They are drained and in large part enclosed by the North (or Belle Fourche) and South forks of the Cheyenne river (at whose junction a fur-trading post was established about 1830); and are surrounded by semi-arid, alkaline plains lying 3000 to 3500 ft.
Of these the Cheyenne is the most important, being formed by two branches, the Belle Fourche and the South Fork, which, after almost completely encircling the Black Hills, unite at a point nearly 350 m.
Under the Reclamation Act passed by Congress in 1902 the irrigation of 10o,000 acres in the Belle Fourche Valley adjacent to the Black Hills region was provided for.