river rising in the Inner Mongolia Autonomous Region of China, on the western slope of the Greater Khingan Range (Da Hinggan Range), where it is known as the Hailar River. Its length is 1,007 miles (1,620 km), of which about 600 miles (965 km) form the boundary between China and Russia. Near Luoguhe, the Argun merges with the Shilka (Shilka River) River to form the Amur River. The Argun flows for the most part through a wide valley; it has a drainage basin of approximately 63,000 square miles (163,000 square km). In years of abundant rainfall, the Argun overflows into the basin of Hulun Lake (Hulun, Lake) in Inner Mongolia. Navigation on the river is irregular.