Visitors to Chaco are immediately entranced by the ruins of the Great Houses, and with good reason. They are stately, regal, and mysterious. But perhaps the most important, life-sustaining feat of engineering and construction is not easily visible to the modern tourist.
A sophisticated system of check dams, ditches, irrigation canals, and gridded fields existed to capture and exploit the runoff of the rainfall from the high cliff walls and smaller side canyons. These smaller side canyons would funnel large volumes of rainwater from thunderstorms, turning them into violent, short-lived torrents of water crashing down into the canyon.
A check dam is a small dam constructed across a waterway to collect, store, or re-direct water. In terrains such as Chaco, violent summer rainstorms strike suddenly, and everything changes just as suddenly. For the Anasazi, water would have come roaring down the side of the canyon, flooding agricultural fields and destroying crops. However, with a check dam, this could be avoided, and the water, while detrimental in flood, could be collected and used wisely.
When the rains exploded from the skies, the women leapt into action. They would take empty ollas to the pools of collected rainwater, fill their jars, store them safely, and repeat until the water supply had been replenished.
After the rains passed, and fields began to dry, they would open the check dam temporarily, to allow water to flow into the fields and serve the thirsty crops.