The channel was located near the left side of the valley at the time of construction. The left abutment and spillway are founded on weathered siltstone bedrock and alluvial deposits. The bedrock at the site is described as a tan to light brown, sandy siltstone containing numerous limy concretions, volcanic ash and bone fossils. Lenses of sandstone and conglomerate also occur. The beds are primarily cemented with calcium carbonate varying from weakly to non-cemented.
Bedding is nearly horizontal and usually massive.
The Niobrara River Fault was mapped along the length of the river valley at the site of Box Butte Dam. The fault zone forms a graben along which the river traverses. The Nebraska Geological Survey indicated the last movement along the Niobrara River Fault occurred approximately 16 million years ago. Drilling for the CAS in 1988 along the left abutment and spillway encountered soil materials ranging from 1.1 to 15.0 ft. in thickness. The soil appears to be derived from the underlying siltstone of the Monroe Creek Formation.