| Alcantarilla Dam | |
| Location Map: | Spain |
| Dam Crosses: | Guajaraz (Tagus basin)[1] |
| Location: | Mazarambroz (Toledo), Spain |
| Dam Length: | >800m (2,600feet) |
| Dam Height: | 20m (70feet) |
| Dam Width Base: | 4m (13feet)? |
| Opening: | 1st century |
| Coordinates: | 39.66°N -4.0578°W |
The Alcantarilla Dam is a Roman gravity dam built to supply water to the Roman city of Toletum –present-day Toledo, Spain–, in the Roman province of Hispania Tarraconensis. It was built in the 2nd century BC on a tributary of the River Tagus. Currently in ruins, it is located in present-day Mazarambroz (Toledo).[2] It is believed to be the oldest dam in Spain, and is possibly the oldest known Roman dam.
The toponym "Alcantarilla" means conduit and is of Arabic origin:[3] its Latin name is unknown. It was 20m (70feet) high and at least 550m (1,800feet) long. The water was conveyed to the city by an aqueduct which passed through Layos.
The structure appears to have been similar to the surviving Proserpina Dam near Merida, an earth dam with a stone retaining wall. The upstream retaining wall consists of two parallel rubble-masonry walls about 1m (03feet) thick, separated by a concrete-filled space approximately 0.6m (02feet) wide. The upstream side of the wall was faced with cut stone blocks.
The structure has been in ruins for a long time. There has been speculation that it was not strong enough to cope with a large volume of water. It was possibly breached in the Roman era. Another possibility is that the masonry collapsed upstream, perhaps by the pressure of the earth fill when the water was low, since, unlike later dams, it was not buttressed on the upstream side.
The remains of Toledo's Roman water supply system are partly protected by a heritage designation (Bien de Interés Cultural).[4]