Arroyo Negro rises near Farm Road 1867 two miles northwest of Loma Vista in southeast Zavala County (at 28°47' N, 99°37' W) and runs southeast twenty-four miles to its mouth on the Nueces River, five miles southwest of Big Wells in northeast Dimmit County (at 28°30' N, 99°35' W). According to local tradition the last Indian fight in Zavala County took place on Arroyo Negro. The streambed begins in an area of variable terrain surfaced by muddy sand and alluvial material. Scrub brush, cacti, and grasses grow there. Downstream, the land is flat with locally shallow depressions and a clay surface that supports water-tolerant hardwoods, conifers, and grasses.