Lavelanet, Bélesta, Ariege
|Intermittent Spring Karst Spring
|Ymax= 1,800 l/s, Ymin=50 L/h.
|As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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The Fontaine de Fontestorbes is the karst spring par excellence. It takes the term intermittent really serious: it changes its yield every 30 minutes from around 50 l/s to 1,800 l/s and back.
But lets start from the beginning. Fontaine de Fontestorbes is a rather typical karst spring, where a cave river reaches the surface out of a cave. The portal of the cave is located really nice below a white limestone cliff. It shows the typical behaviour, low yield in winter, high yield during snow melt in spring and after heavy rains, again low yield during summer and autumn.
But one thing is special, there is an additional change in yield, which changes extremely fast. In an hour the yield changes from almost zero to full yield and back. This means, the cave is almost half an hour dry, and visitors can easily walk in, but then the water rises and the cave is filled with water. The people inside have to wait until the flow drops again, about half an hour later. Such springs are extremely rare and they are called Ebb and Flow Springs or Rhythmic Karst Springs. Actually this is also the meaning of the term Intermittent Spring, unfortunately this term is often mixed up with seasonal or periodic springs.
The explanation for this strange behaviour is rather complicated, and we have linked a very impressive page with a detailed explanation below. Unfortunately it is in French. However, the whole thing is based on a rather simple principle, so we will try to explain this and leave the details out.
A siphon is a tube formed like a U. The bottom of the U is filled with water, and so the air is not able to go through the siphon. 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 with a trick: if one end is inside the water, the tube running above the rim, and the tube is filled completely with water, the water will flow out. The reason is, that 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 by 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 lever becomes higher than the highest point in the U the water flows out. But as it now fills the passage completely, it starts to suck the water aut of the chamber until its water level reaches the entrance into the passage. Now air goes into the passage, and the effekt ends. And the cycle starts over.