Angkor, the capital of the medieval Khmer state, lies at the North West end of the Tonle Sap on its north shore surrounded by the basin of the great lake. When the severe climate change began, first in the 13th century and then in full force in the 14th century, Angkor was adversely affected by its key characteristic – the massive scale of its infrastructure. The water management network is certainly one of the simpler yet most sophisticated and complex systems prior to the 19th century, using minimal gradients and very basic constructions on an enormous scale. The implications of a complex, inertial, stabilising system in a cultural, regional and climatic milieu that was changing will be reviewed in the discussion of the demise of Angkor. The droughts of the 14th century were extremely severe and would surely have crippled the food economy if the water network no longer worked.