Bois de Sioux River (BWAH da soo), river, 30 mi/48 km, in N. and S.Dak. and Minn.; rises in L. Traverse, NE S.Dak., flows N and joins Otter Tail R. at Wahpeton, N.Dak. (opposite Breckenridge, Minn.), to form Red R. of the North. Forms S.Dak-Minn. and N.Dak.-Minn. state lines.