South Poplar Run is a 34 square mile watershed in southern Blair County. It is located directly below the Poplar Run watershed and covers large portions of Greenfield Township and Freedom Township. The South Poplar Run watershed also passes through small sections of Juniata Township and Blair Township. The small, southernmost tip of the watershed, containing the headwaters, lies within Bedford County to the south. This watershed also faces development pressure along the I-99 corridor.
Multiple tributaries, such as Beaverdam Creek, Polecat Run, Pawpaw Run, South Dry Run, and McDonald Run join South Poplar Run as it flows northeast. South Poplar Run then empties into the Frankstown Branch of the Juniata River in Blair Township.