The Amur River or Heilong River is a perennial river in Northeast Asia, forming the natural border between the Russian Far East and Northeast China. The Amur proper is 2,824 km (1,755 mi) long, and has a drainage basin of 1,855,000 km2 (716,000 mi2). If including its main stem tributary, the Argun, the Amur is 4,444 km (2,761 mi) long, making it the world's tenth longest river.