The general slope of the southern part is southward to the river Balsas, or Mescala, which forms its boundary-line with Guerrero.
The Lerma, on the northern frontier, and the Balsas on the southern, are the only rivers of importance of the state, their tributaries within its boundaries being small and swift-flowing.
Of Morelia in a fertile valley of the Rio de las Balsas basin.
The more important rivers of the first division, which are described in more detail under the titles of the Brazilian states through which they flow, are the following: the Gurupy, Tury-assu, Mearim, Itapicuru and Balsas, in the state of Maranhao; the Parnahyba and its tributaries in Piauhy; Jaguaribe in Ceara; and the Apody and Piranhas in Rio Grande do Norte.
It receives only one important tributary from Maranhao - the Rio das Balsas, 447 m.
A southern extension of the Mexican Central, via Cuernavaca, has reached the Balsas river and will be extended to Acapulco, once the chief Pacific port of Mexico and the depot for the rich Philippine trade.
The largest rivers of Mexico are: the Rio Grande de Santiago, called the Lerma above Lake Chapala, rising in the state of Mexico and flowing westward across Guanajuato, Jalisco and Tepic to the Pacific coast, with a total length of 540 m., celebrated for its deep canyons and waterfalls; the Rio de las Balsas, or Mescala, which rises in Tlaxcala and flows south and west to the Pacific with a course of 426 m.; the Yaqui, which rises in western Chihuahua and, after breaking through the northern ranges of the Sierra Madre Occidental, flows south-westerly across Sonora to the Gulf of California, with a length of 390 m.; the Grijalva, also called the Chiapas on its upper course, which has its sources in the state of Chiapas and flows north-west and north across Tabasco to the Gulf of Mexico, with a total length of 350 m.; the Fuerte, which rises in southern Chihuahua and, after breaking through the sierras, flows south-west across Sinaloa to the Gulf of California, with a course of 340 m.; the Usumacinta, which is formed by the confluence of the Chixoy and Pasion on the east frontier of Chiapas, and flows north-west across Tabasco to the Grijalva, with a course of 330 m.; and the Panuco, which has its source in the north-west of the state of Mexico and flows north-eastward to the Gulf of Mexico.
The principal river of the state is the Rio de las Balsas or Mescala, which, having its source in Tlaxcala, flows entirely across the state from W.
The Parnahyba forms the eastern boundary of Maranhao, but it has one large tributary, the Balsas, entirely within the state.