Why is Mexico City (CDMX) the way it is? Why does it get flooded? So many droughts? Such aggressive earthquakes? Why is it sinking? The key is in Tenochtitlan and how it managed its water A thread on how Tenochtitlan explains present-day CDMX, with maps and animations

You can see how a lot of present-day CDMX matches the low-lying areas of the Valley of Mexico, which is where the ancient Lake Texcoco used to be—and where Tenochtitlan emerged How did the Aztecs structure their capital? Handle its water? How does it determine CDMX?

This picture shows everything you need to know: • Tenochtitlan was on a big island in the middle of the lake • 2 urban centers (white) • Greenish island periphery • Causeways that double as dikes that criss-cross the lake (source: Tomas Filsinger)

Why did the Aztecs build their capital in the middle of a lake? Not because it was great land. It was a swamp! They were just escaping enemies, taking worthless land nobody wanted. They took the biggest and built from there Here's its evolution (Tomas J Filsinger):

1. CHINAMPAS The Aztecs gained land on the lake by piling up mud, containing it with tree trunks and reeds Since the mud was from river sediments, they were super fertile The Aztecs could get up to 7 harvests a year! So huge population growth That's how the city got 200k ppl

2. CAUSEWAYS / DIKES To connect the island to land, they needed to build causeways. These causeways doubled up as dikes, sectioning the lake. The dikes protected the city from floods

But they also turned the lake's salty water into a freshwater lake! The salty water flowed to the lowest part of the lake system, while the upper parts were constantly filled by fresh water from the mountains. Over time, the parts split by causeways accumulated the fresh water

This ancient map from around 1540 shows how the artist was aware of this water split: Water above the Dike of Nezahualcoyotl was fresh (clearer), and below it was brackish (fishermen are depicted)

3. TRANSPORTATION Building the city on a lake had another huge benefit: cheap transportation with canoes! Since Aztecs had no beasts of burden and didn't use the wheel for transportation, the alternative was human backs. Canoes could transport goods more easily

The result is that Tlatelolco (absorbed by Tenochtitlan) became the marketplace of the entire Lake Texcoco valley, making it the wealthiest

4. AQUEDUCTS Since it only had fresh water later on, and it was not fresh enough for drinking, Aztecs needed to get fresh water from springs, so they had to build aqueducts So causeways worked as: • Transportation • Flood mgmt • Freshwater mgmt • Aqueducts

5. ADMINISTRATION All these important infrastructure works pushed the state to emerge and centralize power (see for example how every dike was named after a king) This made the Aztecs into a strong state, with its administratino centralized in the Sacred Precinct

6. VOLCANOES The lake existed because water had no way out into the sea That's because the valley is surrounded by volcanoes (These volcanoes made the valley even more fertile) But they exist because of tectonic plates hitting each other This causes eruptions and earthquakes

7. GRID ORIENTATION Tenochtitlan was oriented in a grid because of their cosmology (replicating the grid of the older city of Teotihuacan) and to manage water (fresh water from south and west, brackish water to the east) Here, a sunrise in Tenochtitlan:

This is why Tenochtitlan was this way: • Lake➡️defense • Water➡️irrigation➡️chinampas ➡️population • Water➡️ cheap transportation➡️markets➡️wealth • Causeways ➡️infrastructure (transport+flood mgmt+fresh water+aqueducts) • Infra➡️strong state • Beliefs+geography➡️grid

How did that translate into CDMX? Ppl think the Spaniards destroyed the city in 1521 and didn't know how to rebuild it in harmony with water. But that's false! The Spaniards managed Mexico City's lakes for a longer time than the Aztecs! Map from 1550, and zoom in: Same causeways, canals, dikes...

Map from 1628, and zoom in: still the same

The Spaniards suffered the exact same issues as the Aztecs, with the same results. More floods meant more dikes and water mgmt. Here, the floods of 1629

1690: still the same

1715: still the same

By 1817, a big part of the lake had dried up. That's because the Spaniards had started trying to drain the water in the 1600s, and were finally getting somewhere at this point

We can see how the lakes really dried up in the 1900s, when the Mexican government (not the Spaniards!) was able to complete the Grand Canal to extract water from the valley

Here's where the pbms truly start: 1. FLOODS The water extraction is not fast enough to manage the very heavy rains of the rainy season in Jun-Sep. It rains more in CDMX than London!

2. DROUGHTS No lakes➡️no water reservoir. As tunnels and canals extract rain season water from the Mexico Valley, during the dry season (or dry years), there's not enough water, so CDMX must import water from other valleys, but it's frequently not enough

I ran out of tweets. Read today's article to find out about how all this explains CDMX's: sinking, earthquakes, population, pollution, ash, zocalo, streets, and more! And follow me for more threads like this!

Very quickly: • Water pumping from under the city➡️uneven sinking • The ancient lakebed amplifies earthquake waves • Volcano ring➡️Traps pollution • Active volcano➡️Ash • Spaniards destroyed the Aztec sacred precinct and built the Zocalo there