[2] Its river valley forms the eastern limit of the Khentei-Daur Highlands.

[3] Its main tributaries are the Khurkhyn Gol, Borzya and Unda from the right, and the Agutsa, Kyra and Aga from the left.

The upper Onon is one of the areas that are claimed to be the place where Genghis Khan was born and grew up.

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.

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."