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The Valley of Mexico ( es, Valle de México) is a highlands
plateau In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

plateau
in central Mexico roughly coterminous with present-day
Mexico City Mexico City ( es, link=no, Ciudad de México, ; abbreviated as CDMX; nah, Āltepētl Mēxihco) is the capital city, capital and largest city of Mexico, as well as the List of North American cities by population, most populous city in North Americ ...

Mexico City
and the eastern half of the
State of Mexico The State of Mexico ( es, Estado de México; ), officially only Mexico ( es, México), is one of the 32 federal entities of the United Mexican States Mexico ( es, México ; Nahuan languages: ), officially the United Mexican States (; EU ...
. Surrounded by mountains and volcanoes, the Valley of Mexico was a centre for several
pre-Columbian In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) also called the is the third and last subdivision o ...
civilization  A civilization (or civilisation) is a complex society A complex society is a concept that is shared by a range of disciplines including anthropology, archaeology, history and sociology to describe a stage of social formation. The concep ...

civilization
s, including
Teotihuacan Teotihuacan (Spanish language, Spanish: ''Teotihuacán'') (; ) is an ancient Mesoamerican city located in a sub-valley of the Valley of Mexico, which is located in the State of Mexico, northeast of modern-day Mexico City. Teotihuacan is ...

Teotihuacan
, the
Toltec The Toltec culture () is a pre-Columbian era, pre-Columbian Mesoamerican culture that ruled a state centered in Tula (Mesoamerican site), Tula, Hidalgo (state), Hidalgo, Mexico in the early post-classic period of Mesoamerican chronology (ca. 90 ...
, and the
Aztec The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec peoples included different Indigenous peoples of Mexico, ethnic groups of central Mexico, particularly those g ...

Aztec
. The ancient Aztec term ('Land Between the Waters') and the phrase Basin of Mexico are both used at times to refer to the Valley of Mexico. The Basin of Mexico became a well known site that epitomized the scene of early Classic Mesoamerican cultural development as well. The Valley of Mexico is located in the
Trans-Mexican Volcanic Belt #REDIRECT Trans-Mexican Volcanic Belt The Trans-Mexican Volcanic Belt ( es, Eje Volcánico Transversal), also known as the Transvolcanic Belt and locally as the (''Snowy Mountain Range''), is an active volcanic belt that covers central-southern M ...
. The valley contains most of the
Mexico City Metropolitan Area Mexico ( es, México ; Nahuan languages The Nahuan or Aztecan languages are those languages of the Uto-Aztecan language family A language is a structured system of communication used by humans, including speech ( spoken language), ge ...
, as well as parts of the
State of Mexico The State of Mexico ( es, Estado de México; ), officially only Mexico ( es, México), is one of the 32 federal entities of the United Mexican States Mexico ( es, México ; Nahuan languages: ), officially the United Mexican States (; EU ...
,
Hidalgo Hidalgo may refer to: People * Hidalgo (nobility), members of the Spanish nobility * Hidalgo (surname) Places Mexico * Hidalgo (state), in central Mexico * Hidalgo, Coahuila, a town in the north Mexican state of Coahuila * Hidalgo, Nuevo Leó ...
,
Tlaxcala Tlaxcala (; , ; from nah, Tlaxcallān ), officially the Free and Sovereign State of Tlaxcala ( es, Estado Libre y Soberano de Tlaxcala), is one of the 32 states which comprise the Federal Entities of Mexico Mexico, officially the United ...

Tlaxcala
, and
Puebla Puebla ( en, Colony, settlement), officially Free and Sovereign State of Puebla ( es, Estado Libre y Soberano de Puebla), is one of the 32 states which comprise the Federal Entities of Mexico Mexico ( es, México ; Nahuan languages: ), ...

Puebla
. The Basin of Mexico covers approximately in the NNE-SSW direction with length to width dimensions of approximately to The Valley of Mexico can be subdivided into four basins, but the largest and most-studied is the area that contains
Mexico City Mexico City ( es, link=no, Ciudad de México, ; abbreviated as CDMX; nah, Āltepētl Mēxihco) is the capital city, capital and largest city of Mexico, as well as the List of North American cities by population, most populous city in North Americ ...

Mexico City
. This section of the valley in particular is colloquially referred to as the "Valley of Mexico". The valley has a minimum elevation of
above sea level Above may refer to: *Above (artist) Tavar Zawacki formerly known as 'ABOVE' (born 1981) is an American abstract art Abstract art uses visual language of shape, form, color and line to create a composition which may exist with a degree of ind ...
and is surrounded by mountains and volcanoes that reach elevations of over . It is an enclosed valley with no natural outlet for water to flow to the sea although there is a gap to the north where there is a high
mesa A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by ...

mesa
but no high mountain peaks. Within this vulnerable watershed all the native fishes were extinct by the end of the 20th century. Hydrologically, the valley has three features. The first feature is the lakebeds of five now-extinct lakes, which are located in the southernmost and largest of the four sub-basins. The other two features are
piedmont it, Piemontese , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = , demographics1_title1 = , demographics1_info1 = , demographics1_title2 ...

piedmont
, and the mountainsides that collect the precipitation that eventually flows to the lake area. These last two are found in all four of the sub-basins of the valley. Today, the Valley drains through a series of artificial canals to the
Tula River The Tula River ( es, Río Tula) is a river in Hidalgo (state), Hidalgo State in central Mexico, and a tributary of the Moctezuma River. Geography It runs through the city of Tula de Allende and begins as a drainage channel for the Valley of Mexic ...
, and eventually the
Pánuco River The Pánuco River ( es, Río Pánuco, ), also known as the ''Río de Canoas'', is a river in Mexico Mexico, officially the United Mexican States, is a country A country is a distinct territorial body or political entity A polity ...
and the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin 400px, Diagrammatic cross-section of an ocean basin, showing the various geographic features In hydrology Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning ...

Gulf of Mexico
.
Seismic activity An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object known ...
is frequent here, and the valley is considered an earthquake-prone zone. 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. Civilizations that have arisen in this area include the
Teotihuacan Teotihuacan (Spanish language, Spanish: ''Teotihuacán'') (; ) is an ancient Mesoamerican city located in a sub-valley of the Valley of Mexico, which is located in the State of Mexico, northeast of modern-day Mexico City. Teotihuacan is ...

Teotihuacan
(800 BC to 800 AD) the
Toltec Empire According to the Aztecs The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec peoples included different Indigenous peoples of Mexico, ethnic groups of central ...
(10th to 13th century) and the
Aztec Empire The Aztec Empire, or the Triple Alliance ( nci, Ēxcān Tlahtōlōyān, Help:IPA for Nahuatl, jéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥, was an alliance of three Nahua peoples, Nahua city-states: , , and . These three city-states ruled th ...

Aztec Empire
(1325 to 1521). When the Spaniards arrived in the Valley of Mexico, it had one of the highest population concentrations in the world with about one million people. After the
Conquest Conquest is the act of military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare War is an intense armed conflict between State (polity), states, g ...
, the Spaniards rebuilt the largest and most dominant city here,
Tenochtitlan Tenochtitlan ( nah, Tenōchtitlan ; es, Tenochtitlán), also known as Mexico-Tenochtitlan ( nah, Mēxihco Tenōchtitlan ; es, México-Tenochtitlán), was a large Mexica ''altepetl'' in what is now the historic center of Mexico City. The exact ...

Tenochtitlan
, renaming it Mexico City. The valley used to contain five lakes called
Lake Zumpango Lake Zumpango was an endorheic lake formerly located in the Valley of Mexico. It was important area for Mesoamerican cultural development in central Mexico. The endorheic basin located in the north of State of Mexico, in the borough of municipal ...
, Lake Xaltocan,
Lake Xochimilco Lake Xochimilco (; nah, Xōchimīlco, ) is an ancient endorheic lake An endorheic lake (also called a sink lake or terminal lake) is a collection of water within an endorheic basin An endorheic basin (; also spelled endoreic basin o ...
,
Lake Chalco Lake Chalco was an endorheic lake formerly located in the Valley of Mexico, and was important for Mesoamerican cultural development in central Mexico. The lake was named after the ancient city of Chalco (municipality), Chalco on its former easte ...
and the largest, Texcoco, covering about of the valley floor, but as the Spaniards expanded Mexico City, they began to drain the lakes' waters to control flooding. Although violence and disease significantly lowered the population of the valley after the Conquest, by 1900 it was again over one million people. The 20th and 21st centuries have seen an explosion of population in the valley along with the growth of industry. Since 1900, the population has doubled every fifteen years. Today, around 21 million people live in the Mexico City Metropolitan Area which extends throughout almost all of the valley into the states of Mexico and Hidalgo. The growth of a major urban industrial centre in an enclosed basin has created significant air and water quality issues for the valley. Wind patterns and
thermal inversion , Scotland Scotland ( sco, Scotland, gd, Alba ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a 96-mile (154 km) An ...
s trap contaminants in the valley. Over-extraction of ground water has caused new flooding problems for the city as it sinks below the historic lake floor. This causes stress on the valley's drainage system, requiring new tunnels and canals to be built.


History of human habitation


First human habitation

The Valley of Mexico attracted prehistoric humans because the region was rich in biodiversity and had the capacity of growing substantial crops. Generally speaking, humans in
Mesoamerica Mesoamerica is a historical and important region In geography, regions are areas that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and the interaction of humanity and the ...
, including central Mexico, began to leave a hunter-gatherer existence in favor of agriculture sometime between the end of the
Pleistocene The Pleistocene ( , often referred to as the ''Ice Age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the earth’s most recent period of repeated glaciations. Before a change finally ...
epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-E ...

epoch
and the beginning of the
Holocene The Holocene ( ) is the current geological epoch In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age (geology), age but shorter than a period (geology), period. The current epoch is the Holocene E ...
. The oldest known human settlement in the Valley of Mexico is located in Tlapacoya, located on what was the edge of Lake Chalco in the southeast corner of the valley in contemporary Mexico State. There is reliable archeological evidence to suggest that the site dates as far back as 12,000 BC. After 10,000 BC, the number of artifacts found increases significantly. There are also other early sites such as those in Tepexpan, Los Reyes Acozac, San Bartolo Atepehuacan, Chimalhuacán and Los Reyes La Paz but they remain undated. 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
camel A camel is an even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, ...

camel
s,
bison Bison are large, even-toed ungulates in the genus ''Bison'' within the subfamily Bovinae. Two extant taxon, extant and six extinction, extinct species are recognised. Of the six extinct species, five became extinct in the Quaternary extinction ...

bison
and horses that could be hunted by man. However, the precise dating of these artifacts has been disputed. Giant
Columbian mammoth The Columbian mammoth (''Mammuthus columbi'') is an extinct species of mammoth A mammoth is any species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as wel ...

Columbian mammoth
s once populated the area, and the valley contains the most extensive mammoth kill sites in Mexico. 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 Santa Isabel Ixtapan is a community which is part of the municipality of San Salvador Atenco San Salvador Atenco is the municipal seat Image:Palacio Municipal de Puebla.JPG, 300px, Municipal palace in Puebla City, seat of govertment. A municipal ...
,
Los Reyes Acozac Los Reyes Acozac is a community that belongs to the municipality of Tecámac Tecámac is a Municipalities of Mexico State, municipality in State of Mexico in Mexico. The municipality covers an area of 157.34 km². The municipal seat lies in th ...
,
Tepexpan Tepexpan is the largest town in the Acolman Acolman de Nezahualcóyotl is a town and municipality A municipality is usually a single administrative division having Municipal corporation, corporate status and powers of self-government or jur ...

Tepexpan
and Tlanepantla. Mammoth bones are still occasionally found in farmland here. They have been discovered in many parts of the Federal District itself, particularly during the construction of the city's
Metro lines Metro, short for metropolitan (disambiguation), metropolitan, may refer to: Geography * Metro (city), a city in Indonesia * A metropolitan area, the populated region including and surrounding an urban center Public transport * Rapid transit, a ...

Metro lines
and in the neighborhoods of Del Valle in the center,
Lindavista Lindavista is a neighbourhood in the north of Mexico City, in the administrative district of Gustavo A. Madero, D.F., Gustavo A. Madero. The streets in Lindavista are named after cities in Latin America. History It was founded in 1932 as a re ...
to the center-north and
Coyoacán Coyoacán ( , ) is a borough ('' alcaldía'') of Mexico City Mexico City ( es, link=no, Ciudad de México, ; abbreviated as CDMX; nah, Āltepētl Mēxihco) is the capital city, capital and largest city of Mexico, as well as the List of North A ...

Coyoacán
in the south of the city. 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. However, the richest site for mammoth remains in the valley is at the
Paleontological Museum in Tocuila The Paleontological Museum in Tocuila (Museo Paleontológico en Tocuila) is a fossil museum located in Texcoco, State of Mexico, Municipality of Texcoco, State of Mexico, central Mexico. It displays part of one of the richest deposits of Late Plei ...
, a site located near the town of Texcoco in Mexico State. Although there is some evidence around the old lakeshores that the first populations here survived by hunting, gathering, and possibly by scavenging, evidence from this time period is scarce.


Pre-Teotihuacan

Tlatilco Tlatilco was a large pre-Columbian In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) also called th ...

Tlatilco
was a large
pre-Columbian In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) also called the is the third and last subdivision o ...
village and culture in the Valley of Mexico situated near the modern-day town of the same name in the
Mexican Federal District ) , blank1_name_sec1 = HDI The Human Development Index (HDI) is a statistic composite index of life expectancy Life expectancy is a statistical measure of the average time an organism is expected to live, based on the ...

Mexican Federal District
. It was one of the first significant population centers to arise in the valley, flourishing on the western shore of Lake Texcoco during the Middle Pre-Classic period, between 1200 BC and 200 BC. It was originally classified as a
necropolis A necropolis (plural necropolises, necropoles, necropoleis, necropoli) is a large, designed cemetery A cemetery, burial ground, gravesite or graveyard is a place where the remains of dead people are burial, buried or otherwise interred. The ...

necropolis
when it was first excavated, but it was determined that the many burials there were under houses of which nothing remains. It was then classified as a major chiefdom center. The Tlatilcans were an agricultural people growing beans,
amaranth ''Amaranthus'' is a cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949 ...

amaranth
, squash and
chili pepper The chili pepper (also chile, chile pepper, chilli pepper, or chilli), from Nahuatl Nahuatl (; ),The Classical Nahuatl word (noun stem ''nāhua'', + absolutive ''-tl'' ) is thought to mean "a good, clear sound" This language name has se ...

chili pepper
s, reaching their peak from 1000 to 700 BC. The next-oldest confirmed civilization is in the far south of the valley and is called
Cuicuilco Cuicuilco is an important archaeological site located on the southern shore of Lake Texcoco Lake Texcoco ( es, Lago de Texcoco) was a natural lake A lake is an area filled with water, localized in a basin, surrounded by land, apart fr ...
. This archaeological site is located where crosses the
Anillo Periférico The Anillo Periférico (Spanish for ''peripheral ring'') is the outer beltway of Mexico City. The ''Periferico'' was originally planned by architect Carlos Contreras as early as 1925, together with other major roads such as the Viaducto Migue ...
in the
Tlalpan Tlalpan ( nci, Tlālpan, , place on the earth, ) is one of the 16 administrative boroughs (called ''alcaldías'' in Spanish) of Mexico City. It is the largest borough, with over eighty percent under conservation as forest and other ecologically se ...

Tlalpan
borough of the city. The old settlement once extended far beyond the boundaries of the current site, but it is buried under lava from one of the volcanic eruptions that led to its demise, and much of the modern city is built over this lava. The settlement was located where an old river delta used to form in the valley with waters from Mount Zacatépetl located in what is now the Tlalpan Forest. Cuicuilco was believed to have reached city status by 1200 BC and began to decline around 100 BC - AD 150. 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 Xitle ( Nahuatl Nahuatl (; ),The Classical Nahuatl word (noun stem ''nāhua'', + absolutive ''-tl'' ) is thought to mean "a good, clear sound" This language name has several spellings, among them náhuatl (the standard spelling in the Spanis ...

Xitle
volcano completely covered it.


Teotihuacan and the Toltecs

Around 2,000 years ago, the Valley of Mexico became one of the world's most densely populated areas and has remained so since. 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. Teotihuacan became an organized village around 800 BC but it was around 200 BC that it began to reach its height. When it did, the city had approximately 125,000 inhabitants and covered of territory. It was dedicated primarily to the
obsidian Obsidian (; ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in the lighter element ...

obsidian
trade and at its peak was an important religious center and
pilgrimage A pilgrimage is a journey, often into an unknown or foreign place, where a person goes in search of new or expanded meaning about their self, others, nature, or a higher good, through the experience. It can lead to a personal transformation, aft ...
for the valley. In the early 8th century, with the rise of the
Toltec The Toltec culture () is a pre-Columbian era, pre-Columbian Mesoamerican culture that ruled a state centered in Tula (Mesoamerican site), Tula, Hidalgo (state), Hidalgo, Mexico in the early post-classic period of Mesoamerican chronology (ca. 90 ...
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.


Aztec Empire

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. With this migration came the concept of a city-state based on the Toltec model. 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. These remained intact with a population of about 10,000 each under Aztec rule and survived into the colonial period. All of these city-states, including the largest and most powerful,
Tenochtitlan Tenochtitlan ( nah, Tenōchtitlan ; es, Tenochtitlán), also known as Mexico-Tenochtitlan ( nah, Mēxihco Tenōchtitlan ; es, México-Tenochtitlán), was a large Mexica ''altepetl'' in what is now the historic center of Mexico City. The exact ...

Tenochtitlan
, with more than 150,000 inhabitants, claimed descent from the Toltecs. None of these cities was completely autonomous or self-sufficient, resulting in a conflictive political situation, and a complex system of agriculture in the valley. These city-states had similar governmental structures based on the need to control flooding and store water for irrigating crops. 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. The largest and most dominant city at the time of the Spanish conquest was
Tenochtitlan Tenochtitlan ( nah, Tenōchtitlan ; es, Tenochtitlán), also known as Mexico-Tenochtitlan ( nah, Mēxihco Tenōchtitlan ; es, México-Tenochtitlán), was a large Mexica ''altepetl'' in what is now the historic center of Mexico City. The exact ...

Tenochtitlan
. It was founded by the
Mexica The Mexica (Nahuatl: , ;''Nahuatl Dictionary.'' (1990). Wired Humanities Project. University of Oregon. Retrieved August 29, 2012, frolink/ref> singular ), or Mexicas, were a Nahuatl-speaking indigenous peoples, indigenous people of the Valley ...

Mexica
(
Aztecs The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec peoples included different Indigenous peoples of Mexico, ethnic groups of central Mexico, particularly those ...

Aztecs
) on a small island in the western part of Lake Texcoco in 1325, and was extended with the use of
chinampa ''Chinampa'' ( nah, chināmitl ) is a technique used in Agriculture in Mesoamerica, Mesoamerican agriculture which relied on small, rectangle, rectangular areas of fertility (soil), fertile arable land to grow agriculture, crops on the wikt:shal ...

chinampa
s, human-made extensions of agricultural land into the southern lake system, to increase productive agricultural land, covering about . The inhabitants controlled the lake with a sophisticated system of
dike Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), the Greek goddess of moral justice * Dikes, diagonal pliers, diag ...
s, canals and
sluice A sluice ( ) is a water channel sometimes controlled by a gate or lock Lock(s) may refer to: Common meanings *, a mechanical device used to secure items of importance *, a device for boats to transit between different levels of water, as in ...

sluice
s. Much of the surrounding land in the valley was terraced and farmed as well, with a network of
aqueduct Aqueduct may refer to: Bridges *Aqueduct (bridge), a bridge to convey water over an obstacle, such as a ravine or valley *Navigable aqueduct, or water bridge, a structure to carry navigable waterway canals over other rivers, valleys, railways or r ...
s channeling fresh water from springs in the mountainsides into the city itself. Despite being the dominant power, the need to rely on resources from other parts of the valley led to the
Aztec Triple Alliance The Aztec Empire, or the Triple Alliance ( nci, Ēxcān Tlahtōlōyān, Help:IPA for Nahuatl, jéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥, was an alliance of three Nahua peoples, Nahua city-states: , , and . These three city-states ruled th ...

Aztec Triple Alliance
between Tenochtitlan, Texcoco and
Tlacopan Tlacopan (From Classical Nahuatl, ''tlacōtl'', meaning stem or rod and ''-pan'', meaning place in or on. This roughly translates to "place on the rods"),Siméon, R. (1977). Diccionario de la lengua náhuatl o mexicana. México: Siglo Veintiuno. ...

Tlacopan
at the beginning of the empire. However, by the time the Spanish arrived in 1519, Tenochtitlan had become the dominant power of the three, causing grievances that the Spaniards were able to exploit. However, despite Tenochtitlan's power outside the valley, it never completely controlled all of the valley itself, with the
altepetl The (, plural ''altepeme'' or ''altepemeh'') was the local, ethnic group, ethnically-based political entity, usually translated into English as "city-state," in pre-Columbian era, pre-Columbian societiesSmith 1997 p. 37 in the Americas. The ''a ...
of Tlaxcala the most prominent example. By 1520, the estimated population of the valley was over 1,000,000 people.


Spanish colonial rule and the Mexico City metropolitan area

After the
Spanish conquest of the Aztec empire The Spanish conquest of the Aztec Empire, also known as the Conquest of Mexico or the Spanish-Aztec War (1519–21), was one of the primary events in the Spanish colonization of the Americas. There are multiple 16th-century narratives of the eve ...
in 1521, the Spanish rebuilt and renamed Tenochtitlan as Mexico City. They started with essentially the same size and layout as the Aztec city but as the centuries progressed, the city grew as the lakes shrank. Just after the conquest, disease and violence had decreased the population in the valley, especially of the native peoples, but after that, the population grew all through the colonial period and in the century after
independence Independence is a condition of a person, nation, country, or Sovereign state, state in which residents and population, or some portion thereof, exercise self-government, and usually sovereignty, over its territory. The opposite of independe ...
. By the early 20th century, the population of Mexico City alone had risen to over one million people. A population explosion began early in the 20th century, with the population of the city itself doubling approximately every 15 years since 1900, partly attributed to the fact that the federal government has favored development of the metropolitan area over other areas of the country. This has spurred investment in
infrastructure Infrastructure is the set of fundamental facilities and systems that support the sustainable functionality of households and firms. Serving a country, city, or other area, including the services and facilities necessary for its economy An ec ...

infrastructure
for the city, such as
electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

electricity
, other power sources,
water supply Water supply is the provision of water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for ...
and
drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural Agriculture is the practice of cultivating plants and livestock. Agr ...

drainage
. These have attracted businesses which in turn have attracted more population. Since the 1950s, urbanization has spread out from beyond the bounds of the Federal District to the surrounding jurisdictions, especially to the north into the State of Mexico making for the Mexico City Metropolitan area, which fills most of the valley. Today, this metropolitan area accounts for 45 per cent of the country's industrial activity, 38 percent of GNP, and 25 percent of the population. Much of its industry is concentrated in the northern part of the Federal District and the adjoining cities in the state of Mexico. While population growth has slowed and even declined in the city proper, the outer limits of the metropolitan area keep growing. Much of this growth has occurred on the mountainsides of the valley, in the form of illegal settlements in ecologically sensitive areas. Overall urban settlement in the valley has expanded from about in 1940 to in 1990. The metropolitan area has about 21 million residents and about 6 million cars.


Air pollution

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. The altitude, with its low oxygen levels, makes for poor combustion of
fossil fuel A fossil fuel is a hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structure ...
s leading to unsafe levels of
nitrogen oxide Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral * Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide * Nitrogen dioxide (NO2), nitrogen(IV) oxide * Nitrogen trioxide (NO ...
s,
hydrocarbons In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
, and
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

carbon monoxide
. The valley is surrounded by mountain ranges with one small opening to the north. The surrounding mountains and climate patterns here make it difficult to clear out the smog produced. The valley has internal wind patterns which circulate around the valley without a prevailing wind to push contaminants in a single direction. The most significant climatic phenomena here is "thermal inversion," which is prevalent in the winter months when the cooler air of the valley is trapped by relatively warmer air above. Adding to this is that prevailing winds outside the valley move from north to south, in through the Valley's one opening, where incidentally most of the region's industry is located. These factors diminish in the summer and the situation is helped by the arrival of the rainy season, but the valley's southern latitude and the abundance of sunlight allows for dangerous levels of
ozone Ozone (), or trioxygen, is an inorganic molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a cha ...

ozone
and other dangerous compounds. While still considered one of the most polluted places on the planet, the valley's air pollution problems are not as bad as they were several decades ago. One major problem that was brought under control was the
lead Lead is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elements ...

lead
contamination in the air with the introduction of
unleaded gasoline Gasoline () or petrol () (see the etymology Etymology ()The New Oxford Dictionary of English ''The'' () is a grammatical article Article often refers to: * Article (grammar) An article is any member of a class of dedicated word ...

unleaded gasoline
. Two other contaminants that have been brought under control are carbon monoxide and
sulfur dioxide Sulfur dioxide (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the autho ...
. The contamination problems that remain are primarily with ozone and
fine particles Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM), or suspended particulate matter (SPM) – are microscopic The microscopic scale (from , ''mikrós'', "small" and σκο ...
(
soot Soot ( ) is a mass of impure Carbonaceous, carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the resi ...
) (between 2.5 
micrometers The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
and 10 micrometers). Thirty to fifty percent of the time, Mexico City's levels of
fine particles Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM), or suspended particulate matter (SPM) – are microscopic The microscopic scale (from , ''mikrós'', "small" and σκο ...
of ten micrometers, the most dangerous, exceed levels recommended by the
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each other through the co-ordinating machinery of the Unit ...
. In the 1940s, before large-scale burning of fossil fuels in the area, the visibility of the valley was about , allowing for daily viewing of the mountain ranges that surround the valley, including the snow-capped volcanoes of Popocatepetl and Iztaccihuatl. Since that time, the average visibility has come down to about . Mountain peaks are now rarely visible from the city itself. While reduced visibility in the valley was due to sulfur emissions in the past, it is now due to
fine particles Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM), or suspended particulate matter (SPM) – are microscopic The microscopic scale (from , ''mikrós'', "small" and σκο ...
in the air. The effects on humans living in an enclosed, contaminated environment have been documented, especially by
Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel Alfred Bernhard Nobel ( , ; 21 October 1833 – 10 December 1896) was a Swedish chemist, engineer, inventor, busines ...
winner Dr
Mario J. Molina Mario José Molina-Pasquel Henríquez (19 March 19437 October 2020), known as Mario Molina, was a Mexican chemist. He played a pivotal role in the discovery of the Antarctic ozone hole Ozone depletion consists of two related events observed ...
. He claims
fine particle Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM), or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspension (chemistry), susp ...
pollution is the greatest concern because of lung damage. According to him, the city's residents lose about 2.5 million working days every year due to health problems associated with
fine particles Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM), or suspended particulate matter (SPM) – are microscopic The microscopic scale (from , ''mikrós'', "small" and σκο ...
.


Hydrology

The Valley of Mexico is a closed or
endorheic basin An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin A drainage basin is any area of land where precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheri ...
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. In the piedmont area, these clays become mixed with silts and sands, and in some areas close to the mountains, the piedmont is largely composed of
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
from old lava flows. The valley is enclosed completely by mountain ranges, from which flow rain and melting snow into the valley's hydraulic system. This groundwater flow produces a number of
springs Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a heli ...
in the foothills and
upwelling Upwelling is an oceanographic Oceanography (compound of the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast ...

upwelling
s in the valley floor. This underground flow is the source of the five aquifers that provide much of the drinking water to Mexico City located in Soltepec,
Apan Apan is a city and one of the 84 municipalities of Hidalgo Hidalgo (state), Hidalgo is a Administrative divisions of Mexico#States, state in central Mexico divided into 84 municipalities of Mexico, municipalities. According to the 2020 Mexican Ce ...

Apan
, Texcoco, Chalco-
Amecameca Amecameca is a municipality located in the eastern panhandle of Mexico State The State of Mexico ( es, Estado de México; ), officially only Mexico ( es, México), is one of the 32 federal entities of the United Mexican States Mexico ( ...

Amecameca
and underneath Mexico City itself.


Old lake system

Before the 20th century, the Mexico City portion of the valley contained a series of lakes, with saline lakes to the north near the town of Texcoco and freshwater ones to the south. The five lakes, Zumpango, Xaltoca, Xochimilco, Chalco, and the largest, Texcoco used to cover about of the basin floor. Small mountains such as the Sierra de Guadalupe and Mount Chiconaultla partially separated the lakes from each other. All the other lakes flowed toward the lower Lake Texcoco, which was saline due to evaporation. The lakes were fed by a number of rivers such as the San Joaquin, San Antonio Abad, Tacubaya, Becerra, Mixcoac and Magdalena Contreras, carrying runoff and snowmelt from the mountains. Long before the arrival of the Spanish, the lake system had been shrinking due to climate change. Warmer temperatures had increased evaporation and reduced rainfall in the area so that the lakes’ waters were shallow at about deep as early as the Tlapacoya culture, around 10,000 BC. During the Aztec Empire, the northern lakes were inaccessible by canoe during the dry season from October to May.


History of water control in the valley

For 2000 years, humans have been interfering with and altering the hydraulic conditions of the valley, especially in the lakes region. The Aztecs built
dikes Dyke or dike may refer to: General uses * Dyke (slang) The term ''dyke'' is a slang Slang is language (words, phrases, and usages) of an informal register. It also sometimes refer to the language generally exclusive to the members of ...

dikes
for flood control and to separate the
saline water Saline water (more commonly known as salt water) is water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main con ...
of the northern lakes from the
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...

fresh water
of the southern ones. After the destruction of Tenochtitlan in 1521, the Spaniards rebuilt the Aztec dikes but found they did not offer enough flood protection. The arrival of the Spanish and subsequent efforts to drain the area for flood control was a major infrastructure project, called the , was pursued the entire colonial period. The idea of opening drainage canals first came about after a flood of the colonial city in 1555. The first canal was begun in 1605 to drain the waters of Lake Zumpango north through
Huehuetoca Huehuetoca is a municipio (Mexico), ''municipio'' (municipality) in State of Mexico, central Mexico, and also the name of its largest town and municipal seat. Name origins The name "Huehuetoca" is derived from the Nahuatl ''huehuetocan'', which ...
which would also divert waters from the
Cuautitlán Cuautitlán () is a city and municipality in the State of Mexico The State of Mexico ( es, Estado de México; ), officially the Free and Sovereign State of Mexico ( es, Estado Libre y Soberano de México), is one of the 32 federal entities of ...

Cuautitlán
River away from the lakes and toward the
Tula River The Tula River ( es, Río Tula) is a river in Hidalgo (state), Hidalgo State in central Mexico, and a tributary of the Moctezuma River. Geography It runs through the city of Tula de Allende and begins as a drainage channel for the Valley of Mexic ...
. This project was undertaken by
Enrico MartínezImage:EnricoMartinez.JPG, 200px, Monument to Enrico Martínez in Mexico City Enrico Martínez, Henri Martín or Heinrich Martin, (Born in Hamburg, date unknown; d. in Mexico in 1632) was cosmographer to the King of Spain, interpreter for the Spanish ...
and he devoted 25 years of his life to it. 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. Another canal, which would be dubbed the "Grand Canal" was built parallel to the Nochistongo one ending in
Tequixquiac Tequixquiac is a municipality A municipality is usually a single administrative division Administrative division, administrative unitArticle 3(1). , country subdivision, administrative region, subnational entity, first-level subdivision ...
. The Grand Canal consists of one main canal, which measures in diameter and long. The drainage project was continued after independence, with three secondary canals, built between 1856 and 1867. During the presidency of
Porfirio Díaz José de la Cruz Porfirio Díaz Mori ( or ; ; 15 September 1830 – 2 July 1915), better known as Porfirio Díaz, was a Mexico, Mexican General (Mexico), general and politician who served seven terms as President of Mexico, a total of 31 years, ...

Porfirio Díaz
(r. 1876–1911) drainage again became a priority. Díaz completed it officially in 1894, although work continued thereafter. Despite the Grand Canal's drainage capacity, it did not solve the problem of flooding 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. Along with the pumps, the Grand Canal was expanded with a new tunnel through the low mountains called the Xalpa to take the canal past Tequisquiac. Even so, the city still suffered floods in 1950 and 1951. Despite its age, the Grand Canal can still carry out of the valley, but this is significantly less than what it could carry as late as 1975 because continued sinking of the city (as much as ) weakens the system of water collectors and pumps. As a result, another tunnel, called the Emisor Central, was built to carry wastewater. Although it is considered the most important pipe in the country, it has been damaged by overuse and corrosion of its diameter walls. Because of lack of maintenance and gradual decrease in this tunnel's ability to carry water, there is concern that this tunnel will soon fail. It is continuously filled with water, making it impossible to inspect it for problems. 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. Because of this, another new drainage project is planned that will cost US$1.3 billion. The project includes new pumping stations, a new drainage tunnel and repairs to the current system of pipes and tunnels to clear blockages and patch leaks. Over-pumping of groundwater in the 20th century has hastened the disappearance of the lakes. The old lake beds are almost all paved except for some canals preserved in Xochimilco, mostly for the benefit of visitors who tour them on brightly painted , boats similar to
gondola The gondola (, ; vec, góndoła ) is a traditional, flat-bottomed Venetian rowing Rowing is the act of propelling a boat using the motion of oar An oar is an implement used for water-borne propulsion. Oars have a flat blade at one en ...

gondola
s. Desiccation has had a major environmental impact on the Valley of Mexico.


Drinking water and sinking lands

Historically, Mexico City's potable water supply came via aqueduct from the mountain springs on the valley sides like that in Chapultepec as most of the water in Lake Texcoco was saline. These were originally built by the Aztecs and were rebuilt by the Spaniards. In the mid-1850s, potable groundwater was found underneath the city itself, which motivated the large-scale drilling of wells. Today, 70% of Mexico City's water still comes from five principal aquifers in the valley. These aquifers are fed by water from natural springs and runoff from precipitation. It was only when the population reached about six million that Mexico City started to need to appropriate water from outside the valley. Today, Mexico City faces a serious water deficit. Because of increased demand from a growing population, increasing industry, and ecosystem degradation in the form of deforestation of the surrounding mountains, more water is leaving the system than is entering. It is estimated that of water is needed to support the potable and agricultural irrigation needs of Mexico City's population. The main aquifer is being pumped at a rate of , but is only being replaced at , or about half of the extraction rate, leaving a shortfall of . This over-extraction of groundwater from the old clay lake bed has been causing the land upon which the city rests to collapse and sink. This problem began in the early 20th century as a consequence of the drainage of the valley for flood control. Since the beginning of the 20th century, some areas of Mexico City have sunk . In 1900, the bottom of the lake was lower than the median level of the city center. By 1974, the lake bottom was higher than the city. 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 deep. Today, Mexico City is sinking between five and forty centimeters (0.2 and 1.3 ft) per year, and its effects are visible. El Ángel de la Independencia ("The Angel of Independence") statue, located on
Paseo de la Reforma Paseo de la Reforma (translated as "Promenade of the Reform") is a wide avenue that runs diagonally across the heart of Mexico City. It was designed by Ferdinand von Rosenzweig in the 1860s and modeled after the great boulevards of Europe, such a ...

Paseo de la Reforma
was built in 1910, anchored by a foundation deep beneath what was the surface of the street at that time. However, because the street has sunk around it, steps have been added to allow access to the statue's base. Subsidence of the valley floor beneath has caused flooding problems as now much of the city has sunk below the natural lake floor. Currently, pumps need to work 24 hours a day all year round to keep control of runoff and wastewater. Despite this, flooding is still common, especially in the summer rainy season, in lower-lying neighborhoods such as
Iztapalapa Iztapalapa () is one of Mexico City’s 16 Municipalities of Mexico City , municipalities, located on the east side of the entity. The borough is named after and centered on the formerly independent municipality of Iztapalapa, which is officially ...
, forcing residents to build miniature dikes in front of their houses to prevent heavily polluted rainwater from entering their homes. Subsidence also causes damage to water and sewer lines, leaving the water distribution system vulnerable to contamination which carries risks to public health. Measures other than drainage have been implemented to contain flooding in the city. In 1950, dikes were built to confine storm runoff. Rivers that run through the city were encapsulated in 1950 and 1951. Rivers such as the Consulado River, Churubusco River and the Remedio River are encased in concrete tunnels which take their waters directly to the drainage system to leave the Valley. 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. No water from these rivers is allowed to sink into the ground to recharge the aquifer. While the rivers and streams that flow down from the mountain peaks still begin the way they always have, their passage through the shantytowns lacking city sanitation schemes that surround Mexico City turns them into open
combined sewer A combined sewer is a type of gravity sewer A gravity sewer is a conduit utilizing the energy resulting from a difference in elevation to remove unwanted water. The term ''sewer'' implies removal of sewage or surface runoff rather than water int ...
s. Therefore, their final stages are frequently culverted or added to the existing major culverted rivers to keep this water from contaminating the aquifer.


See also

*
Mexican Plateau The Central Mexican Plateau, also known as the Mexican Altiplano ( es, Altiplanicie Mexicana), is a large arid-to-semiarid plateau In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of ...
*
Trans-Mexican Volcanic Belt #REDIRECT Trans-Mexican Volcanic Belt The Trans-Mexican Volcanic Belt ( es, Eje Volcánico Transversal), also known as the Transvolcanic Belt and locally as the (''Snowy Mountain Range''), is an active volcanic belt that covers central-southern M ...
* Valleys of Mexico


References

{{DEFAULTSORT:Valley Of Mexico Valleys of Mexico, Mexico Mexican Plateau Trans-Mexican Volcanic Belt Regions of Mexico Landforms of Mexico City Landforms of the State of Mexico Landforms of Puebla Landforms of Tlaxcala Geography of Mesoamerica Geography of Hidalgo (state) Geography of Mexico City Geography of Puebla Geography of the State of Mexico Geography of Tlaxcala