Romans are considered as the greatest aqueduct builders of the ancient world, though qanat systems were in use in ancient Persia, India, Egypt, and other Middle Eastern countries thousand of years earlier. Based on history documents and civil engineering studies, this paper summarizes hydrology and hydraulics engineering techniques developed by Roman Engineers. The study case is the Nîmes Aqueduct and its Pont du Gard bridge, the most intact aqueduct bridge remaining today. Despite the existence of superb ruins and conducts' frames, little is known of the hydraulic engineering of these Roman aqueducts and on their water supply and flow rates. This paper explains hydraulic structures and regulations used. It demonstrates the expertise of Roman Engineers on hydraulics of open channel flows.