Although the Belle Fourche River is mainly in South Dakota, a portion of it lies within Wyoming in Crook County. Keyhole Reservoir is situated on this river as well, NE of Moorcroft and right next to Pine Haven. The river is the lowest point in the state at 3,100 ft elevation.
Keyhole Reservoir also supplies water storage for the Belle Fourche Project in South Dakota. The Belle Fourche River Compact, a contract negotiating the appropriation of the river's waters, was signed between Wyoming and South Dakota on March 4, 1943. Wyoming received ten percent of the unappropriated flow, and South Dakota received the other ninety percent.
