For 10 points, name this river that forms the boundary between China and North Korea.
Sup'ung Lake was created along this river at a dam near Sinuiji, and it arises from T'ien Lake in the Ch'ang-pai Mountains. Its main tributaries include the Herchun, Changjin, and Tokro rivers, and it flows south to Hyesan before sweeping 130 kilometers northwest to  Linjiang in the Kirin province. It eventually empties near the city of Dangdong into an arm of the Yellow Sea named for one of the countries it borders. For 10 points, name this river that forms the boundary between China and North Korea.
Sup'ung Lake was created along this river at a dam near Sinuiji, and it arises from T'ien Lake in the Ch'ang-pai Mountains.
{Yalu} River [or {Amnok} River]