COLORADO CANYON. Colorado Canyon, through which the Rio Grande flows at the United States-Mexico border, begins south of Tapado Canyon (at 29°20' N, 104°03' W) and runs southeast of Farm Road 170 to a point west of Santana Mesa entirely within extreme southern Presidio County (at 29°18' N, 103°58' W). The canyon has rugged terrain with high relief surfaced by shallow, stony soils that support scrub brush, sotol, cacti, and sparse grasses.