Archaeologists widely recognize that the development of large communal structures in Chaco Canyon, New Mexico, between ca. AD 850 and 1150, is an example of emergent social complexity. Most assume that this development was based on agricultural intensification manifest by elaborately engineered water control systems. However, recent investigations in the canyon indicate that evidence for prehistoric water control is surprisingly limited, as well as chronologically ambiguous. This presentation outlines the nature of the evidence and the implications for understanding rapid economic growth reflected in the construction of massive masonry buildings.