Jin River (Sichuan)

Jin River is located in the western region of Sichuan Basin and the eastern region of Qinghai Tibet Plateau.^[9] The Fu River and the Nan River, the two components of the Jin River, encircle the ancient city of Chengdu.^[3] The Fu River starts from XinQuan Road (Chinese: 新泉路) and ends in Hexin Village (Chinese: 河心村) inside Chengdu city.^[3] The extension of the Fu River starts from Hexin Village and flows until it joins the Min River (Chinese: 岷江) at Pengshan district (Chinese: 彭山区) in the city of Meishan (Chinese: 眉山), further south of Chengdu. The Fu River is 19.4 km long.^[1] The Nan River occupies a small segment of the Jin River and its length is only 5.63 km.^[1] The Nan River joins the Fu River at the confluence point called He Jiang Ting (Chinese: 合江亭) inside Chengdu city.^[10] From the confluence point to Hexin Village, the Baidu Map calls this reach the Fu River.^[10] Therefore, the Nan River is a tributary of the Fu River. The total length of the Jin River is 97.3 km.^[1] The total watershed area that Jin River flows across is 2090 km².^[5]

Chengdu is located within the Min River tributaries of the Yangtze River and Tuo Jiang Basin.^[6] Both the Fu River and the Nan River are fed by the Min River tributaries.^[6] The origins of the Fu River and the Nan River can be attributed to the history of the earliest dam project, called Dujiangyan, built by a local official, Li Bing, and his son in 256 BC.^[2] The project consisted of artificial canals, conduits, and dams that split the Min River into four inner irrigation rivers and one outer river.^[4] The four inner rivers flow in artificial conduits whereas the outer river, called the Jin Ma River (Chinese: 金马河), flows in a natural channel at the southernmost position.^[10] The four inner irrigation rivers, from north to south, are the Pu Yang River (Chinese: 蒲阳河), the Bai Tiao River (Chinese: 柏条河), the Zou Ma River (Chinese: 走马河), and the Jiang An River (Chinese: 江安河).^[10] The Zou Ma River (Chinese: 走马河) and Bai Tiao River (Chinese: 柏条河) flow for several dozens of kilometers and eventually join together at Xiang Cao Hu Wetland Park (Chinese: 香草湖湿地公园).^[10] Once it reaches the Chengdu's Shidiyan Dam (Chinese: 石堤堰), it diverges into two rivers, the Pihe River and the southward flowing Fu River.^[10]

The Zou Ma River flows from the northwest in a southeast direction.^[10] Not far after Dujiangyan, it spits into the Qingshui River (Chinese: 清水河) and the Zou Ma River.^[10] Further south than the Fu River, the Qingshui River eventually flows into Chengdu.^[10] After the Qingshui River enters Chengdu, it splits into the Mo Di River (Chinese: 摸底河) and the Qingshui River for a short distance until they merge and form the Nan River.^[10] The Nan River is therefore from the Qingshui River. Another inner river derived from Dujiangyan, called the Jiang An River (Chinese: 江安河), flows south of the Zou Ma River and the Qingshui River.^[10] The Jiang An River enters Chengdu further south than the Nan River.^[10] After that, it joins the Fu River near Nan Hu Wetland Park (Chinese: 南湖湿地公园).^[10] Even though the Baidu Map indicates that the river joining the Jiang An River is labelled as Fu River, one Chengdu history book mentions that this part is not considered as the Fu River.^[1] This part belongs to the extension that is considered as the Jin River and it flows further down south to join the Min River.^[1] If we count the length of the Jin River starting from the start point of the Fu River at the Shidiyan Dam until the confluence with the Min River, then the total length of Jin River is 116.9 km.^[1] The Jin Ma River, the only river that flows in a natural channel after Dujiangyan, runs for 70.6 km and eventually joins the Min River shortly after the Jin River ends.^[11]

The Jin River, composed of the Fu River, the Nan River, and the river channel from Hexin Village of Chengdu to the Min River, have been renamed several times over the course of different dynasties. The Fu River was first known as the Pi River, Shiqiao River, or Qingyuan River when the city was called the Chengdu Mansion.^[3] After the Ming Dynasty (1368-1644), the Nan River was known as Wenjian, Dajian, Shengjiang River, Shengqiao Water, Qingjiang, Qingshui River, Fenshui, Fenjian, Guanjin River, and Jinshui.^[3]

In history, the Fu and the Nan River components of the Jin River were understood unofficially by the people as the "Funan River" for short.^[3] This was deemed too informal and was rectified on May 10, 2005, by the Sichuan Provincial People's Government.^[3] They are now officially named as their distinct channels.

The current Jin River has eutrophication issues caused by different types of urban wastewater such as sewage discharge, road surface runoff, and effluent.^[12] Eutrophication is caused by excessive amounts of total dissolved phosphorus (TDP) and total dissolved nitrogen (TDN) in both the Fu River and the Nan River.^[7] Urbanization and wastewater in the City of Chengdu raises the TDP concentration in the Fu River.^[7] Compared to the Fu River, the Nan River has less TDP due to less wastewater input.^[7] Even though the concentration of TDN is considered low in both the Fu River and the Nan River, it is higher than the type 3 international standard requirements for water quality.^[7]

Different types of algae exist in the waterbody of the Jin River. The growth rate of Chlorophyll a is limited by nitrogen but not by phosphorus according to the determined N:P ratio in the Jin River's water column.^[7] Even with high phosphorus inputs, there is no such phenomenon as algal or cyanobacteria blooms because of the small ratio between N and P.^[7] The low ratio of N:P found in the Jin River is consistent with other studies where the phytoplankton community was N-limited. It is also caused by low N:P ratio of the water in the area.^[13] The N-limiting condition in both the Fu River and the Nan River can pose a potential biological hazard. If any wastewater or effluent that contains a large amount of TDN enter the river, it is likely to trigger an algal bloom.^[7] Studies shows that the surface micro-layer, which is the topmost layer of the river that exists in between atmosphere and water body, has larger amounts of P and N than the subsurface layer of the Jin River water body.^[14] Sediment studies in the Jin River show that the accumulated N and P in benthic sediments far exceed the P and N concentration in both the surface micro-layer and the sub-surface layer.^[7]