Houaphanh Province
Chu River (name in Vietnam, ) also known as the Nam Sam River or Nam Xam River (name in Laos) is the largest tributary of Ma River. 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 joins Ma River on the left in Thiệu Hóa.