Gone with the Flow: Ancient Andes canals irrigated farmland

Archaeologists working in a valley on the western slopes of Peru’s Andes mountains have discovered the earliest known irrigation canals in South America, a find that illuminates the origins of large-scale agriculture in the New World.

Tom D. Dillehay of Vanderbilt University in Nashville and his colleagues came across three buried canals, stacked one on top of the next, on the southern side of a river running through the Zana Valley. Radiocarbon measurements of carbon fragments date the bottom canal to about 5,380 years ago, the researchers report in an upcoming Proceedings of the National Academy of Sciences. They estimate the ages of the middle and top canals at around 4,390 years and 1,190 years, respectively.

The researchers also found traces of a fourth canal, about 6,700 years old, beneath the others.

Remnants of at least 51 sites once inhabited by people dot the countryside around the canals, Dillehay says. A majority of the sites, ranging from 4,500 to 6,000 years old, include remains of domesticated cotton, beans, squash, and coca. Preserved bits of both wild and cultivated plants have turned up at earlier sites.

The Zana Valley canals were designed to pipe water from a nearby river into adjoining agricultural fields, Dillehay says. The earliest canal was 2 feet wide and ran for slightly more than 1 mile. Each succeeding canal was built wider, longer, and with a flatter gradient. The top channel was 4 feet wide and stretched about 2.5 miles.

The two older canals were dirt ditches with pebbles and rocks along the bottom. In the third canal, rough stone and burned clay line the channel.

Dillehay’s group has also uncovered probable irrigation canals on Peru’s north coast at three villages inhabited between 6,500 and 4,700 years ago.

“By 5,000 years ago, irrigation canals likely had developed in lots of different places,” Dillehay says.

With a constant supply of water from the canals, farmers could expand their collective food-growing effort to large plots of land.

River sediment eventually filled each ancient canal, according to the researchers. The region’s inhabitants periodically built a new canal on top of the previous one to continue using the best path to draw water from the river to the fields.

Several Andean sites from more than 4,000 years ago contain planting areas coated by what appears to be river sediment, which may have overflowed from irrigation ditches, notes archaeologist Michael E. Moseley of the University of Florida in Gainesville. “However, [Dillehay’s] article is the first report of ancient irrigation canals that have actually survived,” Moseley says.

The Zana Valley lies in an archaeologically neglected region, the arid Andean sierra, which is between about 6,000 and 9,000 feet in elevation. Andean researchers usually focus on coastal areas and high-elevation mountain basins that supported large prehistoric populations, Moseley notes.

Building and maintaining the ancient irrigation canals required a collective effort orchestrated in the name of the gods, he proposes. For religious rituals, inhabitants erected flat-topped mounds just across the river at about the time that they constructed the oldest canal.