Valley of Mexico

Where it is

The Valley of Mexico is a high basin — around 2,200 metres — ringed by mountains and, crucially, closed: water that falls in it has no natural outlet to the sea. Historically it held a chain of shallow lakes, fresher in the south where springs and rainfall fed them and saltier in the north where evaporation concentrated them. That single hydrological fact runs through everything: how the Mexica built, why the Spanish city flooded, why four centuries were spent digging a way out, and why the ground is now sinking.

The land and water around it

• The lake systemlake

  Texcoco, Xochimilco, Chalco, Xaltocan and Zumpango: a connected chain rather than a single body, with a salinity gradient from south to north. Only fragments survive, at Xochimilco most visibly.

• The surrounding sierrasmountains

  Volcanic ranges enclosing the basin, including Popocatépetl and Iztaccíhuatl to the south-east. They are the reason the basin is closed and the reason it is seismically and volcanically active.

• The lake-bed claysbasin

  Soft, water-rich sediments beneath the modern city. They compact when the water is pumped out of them, and they amplify earthquake shaking — the same material behind both the subsidence and the 1985 damage.

Its history, in short

The basin has held large settlements for two thousand years. Teotihuacan, in an adjoining valley, was a planned city of substantial scale that was abandoned centuries before the Mexica arrived. Tenochtitlan, founded on an island in 1325, grew by building into the lake — causeways, an aqueduct, a dyke separating fresh water from salt, and chinampas raised from lake mud. After 1521 the Spanish city was built on the same site and repeatedly flooded, and the response was to give the closed basin an artificial outlet. It worked; the lakes largely went; and the bill arrived in the twentieth century as subsidence.

Timeline

Dates are shown on a timeline rather than as changing shapes on the map. The globe draws modern boundaries only, and tinting a modern outline to stand for a historical one would state something we cannot source.

Places that carry meaning

• Templo Mayor14th–16th centuries

  The main temple precinct of Tenochtitlan, rebuilt in successive enlargements and excavated beneath the centre of the modern city from 1978. Each rebuilding encased the last, so the structure is a stack of its own history.

• XochimilcoPre-Hispanic to present

  The surviving chinampa landscape in the basin's south — raised beds built from lake mud and still farmed. It is the only place the pre-drainage relationship with the water is still legible on the ground.

• Teotihuacanc. 1st–6th centuries

  A planned city on a scale that required organised labour and long-distance exchange, abandoned long before the Mexica, who gave it the name it is known by and treated it as an ancient place.

Learn the language here

1. 1Understand which languages are spoken hereValley of Mexico is multilingual. Start with which languages are used, by whom, and for what — the phrase list only covers one of them.Written by Globe Quest

Check yourself

• What makes the Valley of Mexico a closed basin?

  Water entering it has no natural outlet to the sea

  Water accumulates and leaves mainly by evaporation, which is why the basin held lakes and why draining it required cutting an outlet.

• How does pumping groundwater make flooding worse?

  It compacts the lake-bed clays, so the surface sinks and drainage loses its gradient

  Subsidence flattens or reverses the drainage designed to run downhill, and breaks the pipes, which leak and increase demand.

Worth arguing about

• Tenochtitlan lived with the lake; the modern city removed it. Was the drainage a mistake, or a reasonable choice with a very long bill?
• The soft ground that made the basin habitable amplified the 1985 earthquake. How should a city weigh a hazard that is inseparable from its advantages?

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