This paper summarises the results of an interdisciplinary study by the Jarash Water Project (JWP) into the aqueduct systems servicing the Decapolis city of Gerasa and its hinterland in the Roman-Byzantine period (Study Period). 1 It brings together the key elements in the physical landscape and explains the impacts of differences in geology and climate on the water supply and use in two adjacent valleys. The paper describes the elements of the relatively simple open channel aqueduct networks and their sources, with a focus on the central Jarash valley. It discusses aspects of the intramural water management system and draws attention to the impact of seasonal variances in spring flow rates, seismic damage, and increasing demand on the operation of the aqueduct networks.