 Discharging from the Angosture Reservoir, the Cheyenne River flows northeasterly through the upper part of the county, departing the county through its north boundary line into Custer County.Fall River County has a total area of , of which is land and (0.5%) is water.As of the 2000 census, there were 7,453 people, 3,127 households, and 1,976 families in the county.