Valley of Mexico
Surrounded by mountains and volcanoes, the Valley of Mexico was a centre for several pre-Columbian civilizations including Teotihuacan, the Toltec, and the Aztec Empire.
[10] The valley has been inhabited for at least 12,000 years, attracting humans with its mild climate (average temperatures between 12 and 15 °C, or 54 and 59 °F), abundant game and ability to support large-scale agriculture.
[3] The growth of a major urban industrial centre in an enclosed basin has created significant air and water quality issues for the valley.
[10][13] The Valley of Mexico attracted prehistoric humans because the region was rich in biodiversity and had the capacity of growing substantial crops.
[4] Generally speaking, humans in Mesoamerica, including central Mexico, began to leave a hunter-gatherer existence in favor of agriculture sometime between the end of the Pleistocene epoch and the beginning of the Holocene.
Human remains and artifacts such as obsidian blades have been found at the Tlapacoya site that has been dated as far back as 20,000 BC, when the valley was semi-arid and contained species like camels, bison and horses that could be hunted by man.
Most of the sites are located on what were the shores of Lake Texcoco in the north of the Federal District and the adjacent municipalities of Mexico State such as in Santa Isabel Ixtapan, Los Reyes Acozac, Tepexpan and Tlanepantla.
The symbol for Line 4's Talisman station of the Mexico City Metro is a mammoth, due to the fact that so many bones were uncovered during its construction.
[11] Tlatilco was a large pre-Columbian village and culture in the Valley of Mexico situated near the modern-day town of the same name in the Mexican Federal District.
However, even though the ceremonial pyramid was abandoned, the site remained a location to leave offerings up to AD 400, although lava from the nearby Xitle volcano completely covered it.
[3] After the decline of Cuicuilco, the population concentration shifted north, to the city of Teotihuacan and later to Tula, both outside the lake's region of the valley.
[19] In the early 8th century, with the rise of the Toltec empire, Teotihuacan ceased to be a major urban centre and the population shifted to Tollan or Tula on the northern front of the Valley of Mexico.
[12] After the end of the Toltec empire in the 13th century and the decline of the city of Tula, the population shifted once again, this time to the lakes region of the valley.
By the end of the 13th century, some fifty small urban units, semi-autonomous and with their own religious centers, had sprung up around the lakeshores of the valley.
Many of the institutions created by these hydraulic societies, such as the building and maintenance of chinampas, aqueducts and dikes, were later co-opted by the Spanish during the colonial period.
Much of the surrounding land in the valley was terraced and farmed as well, with a network of aqueducts channeling fresh water from springs in the mountainsides into the city itself.
[3] Despite being the dominant power, the need to rely on resources from other parts of the valley led to the Aztec Triple Alliance between Tenochtitlan, Texcoco and Tlacopan at the beginning of the empire.
[21] Mexico City is vulnerable to severe air pollution problems due to its altitude, its being surrounded by mountains and the winds patterns of the area.
[13] The effects on humans living in an enclosed, contaminated environment have been documented, especially by Nobel Prize winner Dr Mario J. Molina.
[21] The Valley of Mexico is a closed or endorheic basin which geologically divides into three hydrologic zones, the low plain, which is essentially the bed of now-extinct lakes, the piedmont area and the surrounding mountains.
The old lakebeds correspond to the lowest elevations of the valley in the south are mostly clay with a high water content and are almost entirely covered by urban development.
[24] Warmer temperatures had increased evaporation and reduced rainfall in the area so that the lakes’ waters were shallow at about five meters (16 feet) deep as early as the Tlapacoya culture, around 10,000 BC.
[25] The arrival of the Spanish and subsequent efforts to drain the area for flood control was a major infrastructure project, called the desagüe, which was pursued throughout the entire colonial period.
He did succeed in building a canal in this area, calling it Nochistongo, leading waters to the Tula Valley, but the drainage was not sufficient to avoid the Great Flood of 1629 in the city.
From the beginning of the 20th century, Mexico City began to sink rapidly and pumps needed to be installed in the Grand Canal, which before had drained the valley purely with gravity.
If it fails, it would most likely be during the rainy season when it carries the most water, which would cause extensive flooding in the historic center, the airport and the boroughs on the east side.
The old lake beds are almost all paved[3] except for some canals preserved in Xochimilco, mostly for the benefit of visitors who tour them on brightly painted trajineras, boats similar to gondolas.
[8] The first signs of dropping ground water levels was the drying up of natural springs in the 1930s, which coincides with the beginning of intensive exploitation of the aquifer system through wells between 100 and 200 meters (330 and 660 ft) deep.
[3] Despite this, flooding is still common, especially in the summer rainy season, in lower-lying neighborhoods such as Iztapalapa, forcing residents to build miniature dikes in front of their houses to prevent heavily polluted rainwater from entering their homes.
Two other rivers, the San Javier and the Tlalnepantla, which used to feed the old lake system, are diverted before they reach the city and their waters now flow directly into the Grand Canal.
