Yu River, river in southern China. A southern tributary of the Xi River system, it rises in two branches in southeastern Yunnan province and flows about 400 miles (650 km) generally east in Guangxi province to unite at Guiping with the Hongshui River to form the Xun River (which in Guangdong province is known as the Xi). It receives the Zuo River near Nanning. The Yu River is important in southern China’s transportation system.