Bart Schultz
When the Aztecs settled in the area in 1245, a complex of four interconnected lakes had formed in the valley: Zumpango, Xaltocan, Texcoco and Xochimilco. The construction of a dike separated Lake Xaltocan into Lakes Xaltocan and San Cristobal, Lake Texcoco into Lakes Texcoco and Mexico, and Lake Xochimilco into Lakes Xochimilco and Chalco.
When the Aztecs arrived, all the good land was already occupied, and no city would allow these unknown people to settle anywhere near them. They were essentially sent from place to place. The Aztecs tried to settle anyway, which resulted in a brief war that drove them into the swamps of Lake Texcoco. There, they found shelter in the reeds. According to the chronicles, their god advised them to build their new city on Tenochtitlan, a small, uninhabited island in the reed swamps.
After some time, the Aztecs realized that their new location offered several advantages: there were no land claims by third parties, the island was completely surrounded by swamps (which gave the inhabitants a significant defensive advantage in times of war), three mainland cities (offering good trade opportunities) were nearby, there was great potential for expansion, and the surrounding waters offered good transportation. A canoe, steered by a single man, could carry the load of many porters. Floating gardens (chinampas) were constructed from silt mixed with reeds and waste, while the swamps were gradually filled with earth and stones.
From this location, the Aztecs expanded their empire and achieved a high level of civilization. Some examples of their civil engineering skills included the use of canal irrigation, the construction of the 16 kilometres long and 20 metres wide Albarradón de Nezahualcóyotl (a dike that separated Lake Texcoco from Lake Mexico and prevented flooding in Tenochtitlán), the use of discharge sluices in the dike, the construction of a three kilometres long aqueduct for drinking water, the construction of dams, and dredging, using the spoil to further fill the ground.
With the arrival of the Spanish in 1519, many dikes were destroyed. The lakes became a major problem, primarily due to flooding that lasted for several years. After the Spanish conquest of the Aztec Empire, Spanish flood control efforts led to the reclamation of most of the lake area. The area of the original Lake Texcoco is now almost entirely occupied by Mexico City, with Tenochtitlan as a suburb.
Mexico City suffered from periodic flooding, the worst occurring in 1555, 1579-1580, 1604, and 1607. A drainage canal was then constructed to control the lake's level, but in 1629, another flood left most of the city submerged for five years. The Spanish authorities decided to maintain the city at its current location. Although various hydraulic engineering projects were completed in the 17th century, it was decided to completely eliminate the lake complex. Ultimately, the excess water was discharged through canals and a tunnel through the mountains to the Pánuco River, which flows into the Gulf of Mexico. Even this could not stop the flooding, which regularly submerged most of the city.
After another catastrophic flood in 1878, a 50 kilometres long drainage canal (Gran canal del Desagüe) with the Tequixquiac Tunnel (Tunel de Tequixquiac) was built. The project was completed in 1900. This drainage system was considered the most important hydraulic engineering infrastructure built in Mexico in the 19th century.
It wasn't until the 20th century that the floods were brought under control. In 1967, the deep drainage system (Drenaje Profundo), a network of several hundred kilometres of tunnels, was constructed at a depth between 30 and 250 metres. The central tunnel has a diameter of 6.5 metres to enable the discharge of excess water from the area. The 62 kilometres long eastern discharge tunnel only came into operation in 2019. In fact, this system created by far the highest polder in the world, at approximately 2,240 metres above mean sea level.
A problem, however, is that large-scale groundwater extraction and the resulting lowering of the water table are causing significant land subsidence. It is estimated that the central area of Mexico City has subsided by 10 metres over the past 100 years. The clay soil continues to settle, and the city continues to subside, but due to its high elevation, it will always remain within the highest polder.
