The Bighorn River-Tullock Creek drainage divide area is located in Montana, USA. Although detailed topographic maps of the Bighorn River-Tullock Creek drainage divide area have been available for more than fifty years detailed map evidence has not previously been used to interpret the region’s geomorphic history. The interpretation provided here is based entirely on topographic map evidence. The Bighorn River-Tullock Creek drainage divide area is interpreted to have been eroded during immense southeast-oriented flood events, the first of which flowed on a topographic surface at least as high as the highest points in the present-day drainage divide area. Flood erosion across the drainage divide ended when headward erosion of the deep Bighorn River valley captured all southeast-oriented flood flow.