estavelle. (French.) An intermittent resurgence or exsurgence, active only in wet seasons. May act alternatively as a swallow hole and as a rising according to ground-water conditions.
Watson Hiner Monroe (1970): A Glossary of Karst Terminology, Geological Survey Water-supply Paper 1899-k, Library of Congress catalog-card No. 75-607530 DOI online

Estavelles are orifices with a dual function. They either discharge water as a spring or allow water to sink, depending on groundwater conditions.

An estavelle, also called inversac, is a very unusual and somewhat rare karst phenomenon. Nevertheless, it must be considered a typical karst phenomenon, as it can be easily explained by the characteristics of a karst area. In principle, it is a special form of a ponor and an intermittent karst spring.

So what exactly is an estavelle? In short, an access point to the karst groundwater in the form of a cleft or cave, which is either a swallow hole or a spring, depending on the season and/or weather conditions. This means that at certain times it absorbs water, while at other times it pours water. It is also possible that it falls completely dry and has an inactive phase.

The simplest structure of an estavelle is an access to the karst groundwater, which is subject to strong fluctuations that rise above the surface of the earth. This means that when the karstic water level rises and is high, the water flows out of the estavelle and supplies water to either a stream or a lake. For the time being, this is identical to an intermittent karst spring. The peculiarity of the Estavelle is now when the water level falls or is low, when the same karst crevice reabsorbs the water of the karst lake.

Especially interesting, in a way the pure form of an estavelle is when a stream sinks into this dwindling. This means that the stream delivers water above the surface, which is often the case on the border to non-karstified rocks. However, an impermeable layer of marl is also sufficient to provide a spring above the Estavelle. This stream is now supplied with water from the Estavelle when the water level is high. At times when the weather is dry, some or all of the water is swallowed.

A special form of the Estavelle is the submarine Estavelle, i.e. an Estavelle which is located under the sea. At times of high discharge, underwater fresh or brackish water is released. What distinguishes it from a normal submarine spring are the times when the karst water level drops very low and water flows into the cave. However, this inversion is based on different physical principles than in normal estuaries. The karstic water level can generally not sink below sea level, because all fresh water flows into the sea above and below ground, so there must always be a gradient towards the sea. During periods of low inflow, this can fall below sea level for a short period of time by two mechanisms and the seawater flows into the Estavelle.

The simplest mechanism is the tidal range, the water flows out of the estuary at low tide and back again at high tide.

Another mechanism is based on the different specific gravity of salt water and fresh water. In communicating vessels, the water level is normally at the same level, but when seawater with a higher specific gravity is filled in on one side, the water level is lower despite a state of equilibrium. A cave with an undersea outflow and an additional outflow just above sea level can use this effect in a special way. The karstic water level sinks below sea level due to the above-ground outflow, which causes seawater to flow into the cave system at the underwater entrance.