Archaeology Magazine Archive

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An archaeologist strides across chalk outlines that mark the remains of the earliest irrigation system in America. First constructed in 1200, this complex network of canals fed perhaps 100 acres of maize plots near modern-day Tucson. (Courtesy Desert Archaeology Inc.)

On the outskirts of Tucson, Arizona, near a state-of-the-art sewage treatment plant that transforms waste into fertilizer, is another site that once represented the cutting edge of agricultural technology. While excavating ahead of an expansion of the treatment facility, Desert Archaeology Inc. uncovered a complex canal system that dates between 1200 and 800 B.C. Known as Las Capas ("the layers" in Spanish), it was the earliest large-scale system of its kind in the Southwest, and was built by people belonging to what archaeologists call the San Pedro Phase Culture. "It is the best exposure of an early canal system of just about any pre-Columbian time period in North America to date," says project director James Vint. "And this is all the more unusual when you consider how old it is."

The canals supplied water to an agricultural field system of about 60 to 100 acres and were preserved by a series of floods that gradually filled them in with clay and organic matter. The system brought water from the nearby Santa Cruz River in two canals that fed into eight smaller channels that spread it out over a checkerboard of fields with cells of about 250 to 450 square feet each. The early farmers would have used these plots to cultivate maize and the herb amaranth, which was grown for its young greens and mature seeds. It's possible the agriculturalists were ancestors of the Hohokham people, who were famous for building and maintaining a massive network of canals around what is now Phoenix from A.D. 500 to 1450.

The discovery is forcing archaeologists to reassess the idea that people from this period were primarily nomadic hunter-gatherers. "This puts them on the ground and anchors them to a place, and it requires a high level of social organization to operate a large irrigated system," says Vint.

Archaeologists used a specially designed backhoe to remove one to two centimeters of soil at a time from the site, which according to William Doolittle, professor of Geography at the University of Texas, allows researchers to get an unusually close look at the evolution of these fields. "It gives a nice picture of how dynamic land use really is, and how it varies from one year to the next. These are some of the oldest canals in the entire New World."

Vint's team also uncovered several clusters of three to five pit houses in the fields. Vint believes the houses were used by workers from a nearby village who tended the fields, kept sediment out of the canals, and rebuilt the irrigation network when flash floods destroyed it.

The village itself, which has been partially excavated, would have supported between 80 and 150 people. Skeletal remains of the inhabitants confirm they were quite healthy, with no signs of nutritional deficiencies. A catastrophic flood, however, buried the fields in 800 B.C., and the people moved on.

"In 35 years of looking," says project geoarchaeologist Fred Niles, "I've never seen a site, outside of Peru and Chile, that gave us such a detailed, day-to-day look at how early indigenous people used agriculture to make a living."