Nam Sam River

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.
The river is 325 km long (165 km in Laos and 160 km in Vietnam) with a drainage area of about 7,580 km² (4,570 km² in Laos and 3,010 km² in Vietnam).