ATASCOSA RIVER. The Atascosa River heads in two branches known as the North and West Prongs. The North Prong rises four miles west of Atascosa in far southwestern Bexar County (at 29°16' N, 98°47' W) and runs south for six miles. The West Prong rises a mile west of Lytle in eastern Medina County (at 29°15' N, 98°49' W) and runs southeast for four miles. The two prongs flow together two miles southeast of Lytle in northwestern Atascosa County (at 29°12' N, 98°46' W) to form the Atascosa River proper. From that point the river runs ninety-two miles southeast through Atascosa County and into Live Oak County, where it drains into the Frio River two miles northwest of Three Rivers (at 28°29' N, 98°12' W). The river is probably the one called Arroyo de Vino by Alonso De León in 1689. It traverses flat to gently rolling terrain surfaced by clay and sandy loam that supports water-tolerant hardwoods, mesquite, cacti, and grasses.