Chapter 26 Section 4 River Network and Polder Transformation in Tang and Song Dynasties

Since agriculture in the Taihu Lake Basin also needs artificial irrigation, especially in the mountainous and hilly areas that account for 22% of the watershed area, the construction of water storage projects such as ponds during the Sui, Tang and Song Dynasties was still an important aspect of water conservancy construction here.Taking the Tang Dynasty as an example, according to records such as "New Book of Tang Geography" and "Yuanhe County Chronicles", there were as many as a dozen large-scale water storage facilities that were expanded and new.Among them, the most important ones are Jiangyan Pond and Qiantang Lake.Jiangyan Pond was expanded from the Chishan Lake built by Wu State during the Three Kingdoms period. In the Tang Dynasty, it could irrigate 10,000 hectares of farmland. It was the water storage project with the most irrigated fields around Taihu Lake.Qiantang Lake is the Xizi Lake with "the best mountains and rivers in the world". It was built by Bai Juyi during the reign of Tang Muzong (821-824 A.D.) and used the Jiangnan River as the main irrigation canal to irrigate Qiantang (now Hangzhou City) and Yanguan (now Haining County). .

The construction of Tang, Du, Jing and Pu in the Taihu Lake Basin with the main purpose of flood drainage was even more valued by people in the Sui, Tang and Song dynasties.Although there have been many natural rivers in the Taihu Lake Basin since ancient times, such as the one that flows from Taihu Lake to the East China Sea, and the Lou River (Liu River) that flows from Taihu Lake to the Yangtze River; The Grand Canal has also become an important artery for flood drainage.However, since most (62%) of the Taihu Basin are plains and depressions, and typhoons and storms come very suddenly, the floods cannot be discharged in time, and the damage caused by floods is much greater than that of droughts.This necessitates the construction of more flood drainage-based projects.What's more, these projects also benefit from irrigation and shipping.

During the Sui, Tang and Song Dynasties, a large number of waterways were built in the Taihu Lake Basin, which were of great significance for flood drainage.In addition to the Jiangnan Grand Canal in the Sui Dynasty, the large ones include Yuanhetang, Mengdu, Taibodu, Hantang in the Tang Dynasty, and Zhihetang in the Song Dynasty.Yuanhetang, also known as Changshutang, was excavated in the eighth year of Tang Yuanhe (813 A.D.), starting from today's Suzhou City in the south and reaching Changshu County in the north, with a length of 90 miles.It is connected to the Baimao [Mao Mao] River to connect to the Yangtze River.Mengdu, also known as Menghe, is located in the northwest of today's Changzhou City, connecting the Jiangnan River and the Yangtze River, with a length of 40 miles.Taibodu is located in the southeast of today's Wuxi City, betting on Yangcheng Lake, about 80 miles long.Both Mengdu and Taibodu were carved out of the old roads under the auspices of the governor Meng Jian around Tang Yuanhe eight years ago.In the seventh year of Tang Dahe (833 A.D.), the ancient Hantang was also restored.Hantang starts from today's Jiaxing City, Zhejiang Province in the west and ends in Hangzhou Bay in the east.Zhihe Pond, also known as Kunshan Pond, was dug in the second year of Song Zhihe (AD 1055). It started from today's Suzhou City, Jiangsu Province, and went down to Kunshan to flow into the Yangtze River. It was mainly transformed from the natural river Loujiang.During the Song and Yuan dynasties, the Loujiang River was the second largest river in the lower reaches of Taihu Lake, second only to the Wusong River.

In addition to the above-mentioned main waterways, during the Sui, Tang and Song Dynasties, a large number of Jingpu were built based on these main waterways, making the waterways in the Taihu Lake Basin dense like a cobweb.According to the records of Shan E, a famous water conservancy expert in the Song Dynasty, Yixing and other places located in the upper reaches of Taihu Lake have "the benefits of a hundred duns", of which Yixing has 74 duns and Wujin has 26 duns.There are more ponds in the lower reaches of Taihu Lake. Another famous water conservancy expert in the Song Dynasty, Jia Dan (jiadan Jiadan), said that there were as many as 265 ponds in Suzhou.During the Tang and Song Dynasties, the river network in this area has developed to the point of "one vertical pond for five miles, one horizontal pond for seven miles".

Simultaneously with the water conservancy construction of the river network, in terms of farmland construction, it is gradually moving towards polderization, and people have built the square land between the vertical and horizontal ponds into polders.Due to the low terrain here, many places are: when the water rises, it becomes a swamp; when the water recedes, it becomes farmland.Polderization is to transform these lands into fertile fields that can basically guarantee harvests from drought and floods.The most important project of polder construction is to build solid dikes around the polders near the pond.The height, width, and narrowness of the embankment depend on the size of the polder, the terrain and the surrounding water conditions. Generally, it is five feet to two feet high and several feet wide.There are roads on the embankment to facilitate traffic; willows are planted outside the embankment to protect the foot of the embankment.There are gates around the polder to open the gates when there is a drought to irrigate the fields with water from ponds outside the embankment, and to close the gates when floods occur to prevent the intrusion of external water.Vertical and horizontal drainage channels are drilled in the polder, which is shaped like a chessboard; when it is flooded, the field water will be drained into the canal, and when it is dry, the water from the (huhu) canal will be used to irrigate the field.The lowest part of the polder is transformed into a pond to collect water.The length of a polder ranges from a few miles to tens of miles.Although polders cannot resist severe droughts and floods, they are capable of self-defense against general floods and droughts, and their economic benefits are far higher than ordinary farmland.It is a great creation of the people of the water town.In the Southern Song Dynasty, the distribution of polders in the Taihu Lake Basin was already very wide. In the territory of Pingjiang, that is, counties and cities such as Suzhou, Wujiang, Changshu, and Jiading, there were more than 1,500 polders (Figure 7).

The construction of river network and polder has promoted the development of agricultural production in the Taihu Lake Basin. The folk proverb "Suhu is cooked, the world is full" reflects that since the Southern Song Dynasty, the Taihu Lake Basin, represented by Suzhou and Huzhou, has become the main food supply area for "the world" in the Southern Song Dynasty.

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