The Coatzacoalcos is a large river that feeds mainly the south part of the state of Veracruz; it originates in the Sierra de Niltepec and crosses the state of Oaxaca in the region of the Isthmus of Tehuantepec, flowing for toward the Gulf of Mexico.  Tributaries include El Corte, Sarabia, Jaltepec, Chalchijalpa, El Chiquito, Uxpanapa, and Calzadas. The merging of all these rivers creates one of the largest current flows in the entire region. Two-thirds of the streams are navigable.