The rise of Sumer in the arid land between Tigris and Euphrates remains an enigma. Known as the civilization that first brought us complex social organization and urban life, much of it made possible by the invention of writing, Sumer’s beginnings are hidden in pre-history. The foremost issue that remains unresolved is the development of large-scale irrigation and flood protection engineering to alleviate the long temporal lag between flood and agricultural cycles. New drillcores together with high resolution topographic data are used here to propose that geomorphodynamic changes at the head of the Persian Gulf controlled the cultural ecology of the region leading to urbanization.