The Usumacinta River is a river in southeastern Mexico and northwestern Guatemala. It is formed by the junction of the Pasión River, which arises in the Sierra de Santa Cruz (in Guatemala) and the Salinas River, also known as the Chixoy, or the Negro, which descends from the Sierra Madre de Guatemala. It defines part of the border between Guatemala and the Mexican state of Chiapas, then continues its northwesterly course, meandering through the Mexican state of Tabasco to the Gulf of Mexico. The total length of the river, including the Salinas, Chixoy, Negro rivers in Guatemala is approximately 1,000 km (620 mi). It is the only visible natural boundary separating the Yucatán Peninsula from Mexico.