What is the meaning of "Wujin" in "the wall separated by three qins and the water separated by five rivers"?

The Yangtze River from Yan Kui to Qianwei is a section of the five major ferries. Changqu's "Huayang Guozhi Shuzhi": "There are five Tianjin in its great river from the weir down to Qianwei. At first, Bai Huajin, second, Wan, third, Jiang Shoujing, fourth, investing in gold ... fifth, Jiangnan Tianjin. " Wujin is located in Shu, because it is generally used to refer to Shu. Emperor Liang Yuan of the Southern Dynasties wrote "A Monument to Changsha Temple in Jingzhou": "Looking at Wujin, three days away from Qinglian Cave, there is a bright moon." Tang Wang Bo's poem "Send to Ren Shuchuan": "The wall of the Three Qin Dynasties is separated by the fog of the Five Rivers."