NTRODUCTION 



GENERAL TOPOGRAPHY AND HYDROGRAPHY OF MEXICO. 



The greater portion of Mexico is a plateau ranging in elevation 

 from 3,000 to 8,000 feet above the sea. The northern portion is the 

 lower, and the slope to the northeast, especially of that portion drained 

 by the Rio Grande and its tributaries, is more gentle than of any other 

 portion. The eastern border of this plateau is formed by the eastern 

 range of the Sierra Madre Mountains, leaving to the east a plain vary- 

 ing in width from about 150 miles east of Monterey to less than 50 

 miles west of Tampico and Vera Cruz. This plain north of Tampico 

 is chiefly drained by three rivers: the Rio Conchas and the Rio Soto 

 la Marina, which flow into the Gulf; and the Rio Forlon, a tributary 

 of the Rio Panuco. None of these streams drain any portion of the 

 central plateau. Two of the tributaries of the Rio Panuco, the Rio 

 San Juan and the Rio Tula, have their origin on the central plateau, 

 and reach the main river through a gap in the mountains by a series 

 of cascades and falls. South of Tampico this narrow plain is drained 

 by a number of small streams which are wholly confined to the eastern 

 slope. South of Vera Cruz and in the southern portion of the State 

 of Oaxaca this eastern range of the Sierra Madre meets the southern 

 range, the Sierra Madre del Sur, the two forming the low water shed 

 of the Isthmus of Tehuantepec. The northern portion of this Isthmus 

 is drained by the Rio Coatzacoalcos. Between this river and the City 

 of Vera Cruz is a rather broad, low plain on which from 10 to 15 feet 

 of rain falls each year. This plain is mostly drained by the Rio 

 Papaloapam and its tributaries, the Rio Tonto, the Rio Quiotepec, 

 the Rio Tesechoacan, and the Rio San Juan Evangelista. Owing to 

 the great rainfall in this region, these rivers are very large as com- 

 pared with the area they drain. The Sierra Madre del Sur extends 

 parallel to the coast as far as Colima, where it becomes the western 

 range of the Sierra Madre. Between the Sierra Madre del Sur and 

 the zone of recent volcanoes which extends west of Puebla and forms 

 the watershed between the Rio Balsas and the Rio Lerma is a con- 

 siderable plateau drained by the Rio Balsas, the second largest" river 

 in Mexico, which flows into the Pacific Ocean. North of the Rio 

 Balsas basin the central plateau is drained by the Rio Lerma, which, 

 after it leaves Lago de Chapala, is known as the Rio Grande de San- 

 tiago, the two forming the longest river wholly within the republic. 

 The Rio Santiago reaches the sea by a series of cascades and falls. 



