Souders, V. L., 1976. Physiography, Geology, and Water Resources of Boyd County, Nebraska (WSP-42): 113 pp., size 8.5" x 11". Description: The northeastern part of Boyd County drains directly to the Missouri River, and a narrow band along the southern part drains directly to the Niobrara River. Much of the western part drains to the Keya Paha River, a tributary to the Niobrara. The large remaining area drains to Ponca Creek, which heads in South Dakota and flows east-southeasterly and easterly across Boyd County to enter the Missouri River in northwestern Knox County.