derr run begins in a small pond on a hill in penn township. it flows south for several tenths of a mile and enters a valley before turning south-southeast. after several tenths of a mile, the stream turns south for a few tenths of a mile before turning south-southeast again and entering penn township. several tenths of a mile further downstream, it turns southeast for several tenths of a mile. the stream then leaves its valley, crosses pennsylvania route 118, and turns southwest, flowing through the valley of little muncy creek. after a few tenths of a mile, it reaches its confluence with little muncy creek.