It originates at the eastern slope of the
Khentii Mountains. For 298 km it flows within Mongolia, before entering the
Khentei-Daur Highlands region of the
Russian Far East flowing along the
Stanovik Range. In its lower reaches it flows between the
Mogoytuy (Могойтуйский хребет) and the
Borshchovochny (Борщовочный хребет) mountain ranges. It joins with the
Ingoda to form 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 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 ==