How Do You Farm a Lake? Build the Fields Out of the Mud

The method constructs new land in shallow water rather than draining it. Stakes are driven in a rectangle and woven with branches to make a retaining fence, and the enclosure is then filled with alternating layers of vegetation cut from the lake and mud dredged from the channels around it. The plot is raised until it sits a little above the water. Willows planted along the edges root into the structure and hold it together permanently, and the channels between plots are kept open for canoes and for the continuing supply of mud. The result is a grid of long narrow fields separated by water on every side.

Several advantages compound and explain the productivity:

• •Water reaches the roots from the sides constantly, so drought is impossible
• •Dredged mud renews fertility every season at no cost
• •Rotting vegetation in the beds supplies nutrients continuously
• •The surrounding water moderates temperature and reduces frost damage
• •Seedlings are raised separately on rafts and transplanted
• •Transplanting means a new crop starts the day the last is lifted

The system fed Tenochtitlan, which by the early sixteenth century held somewhere between one and two hundred thousand people and was among the largest cities in the world. Estimates suggest that the beds in the southern lakes supplied a substantial share of the maize, beans, squash, chillies and flowers the city consumed, with several harvests a year from each plot. Canoes carried produce along the channels directly into the city, which removed the transport cost that limited how far food could travel to any other city of comparable size. The concentration of population that this permitted is one of the reasons the Spanish accounts of the city are so astonished.

The system is older than the empire most associated with it, which is a common correction in the archaeology of the region. Evidence of the technique appears in the valley well before the Aztec period, with construction dated to several centuries earlier, and the Aztec contribution was expansion on an enormous scale under state direction rather than invention. Building out across the shallow southern lakes required controlling water levels, which meant substantial public works including a long dyke built in the fifteenth century to hold back saltier water from the northern lakes and keep the growing area fresh. Maintaining that separation was a state responsibility and its failure flooded the city more than once.

The system depended on a lake system that was deliberately destroyed. Colonial authorities began draining the lakes from the seventeenth century to stop the flooding that repeatedly damaged the new city, and the work continued for three centuries, culminating in large drainage tunnels and canals. The lakes are now almost entirely gone, the beds mostly dry, and the ground has subsided severely because the city stands on soft lake sediment being pumped dry, with parts of Mexico City having sunk more than ten metres. A remnant of the system survives at Xochimilco, where plots are still worked and the channels still carry boats, and it is protected internationally while being threatened by pollution and by the falling water table.

Fields built up in shallow water from layers of lake weed and dredged mud, held by staked fences and willow roots, are watered from the sides continuously and refertilised every season from the channels. Seedlings raised on rafts and transplanted let a new crop start immediately. The system fed a city of well over a hundred thousand, and draining the lakes ended it and left the ground sinking.