FLAG BRANCH (Erath County). Flag Branch rises five miles southeast of Duffau in extreme southeastern Erath County (at 32°06' N, 97°57' W) and runs southeast for eight miles to its mouth on the East Bosque River, east of Big Lake in northeastern Bosque County (at 32°04' N, 97°50' W). The creek, which is dammed at one point to form Flag Branch Lake, traverses variable terrain surfaced by shallow, stony clay loams that support juniper, oak, grasses, chaparral, and cacti.