Dams in urban environments may preserve valuable records of sedimentological and hydrological fluctuations. An ongoing archaeological excavation at the Birket al-Ḥamra (Siloam Pool) in Jerusalem has revealed an 11 m–high dam, constructed at the end of the Iron Age IIa (ca. 805–795 BCE). Oxygen stable isotopic composition (18O) was used together with detailed sedimentological and geological observations. Comparison of the present-day chemical and isotopic composition of a nearby Gihon spring with those of water preserved at the base of the dam reveals clear differences, indicating distinct sources and urban influences. Sedimentological and geochemical analysis of deposits accumulated behind the dam suggests the presence of a multi-annual water body strongly influenced by human intervention within the stream’s drainage basin. Integrating the hydrological proxies with archaeological evidence suggests that flooding events in the early first millennium BCE played a critical role in shaping site conditions, promoting inhabitants to adapt their construction techniques to an unstable environment. The combined archaeological, geological, mineralogical, and isotopic datasets highlight the value of an interdisciplinary approach for reconstructing past human–environment interactions.