How it began
Jawa is an Early Bronze Age site in the basalt Black Desert of what is now Mafraq Governorate in eastern Jordan. A group of perhaps 2,000 migrants built a walled town there in the late 4th millennium BCE, along with earthworks that turned the winter floods of Wadi Rajil into a water supply. The largest of the dams they built is a masonry gravity dam. Carbon-14 dates published in 2015 put its first construction between 3500 and 3400 BCE, and it is described as the oldest known dam in the world.
Before it existed
The basalt around Jawa lets very little water soak into the ground, and about 150 mm of rain falls in a year. Wadi Rajil is a dry river bed that floods irregularly in winter, with short, violent flash floods. Without a way to hold that water through the four dry summer months, a town of several thousand people and its herds could not have lived there. The excavator Svend Helms judged that the water works had to be finished by the first winter after the settlers arrived.
How it works
Three dams crossed Wadi Rajil at Jawa. Two were deflection dams that steered flood water into a feed canal and on to a series of reservoirs west of the town; the canal also filled a natural cave. The third tried to block the wadi completely. The largest reservoir wall was a double stone wall about 80 meters long and 4 to 5 meters high, filled with sand and ash, with traces of revetments and a simple apron on the upstream face. Small canals also collected local rain runoff into the reservoirs.
What changed first
Research cited by Helms suggests the community could survive on about 3 percent of the wadi's yearly flow if it could be stored. The stored water supported a population estimated at 3,000 to 5,000 people, large herds of sheep and goats, some cattle, and irrigated crops including barley, wheat, chickpeas, lentils and grapes. The town was short lived. Helms thought it was occupied for perhaps a single generation, and the full blocking dam appears not to have survived more than one season.
Where it stands today
The ruins were photographed from the air by Antoine Poidebard in 1931 and mistaken for Roman remains. Lankester Harding suggested an Early Bronze Age date in 1950, and Svend Helms excavated the site between 1972 and 1976. Engineering histories, including a 2009 paper in the Proceedings of the Institution of Civil Engineers, treat the Jawa dam as the starting point of dam building. The remains of the dams, canals and reservoirs still stand in the desert.
