northward-flowing tributary of the Amur River that for a considerable distance forms the boundary between China (Heilungkiang province) and Russia (Siberia). The Ussuri is formed by the confluence of the Ulakhe and Arsenyevka rivers, both of which rise on the southwestern slopes of the Sikhote-Alin mountains. Its length from the source of the Ulakhe is 565 miles (909 km), and its basin is 72,200 square miles (187,000 square km) in area. The Ussuri is navigable from its confluence with the Great (Bolshaya) Ussurka. It joins the Amur River near Khabarovsk. In the 1960s large sections of the river border came under dispute between China and what was then the Soviet Union (Union of Soviet Socialist Republics), and a sharp armed clash erupted between the two countries in 1969. In 1977 they reached limited agreement on rules of navigation on the Ussuri.