Geography of the Valley of Mexico

The Valley of Mexico, home to the Aztec empire, measures approximately 3,000 square miles in size and is completely surrounded by mountain ranges of volcanic origin: the Sierra Nevada to the east, the Sierra de las Cruces to the west, the Sierras de Pachuca and Texontlapan to the north, and the Sierra del Chichinauhtzin to the south.   While the floor of the central Mexican basin lies 7,250 feet above sea level, the highest mountain peaks, located in the Sierra Nevadas, rise to an altitude of over 17,500 feet.  Because there is no natural drainage outlet, the Valley, in prehistoric times, was a lacustrian region comprised of 5 lakes: Lake Zumpango in the north, Lakes Xaltocan and Texcoco in the center, and Lakes Xochimilco and Chalco in the south.  Because the surface level of Lake Texcoco was the lowest, the other lakes would, in effect, empty into it.  During the rainy season (summer), the lakes would merge to form one, which the Aztecs called the “Lake of the Moon.”  While lack of overflow would cause the southern lakes (three meters higher than Texcoco) to recede and separate from Texcoco during the dry season, the northern lakes (3.5-6 meters higher than Texcoco) would “all but disappear” (Moriarty 1969).

Currently, the average annual temperature for Mexico City is 60.1 degrees Fahrenheit, while the average rainfall is twenty three inches per year.  It is believed that during pre-Spanish times, the climate was wetter than it is today (Moriarty 1969).

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Taken from Coe (1964).

Map shows the sixteen kilometer long dike-causeway which stretched
across Lake Texcoco from Atzacoalco to Ixtapalapa.