The Onon—Shilka—Amur water system is one of the world's ten longest rivers, measuring 1,032 km + 560 km + 2,874 km. The upper Onon is one of the areas that are claimed to be the place where Genghis Khan was born and grew up.
The Onon (Mongolian: Онон гол, Onon gol; Russian: Оно́н) is a river in Mongolia and Russia. It is 1,032 kilometres (641 mi) long, and has a drainage basin of 96,200 square kilometres (37,100 sq mi). It originates at the eastern slope of the Khentii Mountains. For 298 km it flows within Mongolia, before entering the Dauria region of the Russian Far East. It joins with the Ingoda to produce the Shilka. Its river valley forms the eastern limit of the Khentei-Daur Highlands. Its main tributaries are the Khurkhyn Gol, Borzya and Unda from the right, and the Agutsa, Kyra and Aga from the left.
The Secret History of the Mongols begins thus: "There came into the world a "Borte Chono" (blue-gray wolf) whose destiny was Heaven's will. His wife was a "Gua Maral" (beautiful fallow doe). They traveled across the inland sea and when they were camped near the source of the Onon River in sight of Burkhan Khaldun, their first son was born, named Bat Tsagaan."