A geoarchaeological coring survey of the Forum Boarium has shed considerable light on Rome's archaic landscape. We present the first empirical evidence that substantiates ancient and modern assumptions about the existence of a river harbour and ford in early Rome. Prior to the growth of the city, the riverbank — reconstructed as a high ledge at the base of the Capitoline Hill and a low-lying shore north of the Aventine — was particularly advantageous for river-related activities. However, the river valley changed significantly in the sixth century b.c.e., as a result of complex fluvial processes that were arguably spurred by urbanisation. Around the beginning of the Republic, Rome's original harbour silted up, and a high, wide riverbank emerged in its place. The siltation continued until the Forum Boarium was urbanised in the mid-Republic. In order to build their city and maintain river harbour operations, the Romans therefore had to adapt to dynamic ecological conditions.