These lands are located along the south side of the Republican River from the dam to a point about 8 miles east of McCook. The reservoir behind the dam is called Swanson Lake. The Republican River heads on the high plains of Colorado, crosses the corner of Kansas, and flows into southern Nebraska where Trenton Dam is located. The bottom of the cutoff trench, the spillway structure and the outlet tunnel together with its aperture structures are founded on shale. Loess compacted by ponding and rolling, underlain in turn by sandstone and then shale form the abutments.
Sandy alluvium underlies the river section of the fill except in the bottom of the cutoff trench.