It originates from Houa peak at 2062 m near Xam Neua town, Houaphanh Province in eastern Laos and travels east to meander through Nghệ An and Thanh Hóa provinces of Vietnam.
It is reputed there is a Communist re-education camp located in Xam Neua, and that it was the Pathet Lao capital during the Laotian Civil War Battle of Lima Site 85 (LS-85) 11 March 1968.