Deaver Dam is located in the northwestern Bighorn Basin in a small basin between Sage and Polecat Creeks, tributaries of the Bighorn River. The foundation of Deaver Dam consists of late Cretaceous Shale overlain by Quaternary pediment deposits. The pediment deposits consist of gravel with some sand, silt and gravel mostly unlithified, but locally cemented by caliche. The gravels contain subangular to subrounded pebbles and some cobbles. Thickness of gravels at the damsite is not known, but Cody Shale is exposed nearby along the reservoir. The Cody Shale contains buff sandy shale and thinly laminated buff sandstone, moderately lithified. These beds dip about 5 degrees southwest.