Here we present the first 14C ages for the Ascope Canal System (ACS), a large prehispanic hydraulic network in the Chicama Valley on the north coast of Peru. Composed of multiple alignments that irrigated areas north of the river, our results indicate that the ACS was constructed and operated in the Late Intermediate Period, ca. a.d. 1000–1400. This overlaps in time with the Chicama-Moche Intervalley Canal that diverted water on the south side of the Chicama River and extended to the city of Chan Chan. Conservative estimates of discharge capacity indicate that the combined flow through the canals would have exceeded stream flow in the Chicama River during half of the year. The ACS appears to have functioned for several centuries and would have been in direct competition with the Intervalley Canal. There was, apparently, insufficient water for both systems and other Chicama Valley canals during most of the year. This study underscores the complexities of understanding the operations and histories of irrigation systems in complex societies.