Ancient Agriculture at Tiwanaku (by Brian Fagan)
Quicktime movies were taken from National Geographic Explorer covered of the Aymara and Tiwanaku.
In the closing centuries of the first millennium AD, the farmers of Tiwanaku in northern Bolivia supported thousands of non-food producers by intensive cultivation of local swamps. When the city was abandoned, the farmers dispersed and their innovative swamp agriculture was forgotten. Today, the local Aymara Indians eke out a living from arid hillsides, where irregular rainfall and winter frosts regularly decimate the meager potato crops from the thin soil. Many of them own lands on the Pampa Koani, the lake floodplain, where boggy conditions and severe frosts
alternately rot and freeze growing tubers. 1000 years ago, the landscape was very different, for the floodplain was covered with rows of lush raised gardens intersected with canals. The fields literally burst with a bounty of potatoes, more than enough to feed 50,000 people, many of them non-farmers. Bolivian archaeologist Oswaldo Rivera and his University of Chicago colleague Alan Kolata teamed up some years ago to investigate the thousands of ridges and depressions that covered the plain around Tiwanaku. They soon discovered that they were looking at a vast, abandoned agricultural system and persuaded a local farmer to allow them to dig out the silted canals on his land, to recreate the ancient raised fields. Despite vigorous opposition from his fellow villagers, the farmer agreed, with dramatic results. The potato plants grew higher than he had ever seen. When a
severe frost descended on the Altiplano, the villagers watched over their fields all night. The crops in the hillside were ruined, but the potatoes on the raised field below were barely damaged. At dawn, a thin, white mist covered the plot, protecting the precious crop, a fog blanket caused by the heat retained by the surrounding canals. The mist soon burnt off in the warm sun, but returned every night the temperature went below zero. Therein lay Tiwanaku's hydrological genius, for her farmers devised a simple, highly effective way of protecting their crops, while planting them in exceptionally productive, well watered and easily fertilized soil.
Rivera and Kolata found that Tiwanaku's rulers invested vast resources in reclaiming flat altiplano land, especially during and after the great drought of the 6th century AD. By creating ridged fields and carefully conserving the soil, the overseers of huge field systems based on state-founded settlements were able to obtain high crop yields from hitherto unproductive land. Their agricultural systems were part of an extensive network of terraced, stone-walled houses and courtyards, many containing burials. The canals were sophisticated constructions, with a base of cobblestone topped with gravel and impermeable clay, which kept salt from the lake's brackish waters from seeping into the overlying topsoil. These large field systems supported a population of 40,000 to 120,000 people in the 32 square mile Tiwanaku valley alone. Their productivity may have been as high as 400% more than current yields. 
The rediscovery of these ancient farming techniques is paying off handsomely among the Aymara. About 1,200 farmers have now redeveloped raised fields and at least another 50 villages want training in prehistoric agriculture. The local diet is improving dramatically, for fish and ducks in the canals provide added nutrition in a country where over half the children suffer from malnutrition.