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Aerial view: canal and field system
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Preconstruction activities for a new wastewater treatment plant at Ina Road and Interstate 10 in Santa Cruz, Ariz., uncovered an elaborate and well-designed irrigation system dating back to ancient times. The resulting $6.8 million archaeological excavation of the prehistoric system, called “Las Capas” (or “The Layers”), is now part of Pima County's facility expansion. Photo: Desert Archaeology Inc.

When plans for Pima County's Regional Wastewater Reclamation Facility in the Sonoran Desert came to fruition early last year, the county uncovered a major hurdle before construction could begin: a 3,000-year-old irrigation system, the most intricate of its kind found to date in North America.

The ancient field system is one of the earliest examples of North American sedentary village life — which now has archaeologists thinking that the inhabitants, rather than being hunter-gatherers, were farmers who depended on agriculture as their primary food source. The system spans 60 to 80 acres and has six major layers of sediment, housing more than 200 maize (corn) fields and 170 canals.

Archaeologists already knew about extensive canal systems dug by the Hohokam Indians, who inhabited the land from about 1 to 1450 A.D. But Pima County's discovery predates those canals by 1,000 years, indicating an agricultural sophistication previously unheard of for that time period in the Southwest. The site's topmost layer dates back to 800 B.C.; and the sixth layer, which is 13 feet underground, dates to about 1200 B.C.

The unearthing of the prehistoric system was no surprise, says Loy Neff, program manager in the Pima County Cultural Resources and Historic Preservation Office.

“If they're going to build on the Santa Cruz River, they're going to find archeology,” he says. A road improvement project along Interstate 10 in the same area a decade ago indicated that something was there. But an exact location of the canals, called “Las Capas,” wasn't discovered until work on the treatment facility began.

State and county regulations require that a site that may contain historical artifacts be excavated before developing the land. The laws, however, didn't affect construction plans. MWH Constructors Project Manager Steven Schauer says that $6.8 million for archaeological investigation had been built into the project's budget; and only a few post-discovery adjustments — mostly pertaining to office setup — were needed.

For the excavation itself, the county contracted Desert Archaeology Inc. Two phases must take place before beginning ground-disturbance-causing projects on archaeological sites, says Neff. In the first phase, artifacts on the surface are analyzed and text excavation is performed to determine if the area is worth moving into phase two. The second phase involves intensive investigations: creating horizontal exposures to bring discoveries beneath the surface into plain view. Their work ended in late September, and Neff granted clearance in November for plant construction to continue.

Whatever's within the construction footprint will be damaged or destroyed. Peripheral areas, however, will be left in a preserved state. “Compliance requirements don't stop construction; instead, the archaeological excavations are completed to preserve the information and artifacts before construction proceeds,” says Neff.

The recovered artifacts will be analyzed, logged, and sent with photos and maps to the Arizona State Museum to be curated. The Pima County Regional Wastewater Reclamation Department also plans to exhibit Las Capas artifacts and maps at their new facility.