evidence: Yalu River, Chinese Yalu Jiang or Ya-lü Chiang Korean Amnok -kang, River, eastern Asia, between northeastern China and North Korea. Some 491 mi (790 km) long, it rises on the northern border of North Korea, then flows to Korea Bay. It is an important source of hydroelectric power and is navigable by smaller vessels for most of its course. It became a political boundary in the 14th century. During the Korean War, as UN forces battled toward it in 1950, Chinese troops crossed it, in effect marking their entry into the war.