Cheyenne

River in northeastern Wyoming and western South Dakota, USA, flowing to Lake Oahe, South Dakota; length 848 km/527 mi. It flows east out of Niobrara County, Wyoming, then northeast through South Dakota, along the south of the Black Hills, into Meade County, where the Belle Fourche River joins it from the west. Continuing east-northeast, it forms the southern boundary of the 11,047-sq km/4,265-sq mi Cheyenne River Indian Reservation before reaching the western arm of Lake Oahe.

The Cheyenne River Indian Reservation (population (1990 est) 7,800, 66% Indian), like the adjacent Standing Rock reservation just to the north, is home to various Sioux groups; Eagle Butte in Dewey and Ziebach Counties (city, population (1990 est) 489) is its headquarters.