 A vast number of streams, among which are the Chixoy, Lacantún, and Ixcán, unite to form the Usumacinta River, which passes along the Mexican frontier, and flowing on through Chiapas and Tabasco, falls into the Bay of Campeche.The Grijalva and its tributaries the Cuilco and San Miguel rivers drain west into the Chiapas Depression, and from there into the Gulf of Mexico.Lake Atitlan is a land-locked basin encompassed with lofty mountains.