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NATURALIST'S GUIDE TO THE AMERICAS 



Owing to the shortness of watersheds 

 and the mountainous character of the 

 more humid portions of Mexico most 

 of the rivers are torrential streams, of 

 which some of the largest on the Gulf 

 side are navigable for small steamers a 

 short distance only from the sea. Aside 

 from the Rio Grande del Norte and the 

 Rio Colorado, both of which are mainly 

 in the United States, the longest river 

 of the Republic is the Rio Lerma or 

 Rio Santiago, as it is termed below 

 Lake Chapala. This river cuts its way 

 from its sources above 9000 ft. in the 

 mountains about the valley of Toluca, 

 northwesterly through the Laguna de 

 Chapala to the Pacific in the state of 

 Nayarit. The Rio de las Balsas drains 

 the southern slopes of the lofty group 

 of mountains forming the southern 

 border of the valley of Mexico and the 

 highlands of Puebla and northern 

 Oaxaca and flows through a broad in- 

 terior valley of arid tropical character. 

 Its general course is westward into the 

 Pacific in Guerrero. The Rio Yaqui 

 flows through great canyons from the 

 high northern part of the western slope 

 of the Sierra Madre Occidental into the 

 Gulf of California. The Rio Mayo, 

 Rio del Fuerte, and many other western 

 slope streams traverse rugged canyons 

 in nearly parallel courses from the 

 highest part of the Sierra Madre to the 

 sea. The rivers of eastern Mexico are 

 short as compared with some western 

 slope streams, owing in part to the more 

 abrupt eastern escarpment of the table- 

 land region. As their watersheds, how- 

 ever, occupy regions of greater precipi- 

 tation the principal streams of the Gulf 

 slope gather an immense volume of 

 water which form torrents in the moun- 

 tains and become sluggish floods in 

 traversing the coastal plain where they 

 afford navigation for short distances by 

 small steamers. The Rio Panuco drains 

 the eastern slopes of the tableland region 

 with many ramifications mainly in the 

 states of Vera Cruz, Hidalgo, Queretaro, 

 and San Luis Potosi and enters the 

 Gulf at Tampico. The Rio Papaloapan, 

 with a very short main trunk, combines 



near its mouth several large streams 

 including the San Juan, flowing from 

 the mountains of northern Oaxaca into 

 the Gulf south of the port of Vera Cruz. 

 Two large rivers, the Rio Grijalva and 

 Rio Usumacinta rise not far apart in 

 the high mountains of southwestern 

 Guatemala and after following widely 

 divergent courses flow through mouths 

 not far apart into the Gulf of Campeche, 

 on the coast of Tabasco. The Rio 

 Nazas, of the northern part of the table- 

 land region carries the drainage of a 

 section of the eastern slope of the 

 Sierra Madre Occidental eastward from 

 Durango to the sink of the Laguna de 

 Meyran, a small body of water with no 

 outlet, in southwestern Coahuila. 



Through long coijtinued erosion the 

 drainage channels of several streams 

 flowing into the Pacific have cut through 

 the Sierra Madre Occidental in the 

 states of Chihuahua and Durango and 

 are now draining parts of the eastern 

 slopes of these mountains and adjacent 

 tablelands in both Durango and 

 Chihuahua. 



The largest lake in Mexico is the 

 Laguna de Chapala in the state of 

 Jalisco. This lake receives the waters 

 of the Rio Lerma which are discharged 

 from an outlet not far from the mouth 

 of this river. The stream below the 

 lake is known as the Rio Santiago. 

 Other notable depressions are filled by 

 lakes Cuitzeo and Patzcuaro in Micho- 

 acan, and lakes Tezcoco, Chalco, and 

 Xochimilco in the valley of Mexico, all 

 of which are comparatively shallow. 



B. CLIMATE 



Climatic conditions in Mexico vary 

 greatly, due not only to differences in 

 latitude between the northern and 

 southern portions, but also in accord- 

 ance with highly diversified local con- 

 ditions of elevation, slope exposure, 

 direction of prevailing winds, and other 

 factors. In general the climate of 

 eastern Mexico is most humid in the 

 southern portion and the region of the 

 higher mountains, the precipitation 

 decreasing_rapidly toward the north, 



