delta, known as Zhenze and Juqu in ancient times, also known as Wuhu and Lize. It is one of the five largest freshwater lakes in China, ranking the third. Its boundary is between 30 ° 55'40 "~ 31 ° 32'58" n and 119 ° 52'32 "~ 120 ° 36'10" e. it spans Jiangsu and Zhejiang provinces, with Wuxi in the north, Huzhou in the south, Yixing in the West and Suzhou in the East.

Taihu Lake covers an area of 2427.8 square kilometers, its water area is 2338.1 square kilometers, and its coastline is 393.2 kilometers. In the West and southwest, there are hills and mountains, and in the East, there are mainly plains and water networks.

Taihu Lake is located in the subtropical zone with mild and humid climate, belonging to monsoon climate. Taihu Lake has many river ports and estuaries. There are more than 50 main rivers in and out of Taihu Lake.

Taihu Lake water system is in the trend of draining from west to East, with an average annual runoff of 7.5 billion cubic meters and water storage of 4.4 billion cubic meters. There are more than 50 islands in Taihu Lake, 18 of which are inhabited.

Formation and evolution

Cause of formation

Lagoon theory

The formation of Taihu Lake was caused by the deposition of the great river. Taihu Lake, Yangcheng Lake, Dianshan Lake and other lake groups in the Yangtze River Delta were originally large bays connected with the sea. Due to the eastward extension and retroflexion of the Yangtze River and Qiantang River, part of the sea surface was encircled in the inner sea, thus forming the inner sea. The water from the mountains on both sides was continuously injected, diluting the water in the inner sea and becoming freshwater lakes.

Structuralism

Because of the neotectonic movement of the crust in the Taihu Lake area, the Taihu Plain subsided, the rivers changed direction, and the Jingxi river system shifted eastward, from a fault basin to a catchment basin, and gradually became several small marsh lakes, and then gradually formed Taihu Lake.

According to meteorology, the continuous heavy rain in ancient times lasted for thousands of years, and the annual precipitation in some years even reached 6 billion tons. Large areas of low-lying land in Taihu Lake area accumulated a lot of water year after year, thus forming the present Taihu Lake.

According to the theory of storm flow, the abnormal climate from 4000 to 6000 years ago, the vortex of big storm flow, and the torrential rain for thousands of years caused the accumulation of water in marshes and shallow depressions in Taihu Lake area to increase, and the water area expanded day by day, forming Taihu Lake.

The theory of river siltation

During the Yumu ice age at the end of the Pleistocene from 20000 to 15000 years ago, the sea water retreated eastward and the ancient Taihu Bay disappeared. The Taihu area was connected with the East China Sea shelf and became a vast alluvial plain of the ancient Yangtze River Delta. The plain vegetation was temperate grassland or sparse forest grassland. In the middle Holocene (7500-2500 B.C.), with the warming of the climate, the sea level rose to near the present sea level. Due to the formation of the Yangtze River and Qiantang River, Taihu Plain has become a large catchment area. Jingxi, which flows into the Yangtze River to the northeast, and Tiaoxi, which flows into the sea to the East in the western mountainous area, are diverted into this dished depression due to the inundation of the lower reaches of the river, the reduction of gradient, the poor discharge of the river into the sea, and the serious deposition of river mud and sand. The backflow of the sea tide and the rise of the groundwater level in the plain lead to the swamping of the low-lying water and the formation of scattered small lakes. The area of each small lake gradually expanded and merged into a large lake. In addition, the river course out of the lake on the east bank was gradually silted up, the lake water volume increased, and the lake bank was eroded by the wind, waves and tidal currents in the lake. As a result, the area of the lake became larger and larger.

The theory of volcanic eruption

Based on several years of sampling and field geological survey in Sanshan Island, Taihu Lake, the existence of volcanoes is found and confirmed: there are eruption breccia, primary secondary eruption tuff breccia and weak fusion vitrinite tuff in Sanshan island. The north and west of Sanshan island are covered with volcanic breccia, volcanic eruptions, and silicified into chert, chalcedony, agate, etc. a volcanic erupting geological section was found in Dongpo hill, and the eruption age was at the end of Cretaceous 50 million years ago. Volcanic activity is the result of deep magmatic activity. The magmatic volcanic activity reduced the underground thermal pressure and bulk density of Taihu Lake and its surrounding areas, and produced negative pressure, which resulted in the slow Cenozoic subsidence of the area. Taihu Lake and its surrounding lakes are all subsidence lakes formed by volcanic eruption of the same origin.

Meteorite strike theory

This theory can be divided into two kinds: meteorite impact theory and comet explosion theory. According to the meteorite impact theory, 50 million years ago, a huge meteorite hit the ground from the northeast direction, causing a huge impact equivalent to the explosion of 10 million Hiroshima atomic bombs, leaving more than 2300 square kilometers of craters, namely Taihu Lake. According to the theory of the cause of the comet explosion, 4800 years ago, a comet with a diameter of 50 km, which is equivalent to a super snowball, crashed into Taihu Lake from northeast to southwest, exploded and split into many pieces in the sky, and then hit the ground. The big one hit Taihu Lake, and the rest hit around Taihu Lake, forming some small lakes. And caused a large number of iron bumps and other meteorite impact materials scattered around Taihu Lake.

The latest research results of the Department of Earth Sciences of Nanjing University: the Shihu stone stick is determined to be a meteorite, and Taihu Lake is determined to be a meteorite impact crater. The impact sputtered material refers to the terrigenous material which is ejected into the air and splashed on the impact crater and its surrounding areas during the process of impact cratering. The impact sputters of Taihu Lake are mainly distributed in the mud layer above the hard loess layer at the bottom of Shihu lake and Taihu Lake, which indicates that the impact crater structure of Taihu Lake was formed after 10000 years of Holocene.

Change of circumstances

Before the spring and Autumn period and the Warring States period, Taihu Lake area was the alluvial plain of land. In Tang Dynasty, the lake water could reach the Bank of Wujiang River.

Dongting Dongshan and Xishan used to be two big islands in the lake. Later, due to the sediment deposition between Dongshan and Mudu, the beach expanded. In the middle of Qing Dynasty, the island was connected with the sandbank, making the East Taihu Lake a great lake bay of Taihu Lake. In the past one or two hundred years, the East Taihu Lake has actually become a long, narrow, shallow and dry lake area because of the siltation on the East and northwest coasts of the lake and the reclamation of the lake beach. The changes of Taihu Lake in modern times were most prominent in the East Taihu area.

In the 1960s and 1970s, the area of Taihu Lake and its surrounding lakes decreased by 13.6% due to the cultivation and cultivation of surrounding lakes. 165 lakes disappeared or almost disappeared, with a total area of 161 square kilometers. Among them, Taihu Lake and Gehu Lake are the most prominent. Majishan island in the northwest of Taihu Lake has been connected with the land due to reclamation. The north, East and south of Gehu Lake have been greatly reduced due to the acceleration of lake reclamation.

Basin characteristics

geographical position

The boundary of Taihu Lake is between 30 ° 55'40 "~ 31 ° 32'58" north latitude and 119 ° 52'32 "~ 120 ° 36'10" east longitude, spanning Jiangsu and Zhejiang provinces, with Wuxi in the north, Huzhou in the south, Yixing in the West and Suzhou in the East.

topographic features

The West and southwest sides of Taihu Lake are hilly, and the east side is mainly plain and water network.

climate

Taihu Lake is located in the subtropical zone with mild and humid climate, belonging to the monsoon climate. In summer, it is affected by the tropical ocean air mass, and the prevailing southeast wind is mild and rainy; in winter, it is controlled by the northern high pressure air mass, and the prevailing northerly wind is cold and dry. The annual average temperature is 16.0 ℃ - 18.0 ℃, and the annual precipitation is 1100-1150 mm.

Water system composition

Overview of the lake area

The water area of Taihu Lake is 2338 ㎞ 2. The shape of the lake is crescent in the West. In the southwest, the lake bank is smooth and arc-shaped. In the northeast, the lake bank is tortuous, with many bays and mountain crests. The lake is 68 km long and 56 km wide. Due to sediment deposition and artificial reclamation, some islands are connected with Dongting mountain and Xiting mountain respectively, while the nearshore islands are connected with the lake bank to form peninsulas. Now there are 48 large and small islands, with the largest area of 75 ㎞ 2 in the west of Dongting mountain.

Upstream water system

The upstream of Taihu Lake has a catchment area of 19000 square kilometers. In ancient times, there were two major water systems, Tiaoxi and Jingxi, flowing into the lake, which has not changed much so far. The water system of Tiaoxi originates from Tianmu Mountain Area in Zhejiang Province. Jingxi water system originates from Yili mountain and the east foot of Maoshan mountain. It can be divided into Nanxi water system, Taoge lake water system and Jiangnan canal water system, and flows into Taihu Lake to the East. There is a north-south regulation channel between the water systems. The area of Huxi area in Jiangsu Province is 6081 ㎞ 2 after the water is cut off to the north of Jiangnan canal.

The main rivers of Nanxi River system are Xuhe River (formerly known as XuXi River), Nanhe River and Nanxi River. The Xuhe river connects Gucheng Lake in the West and connects Taihu Lake system with Shuiyang river system. Dongba and Xiaba were built in Ming Dynasty to block it. To the east of Xiaba, it passes dingbu to wangjiadu, Hexin Township, to Liyang county and connects with Nanhe River, with a total length of 30 km. Nanhe River, formerly known as XuXi, was also known as Yili canal during the period of the Republic of China. After the founding of the people's Republic of China, it was also known as Nanhe River, with a total length of 45.5 kilometers from the juncture of Chunli to Duji bridge at the juncture of Yili. Nanxi River flows from Duji bridge at the junction of Yi and Li, through Xushe to Xiqi, through Yixing county to Dongqi, and into Taihu Lake at Dapu. The total length of Nanxi River is 42 km, and it is successively joined by Jixi River, Taoxi River, Zhonghe River and Li River. After the founding of the people's Republic of China, four large and medium-sized reservoirs were built in the upper reaches of Daxi River, Daibu River and Jixi river. Daxi river is connected with Daxi reservoir (capacity 171 million cubic meters) and qiansong reservoir (capacity 15.96 million cubic meters). Chengshahe reservoir on Daibu River (capacity 109 million cubic meters). Chenghengshan reservoir on Jixi River (capacity 1.02)

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