Cave Creek switched to reservoir storage Wednesday after a clog in its canal intake system forced the town's water treatment plant to shut down.
About 95% of the town's water normally comes from the Central Arizona Project canal, which carries Colorado River water, making the backup supply critical when that source is disrupted.
Town officials asked residents to voluntarily conserve water while crews work to restore normal operations.
The outage underscores Cave Creek's reliance on outside water partners and contingency planning when its main Colorado River-fed supply fails.