An ancient cistern that has been unearthed at the Gihon Spring
The Gihon Spring in Jerusalem has been the only permanent spring in the city for thousands of years. Rainfall that percolates down through the city’s underlying soil and rock eventually encounters a more resistant dolomite rock layer where it is shunted downslope to the southeast until it emerges at an outlet on the western flank of the Kidron Valley. This source of water allowed the adjacent rocky hill to be an excellent fortress for the Jebusites three thousand years ago, where they taunted King David that the blind and the lame could defend their fortress town against any attack or siege. But via a sneak attack (more on this next week) David captured the fortress and made it his base of operations, which grew over the centuries to become Jerusalem—an important center for three of the world’s major religions.
What few people realize is this spring is a truly rare type. My experience as a hydrogeologist got me curious about the reported behavior of the spring’s flow. Most springs have a flow rate that is either constant through time, varies seasonally, or increases only following major rain events. But the Gihon’s different behavior qualified it as a siphon spring—one where hours of little to no flow is interrupted routinely by several minutes of high flow. The word Gihon in Hebrew suggests to surge. What is causing this? There is cave somewhere underground and above the spring that slowly fills with water and is connected. to the spring by a conduit. This conduit possesses a rare occurrence—a section that is shaped like an upside-down U. As water fills the cave it also fills the upstream arm of the U until it tops out, wherein the cave begins emptying through the conduit until air from the cave enters the U, breaking the suction. There have been fewer than twenty siphon springs reported in the entire world. When I was in Jerusalem last year a hydrogeologist colleague of mine at the Geological Survey of Israel told me of another siphon spring he knew of east of Jerusalem in the Judean desert. Most other reported siphon springs in the world are in Croatia.
The Gihon’s odd behavior was reported by Sir Charles Warren in the nineteenth century after he nearly drowned there during the unexpected high flow stage. But the spring no longer behaves like this today. At the end of World War I the occupying British Army reported its surging nature. But the behavior had stopped by later in the twentieth century. Some have suggested the paving of the land surface in and around Jerusalem has reduced infiltration. I believe that an earthquake may have “broken” the siphon. In 1927 there was a large earthquake in the region that cracked the outer wall of the Al-Aqsa Mosque at the Temple Mount. We know the Mount is part of the area over which rainfall contributes water to the spring. The earthquake could have shifted the fractures enough to break the siphon. Even though the Gihon’s enigmatic behavior that lasted for thousands of years no longer exists today, it did during the first century CE. Was this surging behavior the source of the stories that angels would come down and “stir up” waters in the pool before the lame would seek healing? The odd behavior did have an invisible cause.