Tuul Riveri s a river in central and northern Mongolia considered sacred by the Mongols. It is 704 km long and drains an area 49,840 square km. The river is called the "Duluo river" in the Suishu, a Chinese historical work completed in 636 AD. The Secret History of the Mongols (1240 AD) frequently mentions a "Black Forest of the Tuul River" where the palace of Wang Khan was located (Chinggis Khan visited it frequently and later made it his own).
The Tuul river starts in the Khentii Mountains near Terelj National Park and flows through the capital, Ulaanbaatar, before joining the Orkhon.