Flooding and salinity were two problems which plagued the Valley of Mexico during pre-contact times. As a result, the native inhabitants undertook construction of some quite impressive causeway-dikes. Lakes Xochimilco and Chalco were separated by one and so were Texcoco and Xochimilco. Construction of these works was possible because of shallow lake depths. Citing Linne (1948), Moriarty (1969) offers the following depth measurements, all of which are for 1861: 2.4 meters for Chalco, 3.0 meters for Xochimilco, and 0.5-1.0 meters for the rest.
The “highest point
of Indian engineering” was the Albarron de Nezahualcoyotl, a sixteen kilometer
long dike which “cut off” Texcoco from the southern lakes. It was constructed following the 1440’s great
floods as part of a cooperative project between Montezuma I of Tenochtitlan
and king Nezahualcoyotl of Texcoco (Moriarty 1969). This dike was incredibly important in the control of salinity.
The ‘Albarron de Nezahualcoyotl’ created behind it a freshwater lake, fed from the springs of Xochimilco and Chalco, out of what had been part of the highly saline Lake Texcoco . . . Water was manipulated by means of sluice gates which could be closed during the rainy season to keep the salt water from Lake Texcoco from flooding the Lake and the City of Mexico (Moriarty 1969).
Because eastern Lake Texcoco represented the lowest-lying region in the Valley, it is also where the salt accumulated. By preventing the saline waters of Texcoco from flooding and infiltrating both western Lake Texcoco and the southern lakes, the natives were able to create a completely freshwater zone that enabled chinampa agriculture to advance at a larger scale.