
The Valley of Mexico (; ), sometimes also called Basin of Mexico, is a highlands
plateau
In geology and physical geography, a plateau (; ; : plateaus or plateaux), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. ...
in central Mexico. Surrounded by
mountains
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher ...
and
volcanoes
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often fo ...
, the Valley of Mexico was a centre for several
pre-Columbian
In the history of the Americas, the pre-Columbian era, also known as the pre-contact era, or as the pre-Cabraline era specifically in Brazil, spans from the initial peopling of the Americas in the Upper Paleolithic to the onset of European col ...
civilization
A civilization (also spelled civilisation in British English) is any complex society characterized by the development of state (polity), the state, social stratification, urban area, urbanization, and symbolic systems of communication beyon ...
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 ...
, the
Toltec
The Toltec culture () was a Pre-Columbian era, pre-Columbian Mesoamerican culture that ruled a state centered in Tula (Mesoamerican site), Tula, Hidalgo (state), Hidalgo, Mexico, during the Epiclassic and the early Post-Classic period of Mesoam ...
, and the
Aztec Empire
The Aztec Empire, also known as the Triple Alliance (, Help:IPA/Nahuatl, �jéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥ or the Tenochca Empire, was an alliance of three Nahuas, Nahua altepetl, city-states: , , and . These three city-states rul ...
. The valley used to contain five interconnected lakes called
Lake Zumpango
Lake Zumpango (; ) is an endorheic basin located in the Valley of Mexico in the Municipalities of Mexico, municipality of Zumpango and adjacent to the municipality of Teoloyucan. Tzompanco was formerly the northernmost of five interconnected la ...
, Lake Xaltocan (),
Lake Xochimilco
Lake Xochimilco (; , ) is an ancient endorheic lake, located in the present-day Borough of Xochimilco in southern Mexico City. It is the last remaining habitat of the axolotl.
The lake is within the Valley of Mexico hydrological basin, in ...
,
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 on its former eastern shore.
Lake Chal ...
and the largest,
Lake Texcoco
Lake Texcoco (; ) was a natural saline lake within the ''Anahuac'' or Valley of Mexico. Lake Texcoco is best known for an island situated on the western side of the lake where the Mexica built the city of Mēxihco Tenōchtitlan, which would la ...
, covering about of the valley floor.
When the
Spaniards
Spaniards, or Spanish people, are a Romance-speaking ethnic group native to the Iberian Peninsula, primarily associated with the modern nation-state of Spain. Genetically and ethnolinguistically, Spaniards belong to the broader Southern a ...
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 of the Aztec Empire
The Spanish conquest of the Aztec Empire was a pivotal event in the history of the Americas, marked by the collision of the Aztec Triple Alliance and the Spanish Empire. Taking place between 1519 and 1521, this event saw the Spanish conquistad ...
, the Spaniards rebuilt the largest and most dominant city,
Mēxihco Tenōchtitlan, renaming it ''Ciudad de México'' (
Mexico City
Mexico City is the capital city, capital and List of cities in Mexico, largest city of Mexico, as well as the List of North American cities by population, most populous city in North America. It is one of the most important cultural and finan ...
) and over time began to drain the lakes' waters to control flooding.
The Valley of Mexico is located in the
Trans-Mexican Volcanic Belt
The Trans-Mexican Volcanic Belt (), also known as the Transvolcanic Belt and locally as the (''Snowy Mountain Range''), is an active volcanic belt that covers central-southern Mexico. Several of its highest peaks have snow all year long, and dur ...
.
The valley contains most of the
Mexico City metropolitan area
Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
, as well as parts of the
State of Mexico
The State of Mexico, officially just Mexico, is one of the 32 federal entities of the United Mexican States. Colloquially known as Edomex (from , the abbreviation of , and ), to distinguish it from the name of the whole country, it is the mo ...
,
Hidalgo,
Tlaxcala
Tlaxcala, officially the Free and Sovereign State of Tlaxcala, is one of the 32 federal entities that comprise the Political divisions of Mexico, Federal Entities of Mexico. It is divided into Municipalities of Tlaxcala, 60 municipalities and t ...
, and
Puebla
Puebla, officially the Free and Sovereign State of Puebla, is one of the 31 states that, along with Mexico City, comprise the Federal Entities of Mexico. It is divided into 217 municipalities and its capital is Puebla City. Part of east-centr ...
. 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. 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
Height above mean sea level is a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level ...
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, bounded from all sides by steep escarpments and standing distinctly above a surrounding plain. Mesas consist of flat-lying soft sedimentary rocks, such as shales, capped by a ...
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
Piedmont ( ; ; ) is one of the 20 regions of Italy, located in the northwest Italy, Northwest of the country. It borders the Liguria region to the south, the Lombardy and Emilia-Romagna regions to the east, and the Aosta Valley region to the ...
, 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 () is a river in 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 Mexico, which contains the metrop ...
, and eventually the
Páruco River and the
Gulf of Mexico
The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
.
Seismic activity
An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
is frequent here, and the valley is 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 ...
(800 BC to 800 AD) the
Toltec Empire
The Toltec Empire'', ''Toltec Kingdom or Altepetl Tollan was a political entity in pre-Hispanic Mexico. It existed through the classic and post-classic periods of Mesoamerican chronology, but gained most of its power in the post-classic. During t ...
(10th to 13th century) and the
Aztec Empire
The Aztec Empire, also known as the Triple Alliance (, Help:IPA/Nahuatl, �jéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥ or the Tenochca Empire, was an alliance of three Nahuas, Nahua altepetl, city-states: , , and . These three city-states rul ...
(1325 to 1521).
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
In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...
s trap contaminants in the valley. Over-extraction of groundwater 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 region and cultural area that begins in the southern part of North America and extends to the Pacific coast of Central America, thus comprising the lands of central and southern Mexico, all of Belize, Guatemala, El S ...
, including central Mexico, began to leave a hunter-gatherer existence in favor of agriculture sometime between the end of the
Pleistocene
The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided b ...
and the beginning of the
Holocene
The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
.
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
Chimalhuacán () (Nahuatl for "place of those who have shields") is a city and municipality located in the eastern part of State of Mexico, Mexico. It lies just outside the northeast border of Mexico City and is part of the Greater Mexico City u ...
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 (from and () from Ancient Semitic: ''gāmāl'') is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. Camels have long been domesticated and, as livestock, they provid ...
s,
bison
A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised.
Of the two surviving species, the American ...
and horses that could be hunted by man. However, the precise dating of these artifacts has been disputed.

Giant
Columbian mammoths 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,
Los Reyes Acozac,
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 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. The streets in Lindavista are named after cities in Latin America
Latin America is the cultural region of the Americas whe ...
to the center-north and
Coyoacán
Coyoacán ( ; , Otomi: ) is a borough (''demarcación territorial'') in Mexico City. The former village is now the borough's "historic center". The name comes from Nahuatl and most likely means "place of coyotes", when the Aztecs named a pre- ...
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, 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 village in the Valley of Mexico situated near the modern-day town of the same name in the Mexican Federal District. It was one of the first chiefdom centers to arise in the Valley, flourishing on the western sho ...
was a large
pre-Columbian
In the history of the Americas, the pre-Columbian era, also known as the pre-contact era, or as the pre-Cabraline era specifically in Brazil, spans from the initial peopling of the Americas in the Upper Paleolithic to the onset of European col ...
village and culture in the Valley of Mexico situated near the modern-day town of the same name in the
Mexican Federal District
Mexico City is the capital city, capital and List of cities in Mexico, largest city of Mexico, as well as the List of North American cities by population, most populous city in North America. It is one of the most important cultural and finan ...
. 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 (: necropolises, necropoles, necropoleis, necropoli) is a large, designed cemetery with elaborate tomb monuments. The name stems from the Ancient Greek ''nekropolis'' ().
The term usually implies a separate burial site at a distan ...
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 distribution, cosmopolitan group of more than 50 species which make up the genus of annual plant, annual or short-lived perennial plants collectively known as amaranths. Some names include "prostrate pigweed" an ...
, squash and
chili pepper
Chili peppers, also spelled chile or chilli ( ), are varieties of fruit#Berries, berry-fruit plants from the genus ''Capsicum'', which are members of the nightshade family Solanaceae, cultivated for their pungency. They are used as a spice to ...
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 in the southeastern Valley of Mexico, in what is today the borough of Tlalpan in Mexico City.
Construction of the Cuicuilco pyramid began a few centuri ...
.
This archaeological site is located where
Avenida Insurgentes Sur crosses the
Anillo Periférico
The Anillo Periférico known by locals as ''el 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 othe ...
in the
Tlalpan
Tlalpan ( , 'place on the earth') is a Boroughs of Mexico City, borough (''demarcación territorial'') in Mexico City. It is the largest borough, with over 80% under conservation as forest and other ecologically sensitive area. The rest, almost al ...
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 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. Produced from felsic lava, obsidian is rich in the lighter element ...
trade and at its peak was an important religious center and
pilgrimage
A pilgrimage is a travel, journey to a holy place, which can lead to a personal transformation, after which the pilgrim returns to their daily life. A pilgrim (from the Latin ''peregrinus'') is a traveler (literally one who has come from afar) w ...
for the valley.
In the early 8th century, with the rise of the
Toltec
The Toltec culture () was a Pre-Columbian era, pre-Columbian Mesoamerican culture that ruled a state centered in Tula (Mesoamerican site), Tula, Hidalgo (state), Hidalgo, Mexico, during the Epiclassic and the early Post-Classic period of Mesoam ...
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
, also known as Mexico-Tenochtitlan, was a large Mexican in what is now the historic center of Mexico City. The exact date of the founding of the city is unclear, but the date 13 March 1325 was chosen in 1925 to celebrate the 600th annivers ...
, 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
, also known as Mexico-Tenochtitlan, was a large Mexican in what is now the historic center of Mexico City. The exact date of the founding of the city is unclear, but the date 13 March 1325 was chosen in 1925 to celebrate the 600th annivers ...
. It was founded by the
Mexica
The Mexica (Nahuatl: ; singular ) are a Nahuatl-speaking people of the Valley of Mexico who were the rulers of the Triple Alliance, more commonly referred to as the Aztec Empire. The Mexica established Tenochtitlan, a settlement on an island ...
(
Aztecs
The Aztecs ( ) were a Mesoamerican civilization that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec people included different ethnic groups of central Mexico, particularly those groups who spoke the ...
) on a small island in the western part of Lake Texcoco in 1325, and was extended with the use of
chinampa
Chinampa ( ) is a technique used in Agriculture in Mesoamerica, Mesoamerican agriculture which relies on small, rectangle, rectangular areas of fertility (soil), fertile arable land to grow agriculture, crops on the shallow lake beds in the Va ...
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
dikes, canals and
sluice
A sluice ( ) is a water channel containing a sluice gate, a type of lock to manage the water flow and water level. There are various types of sluice gates, including flap sluice gates and fan gates. Different depths are calculated when design s ...
s. 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.
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, also known as the Triple Alliance (, �jéːʃkaːn̥ t͡ɬaʔtoːˈlóːjaːn̥ or the Tenochca Empire, was an alliance of three Nahua city-states: , , and . These three city-states ruled that area in and around the Vall ...
between Tenochtitlan,
Texcoco and
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, ethnically-based political entity, usually translated into English as "city-state", of pre-Columbian Nahuatl-speaking societiesSmith 1997 p. 37 in the Americas. The ''altepetl'' was ...
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 was a pivotal event in the history of the Americas, marked by the collision of the Aztec Triple Alliance and the Spanish Empire. Taking place between 1519 and 1521, this event saw the Spanish conquistad ...
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 nation, country, or state, in which residents and population, or some portion thereof, exercise self-government, and usually sovereignty, over its territory. The opposite of independence is the status of ...
.
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 facilities and systems that serve a country, city, or other area, and encompasses the services and facilities necessary for its economy, households and firms to function. Infrastructure is composed of public and pri ...
for the city, such as
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
, other power sources,
water supply
Water supply is the provision of water by public utilities, commercial organisations, community endeavors or by individuals, usually via a system of pumps and pipes. Public water supply systems are crucial to properly functioning societies. Th ...
and
drainage
Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess water. The internal drainage of most agricultural soils can prevent severe waterlogging (anaerobic conditions that harm root gro ...
. 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 flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geolog ...
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 (), nitrogen(IV) oxide
* Nitrogen trioxide (), o ...
s,
hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
, and
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
.
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 compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
and other dangerous compounds.

While still considered one of the most polluted places on the planet, the valley's
air pollution
Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...
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; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
contamination in the air with the introduction of
unleaded gasoline. Two other contaminants that have been brought under control are carbon monoxide and
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
.
The contamination problems that remain are primarily with ozone and
fine particles (
soot
Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Soot is considered a hazardous substance with carcinogenic properties. Most broadly, the term includes all the particulate matter produced b ...
) (between 2.5
micrometers
The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...
and 10 micrometers).
Thirty to fifty percent of the time, Mexico City's levels of
fine particles of ten micrometers, the most dangerous, exceed levels recommended by the
World Health Organization
The World Health Organization (WHO) is a list of specialized agencies of the United Nations, specialized agency of the United Nations which coordinates responses to international public health issues and emergencies. It is headquartered in Gen ...
.
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 in the air.
The effects on humans living in an enclosed, contaminated environment have been documented, especially by
Nobel Prize
The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
winner Dr
Mario J. Molina
Mario José Molina-Pasquel Henríquez (19 March 19437 October 2020) was a Mexican physical chemist. He played a pivotal role in the discovery of the Antarctic ozone hole, and was a co-recipient of the 1995 Nobel Prize in Chemistry for his rol ...
. He claims
fine particle 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.
Hydrology
The Valley of Mexico is a closed or
endorheic basin
An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...
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 an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
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 in the foothills and
upwelling
Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...
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,
Texcoco,
Chalco-
Amecameca
Amecameca is a municipality located in the eastern panhandle of Mexico State between Mexico City and the Iztaccíhuatl and Popocatépetl volcanos of the Sierra Nevada mountain range. It is located on federal highway 115 which leads to Cuautla, ...
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), a slang word meaning "lesbian"
* Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks
* Dike (mythology), ''Dikē'', the Greek goddess ...
for flood control and to separate the
saline water
Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish wat ...
of the northern lakes from the
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...
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
desagüe
The ''Desagüe'' was the hydraulic engineering project to drain Mexico's central lake system in order to protect the capital from persistent and destructive flooding. Begun in the sixteenth century and completed in the late nineteenth century, it ...
, which was pursued throughout 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 which would also divert waters from the
Cuautitlán
Cuautitlán (, Otomi: ), is a municipality in the State of Mexico, just north of the northern tip of the Federal District (Distrito Federal) within the Greater Mexico City urban area. The city of Cuautitlán is the municipal seat and makes up ...
River away from the lakes and toward the
Tula River
The Tula River () is a river in 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 Mexico, which contains the metrop ...
. This project was undertaken by
Enrico Martínez 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 municipalities of Mexico, municipality located in the Zumpango Region of the State of Mexico in Mexico. The municipality is located north of Mexico City within the valley that connects the Valley of Mexico with the Mezquital Vall ...
. 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 (; ; 15 September 1830 – 2 July 1915) was a General (Mexico), Mexican general and politician who was the dictator of Mexico from 1876 until Mexican Revolution, his overthrow in 1911 seizing power in a Plan ...
(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 wall corrosion of the diameter pipe.
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 (, ; , ) is a traditional, flat-bottomed Venetian rowing boat, well suited to the conditions of the Venetian lagoon. It is typically propelled by a gondolier, who uses a rowing oar, which is not fastened to the hull, in a scul ...
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 (literally "Promenade of La Reforma, the Reform") is a wide avenue that runs diagonally across the heart of Mexico City. It was designed at the behest of Maximilian of Mexico, Emperor Maximilian by Ferdinand von Rosenzweig d ...
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 a borough (''demarcación territorial'') in Mexico City, located on the eastern side of the city. The borough is named after and centered on the formerly independent municipality of Iztapalapa (officially Iztapalapa de Cuitláhua ...
, 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 with a system of pipes, tunnels, pump stations etc. to transport sewage and surface runoff, urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the se ...
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 (), is a large arid-to-semiarid plateau that occupies much of northern and central Mexico. Averaging above sea level, it extends from the United States border in the north to the T ...
*
Trans-Mexican Volcanic Belt
The Trans-Mexican Volcanic Belt (), also known as the Transvolcanic Belt and locally as the (''Snowy Mountain Range''), is an active volcanic belt that covers central-southern Mexico. Several of its highest peaks have snow all year long, and dur ...
*
Valleys of Mexico
References
{{DEFAULTSORT:Valley Of Mexico
Mexico
Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
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