Grijalva (grēhäl`vä), river, c.400 mi (640 km) long, rising in SW Guatemala and flowing NW into S Mexico and N through Chiapas and Tabasco states to the Gulf of Campeche. It is navigable for c.60 mi (100 km) upstream. Several branches of the Usumacinta River flow into it. The Grijalva Project was built in the 1960s along the lower Grijalva and Usumacinta rivers for flood control and sanitation. Named for the Spanish explorer Juan de Grijalva, who discovered it in 1518, the river is also called the Río Grande de Chiapas in the highland region.