The Onon—Shilka—Amur water system is one of the world's ten longest rivers, measuring 1,032 km + 560 km + 2,874 km.
In history
The upper Onon is one of the areas that are claimed to be the place where Genghis Khan was born and grew up. The seat of Genghis Khan was on the bank of river Onon.
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."