A major theoretical issue in studies of prehistoric societies in the Basin of Mexico concerns the relationship between irrigation and the development of prehistoric settlement systems. Locating and dating the remains of prehistoric water control systems, however, presents a major methodological problem. The discovery of a series of stratified prehistoric canals in the exposed profiles of contemporary borrow pits near Santa Clara Coatitlan provided an excellent opportinity for dating a prehistoric irrigation system. Results of recent excavations, reported on in this paper, demonstrate that these canals are part of a middle Formative floodwater irrigation system, which is the earliest confirmed evidence, to date, for the use of irrigatin in the Basin of Mexico.