The Tuscan hill town of Siena, Italy, has been supplied by a system of gravity-fed fountains since at least the twelfth century. Medieval statutes and surviving physical evidence reveal that the city maintained the purity of its urban water supply by a combination of physical and legal structures. The urban water supply embodied the provisions of that legislation in the physical arrangements of the fountain complexes. Laws and architecture imposed a hierarchy whereby those uses of water with greater potential for contamination were kept downstream from the uses that required a supply of pure water. Although not unique to Siena, the city’s hierarchal division of water provides a powerful and useful model for allocating contemporary water resources.