Xi (also Si and Hsi)River, the principal river of southern China. It is formed by the junction of the Hongshui and Yu rivers, about 80 miles (130 km) west of Wuzhou. From the junction the Xi flows eastward and empties into the Zhu River and the South China Sea through numerous branches. The total length from the source of the Hongshui to the mouth is about 1,250 miles (2,000 km). Tributaries of the Xi include the Gui and Bei rivers. Both Macao, a Portuguese possession, and Guangzhou, southern China's largest city, are situated near main branches of the Xi. The river is navigable to Wuzhou, about 150 miles (240 km) inland.