When the Nabataeans first settled in Petra and started building houses, they found that they needed to protect themselves from the flash floods of the area. They also needed a year-round supply of water for their own use. Therefore, to control floodwaters, a dam was built across the mouth of the Siq in the 1st century BC, along with basins to hold water an 82-meter long rock-cut tunnel, which redirected water through Wadi Mudhlim to reservoirs, water cisterns and dams inside and outside Petra. The dam you see today was constructed in 1964 over the original Nabataean one, which was larger, to prevent a recurrence of a tragic flash flood that had earlier swept through the Siq. The illustration shows a hypothetical depiction of the ancient dam. Towards the end of Wadi Mudhlim there are niches that probably held baetyls, or god blocks, to protect the water, which was sacred to the Nabataeans.