Armstrong Creek rises in western Erath County in three branches. The East Fork rises three miles northwest of Mount Zion (at 32°21' N, 98°26' W), and the West Fork rises three miles northeast of School Hill Church (at 32°21' N, 98°29' W). The two branches run southward roughly parallel for eight miles before merging (at 32°15' N, 98°27' W) to form Armstrong Creek two miles east of Lowell. The third branch, the Dry Fork, begins two miles northwest of Lowell (at 32°17' N, 98°29' W) and runs southeast for 4½ miles to its mouth on Armstrong Creek, 2½ miles south of the confluence of the East and West forks (at 32°13' N, 98°27' W). The consolidated stream runs southwest for twelve miles to its mouth on Proctor Reservoir, three miles east of DeLeon (at 32°05' N, 98°30' W). Upstream, the terrain features steep slopes; downstream, the land is generally flat and locally indented. The land surface of shallow sandy soil supports juniper, scattered oaks, and grasses.