Argun (är´gōōn´), Mandarin Ergun, river, 950 mi (1,529 km) long, rising in the Da Hinggan Mts., Heilongjiang prov., NE China, as the Hailar River and flowing W to the Russian border, then NE along the Russian-China frontier, where it joins the Shilka River to form the Amur. The Chinese Eastern RR, a branch of the Trans-Siberian RR, follows the upper Argun valley. Hailar, China, is the largest city on the river. Corn, grains, and sugar beets are grown in the fertile Argun valley. Silver, lead, and coal are found along the river banks.