The Las Capas Canals: Three Thousand Five Hundred Years of Irrigation

About a thousand years after the Egyptian pyramids were completed, people were still digging irrigation canals in the Santa Cruz River valley.

That is not a rhetorical flourish. At the Las Capas site excavated in Marana, north of Tucson, archaeologists discovered more than two hundred and fifty canals in multiple stratigraphic levels, along with the bordered fields they served. Radiocarbon dating placed the oldest of those canals at three thousand five hundred years before the present. These are the earliest confirmed irrigation agriculture canals in North America.

The people who built them are called the Early Agricultural period people, predecessors of the Hohokam who would later build the more famous canal network in the Salt River Valley near Phoenix. In the Santa Cruz valley, the engineering ran for more than eight hundred years at the Las Capas site alone, a layered record of a society that adapted, expanded, and refined its water management across dozens of generations. The canals were not crude ditches. They were graded to maintain flow rates across the variable terrain of the valley floor.

The Hohokam who followed built on this tradition. Their canal network in the Tucson basin, dating from roughly nine hundred to eleven hundred CE, was a direct continuation of a farming adaptation that had been evolving here for two millennia before they arrived. When Spanish missionaries and then Anglo settlers came to this valley and found productive irrigated fields, they were looking at the end product of three and a half thousand years of engineering refinement.

The Las Capas site is now largely beneath an interchange in Marana. The canals were excavated before construction, documented, and then paved over. Their record lives in the Arizona State Museum.