Why Is Mexico City the Way It Is?

How Tenochtitlan Became Mexico City

This is the last article in the Mexico series. I lingered on the topic, but I really did want to get to this one because you can’t understand Mexico City (the biggest city in the American continent) otherwise. I think the link between Tenochtitlan and Mexico City is fascinating! I hope you enjoy it as much as I did writing it. Again, thanks to Shoni, Heidi, and MajoraZ for their help editing this article!

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Why is Mexico city so populated?
Why is it sinking?
Why is it so prone to earthquakes?
Did the Spanish really destroy Tenochtitlan and rebuild a completely different city?
How can we see the ancient Tenochtitlan in present-day Mexico City?

The answers lie in why the Aztecs built Tenochtitlan the way they did, and the key to everything is water.

Ancient Lake Texcoco, with its banks outlined over the present-day CDMX and the topography of the area

Why Was Tenochtitlan the Way It Was?

Here’s an overview of Tenochtitlan at the time of the Spaniards, to give you a feel for the city:

Source: Isaac Zuren

And this static picture contains nearly everything you need to understand why Tenochtitlan was built the way it was.

Here’s what you should notice:

• Tenochtitlan is on a big island in the middle of Lake Texcoco

• It has two urban centers (in white)

• The periphery of the island is rather greenish

• Causeways that double as dikes criss-cross the lake, connecting the island to dry land

• There are additional dikes, notably the one to the right (east)

Why did the Aztecs build their city this way? You can tell by looking at the topography of the area:

Nearly all the blue areas in the middle—the lowest-lying ones—were covered by lakes 500 years ago. 

The Water of the Valley of Mexico

Lake Texcoco was the largest. You can see the lakes to the south, Xochimilco and Chalco, and those to the north, Zumpango and Xaltocan. Note that Lake Texcoco has the same name as the city of Texcoco to the east. I assume it was the most important city on the lake when the Mexicas arrived. Similarly, Lakes Xochimilco and Chalco take their name from nearby cities. Note that this just shows the most important cities. There were many more towns and villages that you can see here.

As we have seen, this region was rich with lakes because it was surrounded by ancient volcanoes that prevented water from escaping. 

The lowest-lying lake was the central one, Lake Texcoco, which means all the water eventually ended up there.

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This also means that lakes like Chalco and Xochimilco had fresh water, while Texcoco’s water was brackish: All these lakes were fed fresh water by surrounding rivers—bringing some salt with them—and as water evaporated, salt concentration increased. This saltier water would flow from smaller lakes into Lake Texcoco, continuously adding salt, while the upper lakes were replenished by fresh water.

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Having fresh water, the cities on the southern lakes built chinampas, plots of land built on the lake, for agriculture and residential land:

This is a more accurate representation:

Model of chinampa agriculture, National Museum of Anthropology, Mexico City. Source.

Some of these chinampas can be seen to this day!

Modern day chinampas on the southern end of Mexico City, in the Xochimilco area. Source.

As we have seen, the chinampas were extremely fertile, yielding up to seven harvests a year. They were possible because these lakes were generally quite shallow (about 4 m, or 13 ft). It was easy to pile up mud and anchor it with reeds and by planting trees on the banks.

Close-up of a chinampa. Source: Ian Mursell/Mexicolore. Via this.

The chinampas had a beneficial side-effect: Transportation between them was by canoe, promoting a canoe culture. Since the Aztecs had no draft beasts or wheels for transportation, their only alternative on land was carrying goods on their backs. Canoe transportation was much better. 

Canoes between chinampas in Xochimilco today. Source.

But for these laketop gardens to survive, the cities on the southern lakes—Xochimilco and Chalco—needed fresh water, yet sometimes their lakes became salty when the rainy season swelled Lake Texcoco and its brackish waters flooded them.

Heavy rains over Lake Texcoco, Thomas Kole

The southerners realized they had to stop Lake Texcoco from overflowing and built a causeway that doubled as a dike to cut off Lake Texcoco from Lake Xochimilco.¹

Artist’s impression of building a causeway near Tenochtitlan; illustration by Alberto Beltrán, from The Sun Kingdom of the Aztecs by Victor W. von Hagen, Brockhampton Press, 1958. Via this.

This is how large they could be:

This is a cross-section of a causeway:

Here with a bit more detail:

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They were mixes of sand, lime, stone, and wood. Here’s a zoom into the dike core.

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This causeway was located here:

This dike likely required the collaboration of thousands of people, calling for some sophisticated coordination, which always builds up states.

This so-called dike of Mexicaltzinco sealed off and protected their freshwater system.

Map of a chinampas in the Chalco region (the southernmost lake) showing how they were organized in the sixteenth century. A large blue canal, filled with fresh water, rings the perimeter of the fields; large trees served as boundary markers and to hold the soil. The grey stripes, running both vertically and horizontally to slow the flow of water, are the chinampas. Among them are small “islands” ringed in green. These are the high grounds, and the houses drawn on the three lower islands and the church on the top represent the small settlements that were interspersed among the irrigated fields. Source: Archivo General de la Nación de Mexico.

Now imagine you’re a Mexica, and you escape your enemies by settling on a forsaken island in the middle of a brackish, swampy lake.

What’s the very first thing you need to do?
Get fresh water to drink. 

As you can see on the map, there are a few rivers close to the coast, to the west. Early on, the Mexicas had to canoe to the rivers, but eventually they wanted to build an aqueduct into the city. As we have seen, this request from the coastal neighbors sparked a war, which led to the formation of the Triple Alliance between Tenochtitlan, Tlacopan, and Texcoc, and the emergence of the Aztec state. What this means is that water was the direct cause of development, infrastructure, and conflict here. In other words, the local water setup demanded the emergence of a strong state, especially if it came from an island that didn’t have access to fresh water.²

As the Triple Alliance gained power, an obvious next target were the freshwater lakes to the south, full of chinampas that fed the valley. So they conquered them. Throughout this time, Tenochtitlan had been copying these chinampas, and I assume that accelerated after the conquest.

Chinampas in Tenochtitlan. Thomas Kole.

Over time, Mexicas had also been building their own causeways to reach the coast:

Aztec lands around 1390. Source: Tomas Filsinger.

They would have realized that the water in enclosed parts of the lake between causeways became fresher over time, which accommodated chinampas. They also learned from the example of the southern lakes. So they started building more causeways for transportation and to section off parts of the lake to transform their salt water into fresh water.

The Dike of Mexicaltzingo is at the bottom left, running east-west. Artist: Tomas Filsinger.

Notice the dike at the right (east) of this picture, running north to south, and how it separates waters. That’s the famous dike of Nezahualcoyotl, which split Lake Texcoco in half and started making the waters of all Tenochtitlan even fresher:

Dike of Nezahualcoyotl. It was 16 km long! (10 mi). Source.

In order not to impede the flow of water from western rivers into the Lake Texcoco, the Mexica ran canals west to east through their city; these proved useful for transport through the growing metropolis. Such a system of dikes and canals required constant maintenance and upgrading. This work was part of the labor obligation—a kind of tax paid by working—of many towns in the Valley.

Now remember I said they also had aqueducts to transport water from the springs to Tenochtitlan. Indeed, the water in the lakes became fresher with these dikes—good enough for agriculture, but not for drinking. So these causeways also transported fresh water!

Some of the Tenochtitlan waterworks. Here the aqueduct that brought potable water to Tenochtitlan. It had two tubes so that one could be closed for maintenance while the other one remained in service. It’s not clear whether the aqueduct had twin ceramic pipes or stone channels. This doubles up as a good representation of the causeway. Source: Voyages d’Alix: Les Azteques. 

This is a cross-section:

Causeway construction: illustration by Professor Michael E. Smith. Note this shows the aqueduct as a channel rather than a pipe. We just don’t know which one it was. Via this.

Notice in the previous illustration how there were bridges allowing canoes to pass—and to regulate the water levels.

In other words, the beauty of these causeways was that they had many functions:

• Allow walking transportation across the lake

• Shrink the area of the lake that was salt water and replace it with fresh water, which then allowed for chinampas, hence agriculture, and a much bigger population

• Control floods, allowing swelling to be contained in Lake Texcoco

• Bring freshwater from springs via aqueducts

• Facilitate canoe traffic, crucial to cheap transportation (and therefore trade)

The importance of all this water management was evident to Aztec citizens, who experienced water every day—from canoe transportation, chinampa irrigation, fresh or salt water, and even floods that could swallow their streets, kill people, and carry away the adobe walls of their dwellings.

In the rainy summer months, Chalchiuhtlicue’s waters could break like an uncontrollable surge of amniotic fluid, invading the city, rushing down its streets, uprooting the carefully planted chinampas, washing away the adobe buildings that many called home.—The Death of Aztec Tenochtitlan, the Life of Mexico City, Barbara Mundy

This is a section of a map of Tenochtitlan painted perhaps in the 1540s and conserved at the University of Uppsala. There is a stone wall—the dike of Ahuitzotl—which runs along the border of the city. Another wall, the dike of Nezahualcoyotl, runs through the middle of the lake. The artist has been careful to indicate the differences in water quality using pigment. The zone between these two dikes is fresh water, shown by the application of a rich blue pigment. The zone at the bottom of the image, the salty water of the Lake of Texcoco, is shown in a muddier, greenish color. The artist also shows the richness that the lakes provided, as figures in boats trap birds and fish with exuberance. Fun fact: The dike of Ahuitzotl was likely built later, and was not meant to stop saltwater from Lake Texcoco (The bigger Nezabualcoyotl dike did that), but rather to stop the swellings of fresh water from southern lakes from flooding the city of Tenochtitlan. Via this.

And as I mentioned earlier, this type of public works was crucial to justify a quick emergence of a powerful state. Here’s one additional clue to that mechanism: Each one of these causeways carries the name of the ruler behind it. 

This is one of the most detailed maps of Tenochtitlan at the time Cortés arrived. Notice all the small green squares: They are chinampas. Source: BigRedHair.

Tlatelolco

Remember there were two urban centers in Tenochtitlan? The northern one was not Tenochtitlan, but Tlatelolco. It used to be a different island, inhabited by a different Mexica group, but eventually the populations merged and Tenochtitlan prevailed.

Source: Tomas Filsinger

But the previous water barrier between the two was handy for traffic flow, as the Tlatelolco market became the biggest one in the Americas:

We’re seeing this from the east looking west, so the north is right. Notice how Tenochtitlan’s bigger island is to your left (south), and there’s a water channel between the two, ideal for plenty of canoes to easily access the Tlatelolco market. Thomas Kole.

That market was huge:

We again turned our eyes toward the great market, and beheld the vast numbers of buyers and sellers who thronged there. The bustle and noise occasioned by this multitude of human beings was so great that it could be heard at a distance of more than four miles. Some of our men, who had been at Constantinople and Rome, and travelled through the whole of Italy, said that they never had seen a market-place of such large dimensions, or which was so well regulated, or so crowded with people as this one at Mexico.—Bernal Diaz del Castillo, one of Cortés’s soldiers.

According to him, you could find thousands of items in the market.³ 

Between 20,000 and 40,000 people visited the market daily—up to 60,000 on the special weekly market day.⁴ 

We’ve seen in previous articles that the best places for markets to emerge are where transportation is easy across regions. And since the Valley of Mexico was one of the most populous, with many cities on the lake banks and cheap water transportation, it turns out that an island in the middle of Lake Texcoco was the ideal place for a huge market to emerge—as long as the canoes could dock at the market itself.

Tlatelolco market. Can you see the docks?

My guess is that Tlatelolco specialized in the market, whereas Tenochtitlan specialized in administration, with the lake acting as a formidable defense for both.⁵

The rest of buildings in the Sacred Precinct of Tenochtitlan housed temples for different gods, shrines, residences for priests, rows of skulls from enemies and sacrifices, and schools. The city also had a botanical garden, a zoo, and an aviary.

The Templo Mayor (a pyramid on a swamp!) was the main building in the city:

It’s very telling of the Aztec civilization that the two temples on top of this pyramid were dedicated to the gods of rain and war. 

The palace of the rulers, last used by Moctezuma II, was adjacent to it.

Thomas Kole

Of course, it was lavishly decorated:

Scott and Stuart Gentling, Amon Carter museum, via this

The city had been organized along a north-south / east-west grid for two reasons:

1. The orientation of the lakes, as Lake Texcoco was north of Lake Xochimilco, so fresh water flowed south to north from there, while the springs to the west meant fresh water flowed west to east.

2. Cosmology, which was very important to Aztecs⁶, and focused on natural phenomena like volcanoes (hence pyramids), rains, the Sun, and the Moon.

It’s believed that, during or near the equinoxes, sunlight would filter between the two shrines of the Templo Mayor, either during sunrise or sunset (which means this picture shows the sunrise, and east is up). Source: Thomas Kole.

So now we have the key ingredients to understand why Tenochtitlan, a city of about 200k people, was the way it was:

• Strong defenses from the lake

• Water for irrigation, which allowed lots of agriculture through chinampas, which increased the size of the city

• Causeways built for transportation, but doubling as dikes to manage water

• Infrastructure works that facilitated the emergence of a strong state

• Cheap transportation by canoes in a central part of the Valley of Mexico, making Tenochtitlan its central marketplace⁷

• An orientation along north-south/east-west lines due to local geography and astronomy

Unfortunately, Spain is a dry country, astronomy is not as important in Christianity, and Spaniards barely cohabited with the Aztecs enough to understand how Tenochtitlan worked. Therefore, Spaniards understood very little of why the city worked the way it did. And so they changed the city, from a Tenochtitlan perfectly adapted to its surroundings, to a Mexico City that wasn’t.

Or so most people think.