We had passed by the Fontaine de Fontestorbes a number of times but had never stopped to look at it. Today we finally did. Incredible. We noticed stepping stones that were under water and a handrail. We decided that the water level must be quite high. We looked at it for a little while and appreciated its beauty but we didn't really understand the true significance of the fountain until we went into the tiny visitor center and talked the very informative individual working there. During the summer, the water level of the fountain rises and falls very regularly in a cycle that can be, remarkably, 30 to 90 minutes depending on the incoming flow to an interior chamber far inside the cave. What we learned follows. (I pirated the information from a website but I edited it heavily, so I probably won’t be prosecuted.)
The Fontaine de Fontestorbes is a karst spring par excellence. It takes the term intermittent very seriously: it changes its output every 30 minutes from around 50 l/s to 1,800 litres per second and back.
But let’s start from the beginning. Fontaine de Fontestorbes is a rather typical karst spring where a cave river comes to the surface. The portal of the cave is located below a white limestone cliff. It exhibits very typical behavior, low water in winter, high flow during snow melt in spring and heavy rains, and again low production during summer and autumn. But one thing is special; there is an additional change in flow, which changes extremely fast. In an hour, the flow changes from almost zero to full production and back again. This means, the cave is almost dry for half an hour and visitors can easily walk in, but then the water rises and the cave is filled with water. The people inside the cave have to wait until the flow drops again, which is about half an hour later.
The explanation for this strange behavior is rather complicated, but based on a rather simple principle. A siphon is basically a tube formed like a U. The bottom of the U is filled with water, and so air is not able to go through the siphon. Think of the pea trap under your sink. Now think of the opposite, a tube looking like an upside down U, with the bow on top. People use this for getting water out of a tank, or gasoline from a car’s tank. If one end of the tube is in the water, the tube runs above the rim of the tank, and when the tube is filled completely with water, the water will flow out as long as the end of the tube is below the surface of the water. That’s because the water on the outside pulls with more energy (if the opening of the tube lies below the surface of the water inside).
The same thing happens inside the cave. A water filled chamber is continually filled from a subterranean river. The exit passage is formed like an upside down U. When the passage is filled with air, no water flows out and the water level in the chamber rises. The water level in the passage rises too, as they are connected. When the water level becomes higher than the highest point in the U, the water flows out. Because water now fills the passage completely, it starts to suck more water out of the chamber until its water level reaches the opening of the other passage. Now air goes into the passage, and the siphon effect ends.