Poplar Run is a 17 square mile watershed located in southwestern Blair County. A very small section of the watershed falls within Cambria County to the west. The watershed encompasses parts of Juniata Township, Freedom Township, Blair Township, and all of Newry Borough. Portions of State Games Lands 198 also lie within the watershed. Forested land is the primarily land use at 70%, so logging is a major industry in the area.
Poplar Run flows east from its headwaters in Cambria County before emptying into the Frankstown Branch of the Juniata River in Blair Township. Poplar Run is a designated Cold Water Fishery, but faces development pressure along the I-99 corridor and sediment erosion from logging activities.