The dam, connected to a nine mile long wooden aqueduct, has been unearthed near Dorchester in Dorset. It held back a 600 metre long reservoir containing 270 million litres of water.
Constructed in about 80AD, the complex was designed to supply water to the newly built settlement of Durnovaria, forerunner of modern Dorchester.
Two years of archaeological investigations, led by Bill Putnam of Bournemouth University, have revealed the existence of the dam and reservoir, the timber aqueduct and even the identity of the engineers who designed and built the complex.
Recent excavations have uncovered a Roman military fort at the site, indicating that the dam and the aqueduct were almost certainly constructed by the Roman army - probably on a commercial basis - on behalf of the newly urbanised and Romanised inhabitants of Durnovaria. The engineers and troops were from the Second Augusta Legion, a regiment created by the Emperor Augustus in southern Gaul a century earlier.
The fort would have accommodated about 200 troops and military engineers together with their equipment; and the construction of the dam and aqueduct would have taken a trained workforce of that size between three and four months to complete.
The dam and aqueduct supplied Durnovaria with 3,600 litres of water per minute. This would have been used to fill the town's newly constructed public baths and to serve those individuals who paid water rates.
The reservoir and aqueduct remained in use for about 70 years. But in about 150AD the system was abandoned - possibly following a collapse of the dam.
Excavations in front of the surviving remnants of the dam have revealed a one-and-a-half metre thick layer of mud deposited during the disaster.